Agda

A dependently typed functional programming language and proof assistant http://wiki.portal.chalmers.se/agda/

LTS Haskell 12.18:	2.5.4.2
Stackage Nightly 2018-09-28:	2.5.4.1
Latest on Hackage:	2.5.4.2

See all snapshots Agda appears in

LicenseRefOtherLicense licensed by Agda 2 was originally written by Ulf Norell, partially based on code from Agda 1 by Catarina Coquand and Makoto Takeyama, and from Agdalight by Ulf Norell and Andreas Abel. Agda 2 is currently actively developed mainly by Andreas Abel, Guillaume Allais, Jesper Cockx, Nils Anders Danielsson, Philipp Hausmann, Fredrik Nordvall Forsberg, Ulf Norell, Víctor López Juan, Andrés Sicard-Ramírez, and Andrea Vezzosi. Further, Agda 2 has received contributions by, amongst others, Stevan Andjelkovic, Marcin Benke, Jean-Philippe Bernardy, Guillaume Brunerie, James Chapman, Dominique Devriese, Péter Diviánszki, Olle Fredriksson, Adam Gundry, Daniel Gustafsson, Kuen-Bang Hou (favonia), Patrik Jansson, Alan Jeffrey, Wolfram Kahl, Wen Kokke, Fredrik Lindblad, Francesco Mazzoli, Stefan Monnier, Darin Morrison, Guilhem Moulin, Nicolas Pouillard, Nobuo Yamashita, Christian Sattler, and Makoto Takeyama and many more.

Maintained by Ulf Norell

Module documentation for 2.5.4.2

Agda

Agda 2

Note that this README is only about Agda, not its standard library. See the Agda Wiki for information about the library.

Documentation

Getting Started

Hacking on Agda

Head to HACKING

Changes

Release notes for Agda version 2.5.4.2

Installation and infrastructure

Fixed installation with some old versions of cabal-install [Issue #3225].
Using cpp instead of cpphs as the default preprocessor [Issue #3223].
Added support for GHC 8.4.4.

Other closed issues

For 2.5.4.2 the following issues have also been closed (see bug tracker):

#3199: Panics when serialising absolute paths
#3312: Crash in Substitute.hs

Release notes for Agda version 2.5.4.1

Installation and infrastructure

Generated the interface file for the Sigma.agda built-in when installing Agda [Issue #3128].

Emacs mode

Light highlighting is no longer applied continuously, but only when the file is saved [Issue #3119].

Release notes for Agda version 2.5.4

Installation and infrastructure

Added support for GHC 8.2.2 and GHC 8.4.3.

Note that GHC 8.4.* requires cabal-install ≥ 2.2.0.0.
Removed support for GHC 7.8.4.
Included user manual in PDF format in doc/user-manual.pdf.

Language

Call-by-need reduction.

Compile-time weak-head evaluation is now call-by-need, but each weak-head reduction has a local heap, so sharing is not maintained between different reductions.

The reduction machine has been rewritten from scratch and should be faster than the old one in all cases, even those not exploiting laziness.
Compile-time inlining.

Simple definitions (that don’t do any pattern matching) marked as INLINE are now also inlined at compile time, whereas before they were only inlined by the compiler backends. Inlining only triggers in function bodies and not in type signatures, to preserve goal types as far as possible.
Automatic inlining.

Definitions satisfying the following criteria are now automatically inlined (can be disabled using the new NOINLINE pragma):
- No pattern matching.
- Uses each argument at most once.
- Does not use all its arguments.
Automatic inlining can be turned off using the flag --no-auto-inline. This can be useful when debugging tactics that may be affected by whether or not a particular definition is being inlined.

Syntax

Do-notation.

There is now builtin do-notation syntax. This means that do is a reserved keyword and cannot be used as an identifier.

Do-blocks support lets and pattern matching binds. If the pattern in a bind is non-exhaustive the other patterns need to be handled in a where-clause (see example below).

Example:
```
filter : {A : Set} → (A → Bool) → List A → List A
filter p xs = do
  x    ← xs
  true ← return (p x)
    where false → []
  return x
```
Do-blocks desugar to _>>=_ and _>>_ before scope checking, so whatever definitions of these two functions are in scope of the do-block will be used.

More precisely:
- Simple bind
```
do x ← m
   m'
```
  desugars to m >>= λ x → m'.
- Pattern bind
```
do p ← m where pᵢ → mᵢ
   m'
```
  desugars to m >>= λ { p → m'; pᵢ → mᵢ }, where pᵢ → mᵢ is an arbitrary sequence of clauses and follows the usual layout rules for where. If p is exhaustive the where clause can be omitted.
- Non-binding operation
```
do m
   m'
```
  desugars to m >> m'.
- Let
```
do let ds
   m
```
  desugars to let ds in m, where ds is an arbitrary sequence of valid let-declarations.
- The last statement in the do block must be a plain expression (no let or bind).
Bind statements can use either ← or <-. Neither of these are reserved, so code outside do-blocks can use identifiers with these names, but inside a do-block they would need to be used qualified or under different names.
Infix let declarations. [Issue #917]

Let declarations can now be defined in infix (or mixfix) style. For instance:
```
  f : Nat → Nat
  f n = let _!_ : Nat → Nat → Nat
            x ! y = 2 * x + y
        in n ! n
```

Overloaded pattern synonyms. [Issue #2787]

Pattern synonyms can now be overloaded if all candidates have the same shape. Two pattern synonym definitions have the same shape if they are equal up to variable and constructor names. Shapes are checked at resolution time.

For instance, the following is accepted:

  open import Agda.Builtin.Nat

  data List (A : Set) : Set where
    lnil  : List A
    lcons : A → List A → List A

  data Vec (A : Set) : Nat → Set where
    vnil  : Vec A 0
    vcons : ∀ {n} → A → Vec A n → Vec A (suc n)

  pattern [] = lnil
  pattern [] = vnil

  pattern _∷_ x xs = lcons x xs
  pattern _∷_ y ys = vcons y ys

  lmap : ∀ {A B} → (A → B) → List A → List B
  lmap f []       = []
  lmap f (x ∷ xs) = f x ∷ lmap f xs

  vmap : ∀ {A B n} → (A → B) → Vec A n → Vec B n
  vmap f []       = []
  vmap f (x ∷ xs) = f x ∷ vmap f xs

If the file has no top-level module header, the first module cannot have the same name as the file. [Issues #2808 and #1077]

This means that the following file File.agda is rejected:
```
  -- no module header
  postulate A : Set
  module File where -- inner module with the same name as the file
```
Agda reports Illegal declarations(s) before top-level module at the postulate. This is to avoid confusing scope errors in similar situations.

If a top-level module header is inserted manually, the file is accepted:
```
  module _ where    -- user written module header
  postulate A : Set
  module File where -- inner module with the same name as the file, ok
```

Pattern matching

Forced constructor patterns.

Constructor patterns can now be dotted to indicate that Agda should not case split on them but rather their value is forced by the type of the other patterns. The difference between this and a regular dot pattern is that forced constructor patterns can still bind variables in their arguments. For example,

  open import Agda.Builtin.Nat

  data Vec (A : Set) : Nat → Set where
    nil  : Vec A zero
    cons : (n : Nat) → A → Vec A n → Vec A (suc n)

  append : {A : Set} (m n : Nat) → Vec A m → Vec A n → Vec A (m + n)
  append .zero    n nil            ys = ys
  append (.suc m) n (cons .m x xs) ys = cons (m + n) x (append m n xs ys)

Inferring the type of a function based on its patterns

Agda no longer infers the type of a function based on the patterns used in its definition. [Issue #2834]

This means that the following Agda program is no longer accepted:
```
  open import Agda.Builtin.Nat

  f : _ → _
  f zero    = zero
  f (suc n) = n
```
Agda now requires the type of the argument of f to be given explicitly.
Improved constraint solving for pattern matching functions

Constraint solving for functions where each right-hand side has a distinct rigid head has been extended to also cover the case where some clauses return an argument of the function. A typical example is append on lists:
```
  _++_ : {A : Set} → List A → List A → List A
  []       ++ ys = ys
  (x ∷ xs) ++ ys = x ∷ (xs ++ ys)
```
Agda can now solve constraints like ?X ++ ys == 1 ∷ ys when ys is a neutral term.
Record expressions translated to copatterns

Definitions of the form
```
  f ps = record { f₁ = e₁; ..; fₙ = eₙ }
```
are translated internally to use copatterns:
```
  f ps .f₁ = e₁
  ...
  f ps .fₙ = eₙ
```
This means that f ps does not reduce, but thanks to η-equality the two definitions are equivalent.

The change should lead to fewer big record expressions showing up in goal types, and potentially significant performance improvement in some cases.

This may have a minor impact on with-abstraction and code using --rewriting since η-equality is not used in these cases.
When using with, it is now allowed to replace any pattern from the parent clause by a variable in the with clause. For example:
```
  f : List ℕ → List ℕ
  f [] = []
  f (x ∷ xs) with x ≤? 10
  f xs | p = {!!}
```
In the with clause, xs is treated as a let-bound variable with value .x ∷ .xs (where .x : ℕ and .xs : List ℕ are out of scope) and p : Dec (.x ≤ 10).

Since with-abstraction may change the type of variables, instantiations of variables in the with clause are type checked again after with-abstraction.

Builtins

Added support for built-in 64-bit machine words.

These are defined in Agda.Builtin.Word and come with two primitive operations to convert to and from natural numbers.
```
  Word64 : Set
  primWord64ToNat   : Word64 → Nat
  primWord64FromNat : Nat → Word64
```
Converting to a natural number is the trivial embedding, and converting from a natural number gives you the remainder modulo 2^64. The proofs of these theorems are not primitive, but can be defined in a library using primTrustMe.

Basic arithmetic operations can be defined on Word64 by converting to natural numbers, peforming the corresponding operation, and then converting back. The compiler will optimise these to use 64-bit arithmetic. For instance,
```
  addWord : Word64 → Word64 → Word64
  addWord a b = primWord64FromNat (primWord64ToNat a + primWord64ToNat b)

  subWord : Word64 → Word64 → Word64
  subWord a b = primWord64FromNat (primWord64ToNat a + 18446744073709551616 - primWord64ToNat b)
```
These compiles (in the GHC backend) to addition and subtraction on Data.Word.Word64.
New primitive primFloatLess and changed semantics of primFloatNumericalLess.

primFloatNumericalLess now uses standard IEEE <, so for instance NaN < x = x < NaN = false.

On the other hand primFloatLess provides a total order on Float, with -Inf < NaN < -1.0 < -0.0 < 0.0 < 1.0 < Inf.
The SIZEINF builtin is now given the name ∞ in Agda.Builtin.Size [Issue #2931].

Previously it was given the name ω.

Reflection

New TC primitive: declarePostulate. [Issue #2782]
```
  declarePostulate : Arg Name → Type → TC ⊤
```
This can be used to declare new postulates. The Visibility of the Arg must not be hidden. This feature fails when executed with --safe flag from command-line.

Pragmas and options

The --caching option is ON by default and is also a valid pragma. Caching can (sometimes) speed up re-typechecking in --interaction mode by reusing the result of the previous typechecking for the prefix of the file that has not changed (with a granularity at the level of declarations/mutual blocks).

It can be turned off by passing --no-caching to agda or with the following at the top of your file.
```
  {-# OPTIONS --no-caching #-}
```
The --sharing and --no-sharing options have been deprecated and do nothing.

Compile-time evaluation is now always call-by-need.

BUILTIN pragmas can now appear before the top-level module header and in parametrized modules. [Issue #2824]

  {-# OPTIONS --rewriting #-}
  open import Agda.Builtin.Equality
  {-# BUILTIN REWRITE _≡_ #-}  -- here
  module TopLevel (A : Set) where
  {-# BUILTIN REWRITE _≡_ #-}  -- or here

Note that it is still the case that built-ins cannot be bound if they depend on module parameters from an enclosing module. For instance, the following is illegal:

  module _ {a} {A : Set a} where
    data _≡_ (x : A) : A → Set a where
      refl : x ≡ x
    {-# BUILTIN EQUALITY _≡_ #-}

Builtin NIL and CONS have been merged with LIST.

When binding the LIST builtin, NIL and CONS are bound to the appropriate constructors automatically. This means that instead of writing
```
{-# BUILTIN LIST List #-}
{-# BUILTIN NIL  []   #-}
{-# BUILTIN CONS _∷_  #-}
```
you just write
```
{-# BUILTIN LIST List #-}
```
Attempting to bind NIL or CONS results in a warning and has otherwise no effect.
The --no-unicode pragma prevents Agda from introducing unicode characters when pretty printing a term. Lambda, Arrows and Forall quantifiers are all replaced by their ascii only version. Instead of resorting to subscript suffixes, Agda uses ascii digit characters.
New option --inversion-max-depth=N.

The depth is used to avoid looping due to inverting pattern matching for unsatisfiable constraints [Issue #431]. This option is only expected to be necessary in pathological cases.
New option --no-print-pattern-synonyms.

This disables the use of pattern synonyms in output from Agda. See [Issue #2902] for situations where this might be desirable.
New fine-grained control over the warning machinery: ability to (en/dis)able warnings on a one-by-one basis.
The command line option --help now takes an optional argument which allows the user to request more specific usage information about particular topics. The only one added so far is warning.
New pragma NOINLINE.
```
{-# NOINLINE f #-}
```
Disables automatic inlining of f.
New pragma WARNING_ON_USAGE
```
{-# WARNING_ON_USAGE QName Message #}
```
Prints Message whenever QName is used.

Emacs mode

Banana brackets have been added to the Agda input method.

  \((   #x2985  LEFT  WHITE PARENTHESIS
  \))   #x2986  RIGHT WHITE PARENTHESIS

Result splitting will introduce the trailing hidden arguments, if there is nothing else todo [Issue #2871]. Example:
```
  data Fun (A : Set) : Set where
    mkFun : (A → A) → Fun A

  test : {A : Set} → Fun A
  test = ?
```
Splitting on the result here (C-c C-c RET) will append {A} to the left hand side.
```
  test {A} = ?
```
Light highlighting is performed dynamically, even if the file is not loaded [Issue #2794].

This light highlighting is based on the token stream generated by Agda’s lexer: the code is only highlighted if the file is lexically correct. If the Agda backend is not busy with something else, then the code is highlighted automatically in certain situations:
- When the file is saved.
- When Emacs has been idle, continuously, for a certain period of time (by default 0.2 s) after the last modification of the file, and the file has not been saved (or marked as being unmodified). This functionality can be turned off, and the time period can be customised.
Highlighting of comments is no longer handled by Font Lock mode [Issue #2794].
The Emacs mode’s syntax table has been changed.

Previously _ was treated as punctuation. Now it is treated in the same way as most other characters: if the standard syntax table assigns it the syntax class “whitespace”, “open parenthesis” or “close parenthesis”, then it gets that syntax class, and otherwise it gets the syntax class “word constituent”.

Compiler backends

The GHC backend now automatically compiles BUILTIN LIST to Haskell lists.

This means that it’s no longer necessary to give a COMPILE GHC pragma for the builtin list type. Indeed, doing so has no effect on the compilation and results in a warning.
The GHC backend performance improvements.

Generated Haskell code now contains approximate type signatures, which lets GHC get rid of many of the unsafeCoerces. This leads to performance improvements of up to 50% of compiled code.

The GHC backend now compiles the INFINITY, SHARP and FLAT builtins in a different way [Issue #2909].

Previously these were compiled to (basically) nothing. Now the INFINITY builtin is compiled to Infinity, available from MAlonzo.RTE:

data Inf a            = Sharp { flat :: a }
type Infinity level a = Inf a

The SHARP builtin is compiled to Sharp, and the FLAT builtin is (by default) compiled to a corresponding destructor.

Note that code that interacts with Haskell libraries may have to be updated. As an example, here is one way to print colists of characters using the Haskell function putStr:

open import Agda.Builtin.Char
open import Agda.Builtin.Coinduction
open import Agda.Builtin.IO
open import Agda.Builtin.Unit

data Colist {a} (A : Set a) : Set a where
  []  : Colist A
  _∷_ : A → ∞ (Colist A) → Colist A

{-# FOREIGN GHC
  data Colist a    = Nil | Cons a (MAlonzo.RTE.Inf (Colist a))
  type Colist' l a = Colist a

  fromColist :: Colist a -> [a]
  fromColist Nil         = []
  fromColist (Cons x xs) = x : fromColist (MAlonzo.RTE.flat xs)
  #-}

{-# COMPILE GHC Colist = data Colist' (Nil | Cons) #-}

postulate
  putStr : Colist Char → IO ⊤

{-# COMPILE GHC putStr = putStr . fromColist #-}

COMPILE GHC pragmas have been included for the size primitives [Issue #2879].

LaTeX backend

The code environment can now take arguments [Issues #2744 and #2453].

Everything from \begin{code} to the end of the line is preserved in the generated LaTeX code, and not treated as Agda code.

The default implementation of the code environment recognises one optional argument, hide, which can be used for code that should be type-checked, but not typeset:
```
\begin{code}[hide]
  open import Module
\end{code}
```
The AgdaHide macro has not been removed, but has been deprecated in favour of [hide].
The AgdaSuppressSpace and AgdaMultiCode environments no longer take an argument.

Instead some documents need to be compiled multiple times.
The --count-clusters flag can now be given in OPTIONS pragmas.
The nofontsetup option to the LaTeX package agda was broken, and has (hopefully) been fixed [Issue #2773].

Fewer packages than before are loaded when nofontsetup is used, see agda.sty for details. Furthermore, if LuaLaTeX or XeLaTeX are not used, then the font encoding is no longer changed.
The new option noinputencodingsetup instructs the LaTeX package agda to not change the input encoding, and to not load the ucs package.
Underscores are now typeset using \AgdaUnderscore{}.

The default implementation is \_ (the command that was previously generated for underscores). Note that it is possible to override this implementation.
OtherAspects (unsolved meta variables, catchall clauses, etc.) are now correctly highlighted in the LaTeX backend (and the HTML one). [Issue #2474]

HTML backend

An identifier (excluding bound variables), gets the identifier itself as an anchor, in addition to the file position [Issue #2756]. In Agda 2.5.3, the identifier anchor would replace the file position anchor [Issue #2604].

Symbolic anchors look like

<a id="test1">
<a id="M.bla">

while file position anchors just give the character position in the file:

<a id="42">

Top-level module names do not get a symbolic anchor, since the position of a top-level module is defined to be the beginning of the file.

Example:

module Issue2604 where   -- Character position anchor

test1 : Set₁             -- Issue2604.html#test1
test1 = bla
  where
  bla = Set              -- Only character position anchor

test2 : Set₁             -- Issue2604.html#test2
test2 = bla
  where
  bla = Set              -- Only character position anchor

test3 : Set₁             -- Issue2604.html#test3
test3 = bla
  module M where         -- Issue2604.html#M
  bla = Set              -- Issue2604.html#M.bla

module NamedModule where -- Issue2604.html#NamedModule
  test4 : Set₁           -- Issue2604.html#NamedModule.test4
  test4 = M.bla

module _ where           -- Only character position anchor
  test5 : Set₁           -- Only character position anchor
  test5 = M.bla

List of closed issues

For 2.5.4, the following issues have been closed (see bug tracker):

#351: Constraint solving for irrelevant metas
#421: Higher order positivity
#431: Constructor-headed function makes type-checker diverge
#437: Detect when something cannot be a function type
#488: Refining on user defined syntax mixes up the order of the subgoals
#681: Lack of visual state indicators in new Emacs mode
#689: Contradictory constraints should yield error
#708: Coverage checker not taking literal patterns into account properly
#875: Nonstrict irrelevance violated by implicit inference
#964: Allow unsolved metas in imported files
#987: –html anchors could be more informative
#1054: Inlined Agda code in LaTeX backend
#1131: Infix definitions not allowed in let definitions
#1169: Auto fails with non-terminating function
#1268: Hard to print type of variable if the type starts with an instance argument
#1384: Order of constructor arguments matters for coverage checker
#1425: Instances with relevant recursive instance arguments are not considered in irrelevant positions
#1548: Confusing error about ambiguous definition with parametrized modules
#1884: what is the format of the libraries and defaults files
#1906: Possible performance problem
#2056: Cannot instantiate meta to solution…: Pattern checking done too early in where block
#2067: Display forms in parameterised module too general
#2183: Allow splitting on dotted variables
#2226: open {{…}} gets hiding wrong
#2255: Performance issue with deeply-nested lambdas
#2306: Commands in the emacs-mode get confused if we add question marks to the file
#2384: More fine-grained blocking in constraint solver
#2401: LaTeX backend error
#2404: checkType doesn’t accept a type-checking definition checked with the same type
#2420: Failed to solve level constraints in record type with hole
#2421: After emacs starts up, Agda does not process file without restart of Agda
#2436: Agda allows coinductive records with eta-equality
#2450: Irrelevant variables are pruned too eagerly
#2474: The LaTeX and HTML backends do not highlight (all) unsolved metas
#2484: Regression related to sized types
#2526: Better documentation of record modules
#2536: UTF8 parsed incorrectly for literate agda files
#2565: Options for the interaction action give to keep the overloaded literals and sections?
#2576: Shadowing data decl by data sig produces Missing type signature error
#2594: Valid partial cover rejected: “Cannot split on argument of non-datatype”
#2600: Stack complains about Agda.cabal
#2607: Instance search confused when an instance argument is sourced from a record
#2617: Installation instructions
#2623: Incorrect indentation when \AgdaHide is used
#2634: Fixity declaration ignored in definitions in record
#2636: The positivity checker complains when a new definition is added in the same where clause
#2640: Unifier dots the relevant pattern variables when it should dot the irrelevant ones
#2668: Changing the visibility of a module parameter breaks with
#2728: Bad interaction between caching and the warning machinery
#2738: Update Stackage LTS from 9.1 to version supporting Alex 3.2.3
#2744: It should be possible to give arguments to the code environment
#2745: Broken build with GHC 7.8.4 due to (new) version 1.2.2.0 of hashtables
#2749: Add –no-unicode cli option to Agda
#2751: Unsolved constraints, but no highlighting
#2752: Mutual blocks inside instance blocks
#2753: Unsolved constraint, related to instance arguments and sized types
#2756: HTML backend generates broken links
#2758: Relevant meta is instantiated with irrelevant solution
#2759: Empty mutual blocks should be warning rather than error
#2762: Automatically generate DISPLAY pragmas to fold pattern synonyms
#2763: Internal Error at “src/full/Agda/TypeChecking/Abstract.hs:138”
#2765: Inferred level expressions are often “reversed”
#2769: Agda prints ill-formed expression, record argument dropped
#2771: Erroneous ‘with’ error message
#2773: The nofontsetup option does not work as advertised
#2775: Irrelevance to be taken into account in ‘with’ abstraction.
#2776: Dotted variable in inferred type
#2780: Improve level constraint solving for groups of inequality constraints
#2782: Extending Agda reflection to introduce postulates
#2785: internal error @ ConcreteToAbstract.hs:721
#2787: Overloaded pattern synonyms
#2792: Safe modules can sometimes not be imported from unsafe modules
#2794: Using \texttt{-} destroys code coloring in literate file
#2796: Overloaded (inherited) projection resolution fails with parametrized record
#2798: The LaTeX backend ignores the “operator” aspect
#2802: Printing of overloaded functions broken due to eager normalization of projections
#2803: Case splitting loses names of hidden arguments
#2808: Confusing error when inserting declaration before top-level module
#2810: Make --caching a pragma option
#2811: OPTION –caching allowed in file (Issue #2810)
#2819: Forcing analysis doesn’t consider relevance
#2821: BUILTIN BOOL gremlin
#2824: Allow {-# BUILTIN #-} in preamble and in parametrized modules
#2826: Case splitting on earlier variable uses duplicate variable name
#2827: Variables off in with-clauses. Parameter refinement?
#2831: NO_POSITIVITY_CHECK pragma can be written before a mutual block without data or record types
#2832: BUILTIN NIL and CONS are not needed
#2834: Disambiguation of type based on pattern leads to non-unique meta solution
#2836: The Emacs mode does not handle .lagda.tex files
#2840: Internal error in positivity with modules/datatype definitions
#2841: Opting out of idiom brackets
#2844: Root documentation URL redirects to version 2.5.2
#2849: Internal error at absurd pattern followed by rewrite
#2854: Agda worries about possibly empty type of sizes even when no builtins for size are active
#2855: Single-clause definition is both unreachable and incomplete
#2856: Panic: unbound variable
#2859: Error “pattern variable shadows constructor” caused by parameter refinement
#2862: inconsistency from a mutual datatype declaration and module definition
#2867: Give does not insert parenthesis for module parameters
#2868: With –postfix-projections, record fields are printed preceded by a dot when working within the record
#2870: Lexical error for - (hyphen)
#2871: Introduce just trailing hidden arguments by result splitting
#2873: Refinement problem in presence of overloaded constructors
#2874: Internal error in src/full/Agda/TypeChecking/Coverage/Match.hs:312
#2878: Support for GHC 8.4.1
#2879: Include COMPILE GHC pragmas for size primitives
#2881: Internal error in BasicOps
#2883: “internal error in TypeChecking/Substitute.hs:379”
#2884: Missing PDF user manual in the tarball
#2888: Internal error caused by new forcing translation
#2894: Unifier tries to eta expand non-eta record
#2896: Unifier throws away pattern
#2897: Internal error for local modules with refined parameters
#2904: No tab completion for GHCNoMain
#2906: Confusing “cannot be translated to a Haskell type” error message
#2908: primForce is compiled away
#2909: Agda uses newtypes incorrectly, causing wellformed programs to loop
#2911: Inferring missing instance clause panics in refined context
#2912: Add fine-grained control over the displayed warnings
#2914: Slicing ignores as pragma?
#2916: The GHC backend generates code with an incorrect number of constructor arguments
#2917: Very slow due to unsolved size?
#2919: Internal error in Agda.TypeChecking.Forcing
#2921: COMPILE data for data types with erased constructor arguments
#2923: Word.agda not included as builtin
#2925: Allow adding the same rewrite rules multiple times
#2927: Panic related to sized types
#2928: Internal error in Agda.TypeChecking.Rules.LHS
#2931: Rename Agda.Builtin.Size.ω to ∞?
#2941: “coinductive” record inconsistent
#2944: Regression, seemingly related to record expressions
#2945: Inversion warning in code that used to be accepted
#2947: Internal error in Agda.TypeChecking.Forcing
#2952: Wrong compilation of pattern matching to Haskell
#2953: Generated Haskell code does not typecheck
#2954: Pattern matching on string gives unexpected unreachable clause
#2957: Support for async 2.2.1
#2958: as names being duplicated in buffer after with
#2959: Repeating a successful command after revert + reload fails with caching enabled
#2960: Uncommenting indented lines doesn’t work
#2963: Extended lambdas bypass positivity checking in records
#2966: Internal error in Auto
#2968: Bad Interaction with copatterns and eta?, leads to ill-typed terms in error messages.
#2971: Copattern split with --no-irrelevant-projections panics
#2974: Copatterns break canonicity
#2975: Termination checker runs too early for definitions inside record (or: positivity checker runs too late)
#2976: Emacs mode reports errors in connection with highlighting comments
#2978: Double solving of meta
#2985: The termination checker accepts non-terminating code
#2989: Internal error when checking record match in let expr
#2990: Performance regression related to the abstract machine
#2994: Solution accepted in hole is subsequently rejected on reload
#2996: Internal error with -v tc.cover:20
#2997: Internal error in Agda.TypeChecking.Rules.LHS
#2998: Regression: With clause pattern x is not an instance of its parent pattern “eta expansion of x”
#3002: Spurious 1 after simplification
#3004: Agda hangs on extended lambda
#3007: Internal error in Parser
#3012: Internal Error at : “src/full/Agda/TypeChecking/Reduce/Fast.hs:1030”
#3014: Internal error in Rules.LHS
#3020: Missing highlighting in record modules
#3023: Support for GHC 8.4.2
#3024: Postfix projection patterns not highlighted correctly with agda –latex
#3030: [ warning ] user defined warnings
#3031: Eta failure for record meta with irrelevant fields
#3033: Giving and solving don’t insert parenthesis for applications in dot pattern
#3044: Internal error in src/full/Agda/TypeChecking/Substitute/Class.hs:209
#3045: GHC backend generates type without enough arguments
#3046: do-notation causes parse errors in subsequent where clauses
#3049: Positivity unsoundness
#3050: We revert back to call-by-name during positivity checking
#3051: Pattern synonyms should be allowed in mutual blocks
#3052: Another recent inference change
#3062: Literal match does not respect first-match semantics
#3063: Internal error in Agda.TypeChecking.Forcing
#3064: Coverage checker bogus on literals combined with copatterns
#3065: Internal error in coverage checker triggered by literal dot pattern
#3067: checking hangs on invalid program
#3072: invalid section printing
#3074: Wrong hiding causes internal error in LHS checker
#3075: Automatic inlining and tactics
#3078: Error building with GHC 7.10.2: Missing transformers library
#3079: Wrong parameter hiding for instance open
#3080: Case splitting prints out-of-scope pattern synonyms
#3082: Emacs mode regression: a ? inserted before existing hole hijacks its interaction point
#3083: Wrong hiding in module application
#3084: Changes to mode line do not take effect immediately
#3085: Postpone checking a pattern let binding when type is blocked
#3090: Internal error in parser when using parentheses in BUILTIN pragma
#3096: Support GHC 8.4.3

Release notes for Agda version 2.5.3

Installation and infrastructure

Added support for GHC 8.0.2 and 8.2.1.
Removed support for GHC 7.6.3.
Markdown support for literate Agda [PR #2357].

Files ending in .lagda.md will be parsed as literate Markdown files.
- Code blocks start with ``` or ```agda in its own line, and end with ```, also in its own line.
- Code blocks which should be type-checked by Agda but should not be visible when the Markdown is rendered may be enclosed in HTML comment delimiters ().
- Code blocks which should be ignored by Agda, but rendered in the final document may be indented by four spaces.
- Note that inline code fragments are not supported due to the difficulty of interpreting their indentation level with respect to the rest of the file.

Language

Pattern matching

Dot patterns.

The dot in front of an inaccessible pattern can now be skipped if the pattern consists entirely of constructors or literals. For example:
```
  open import Agda.Builtin.Bool

  data D : Bool → Set where
    c : D true

  f : (x : Bool) → D x → Bool
  f true c = true
```
Before this change, you had to write f .true c = true.

With-clause patterns can be replaced by _ [Issue #2363]. Example:

  test : Nat → Set
  test zero    with zero
  test _       | _ = Nat
  test (suc x) with zero
  test _       | _ = Nat

We do not have to spell out the pattern of the parent clause (zero / suc x) in the with-clause if we do not need the pattern variables. Note that x is not in scope in the with-clause!

A more elaborate example, which cannot be reduced to an ellipsis ...:

  record R : Set where
    coinductive -- disallow matching
    field f : Bool
          n : Nat

  data P (r : R) : Nat → Set where
    fTrue  : R.f r ≡ true → P r zero
    nSuc   : P r (suc (R.n r))

  data Q : (b : Bool) (n : Nat) →  Set where
    true! : Q true zero
    suc!  : ∀{b n} → Q b (suc n)

  test : (r : R) {n : Nat} (p : P r n) → Q (R.f r) n
  test r nSuc       = suc!
  test r (fTrue p)  with R.f r
  test _ (fTrue ()) | false
  test _ _          | true = true!  -- underscore instead of (isTrue _)

Pattern matching lambdas (also known as extended lambdas) can now be nullary, mirroring the behaviour for ordinary function definitions. [Issue #2671]

This is useful for case splitting on the result inside an expression: given
```
record _×_ (A B : Set) : Set where
  field
    π₁ : A
    π₂ : B
open _×_
```
one may case split on the result (C-c C-c RET) in a hole
```
  λ { → {!!}}
```
of type A × B to produce
```
  λ { .π₁ → {!!} ; .π₂ → {!!}}
```
Records with a field of an empty type are now recognized as empty by Agda. In particular, they can be matched against with an absurd pattern (). For example:
```
  data ⊥ : Set where

  record Empty : Set where
    field absurdity : ⊥

  magic : Empty → ⊥
  magic ()
```
Injective pragmas.

Injective pragmas can be used to mark a definition as injective for the pattern matching unifier. This can be used as a version of --injective-type-constructors that only applies to specific datatypes. For example:
```
  open import Agda.Builtin.Equality
  data Fin : Nat → Set where
    zero : {n : Nat} → Fin (suc n)
    suc  : {n : Nat} → Fin n → Fin (suc n)

  {-# INJECTIVE Fin #-}

  Fin-injective : {m n : Nat} → Fin m ≡ Fin n → m ≡ n
  Fin-injective refl = refl
```
Aside from datatypes, this pragma can also be used to mark other definitions as being injective (for example postulates).
Metavariables can no longer be instantiated during case splitting. This means Agda will refuse to split instead of taking the first constructor it finds. For example:
```
  open import Agda.Builtin.Nat

  data Vec (A : Set) : Nat → Set where
    nil : Vec A 0
    cons : {n : Nat} → A → Vec A n → Vec A (suc n)

  foo : Vec Nat _ → Nat
  foo x = {!x!}
```
In Agda 2.5.2, case splitting on x produced the single clause foo nil = {!!}, but now Agda refuses to split.

Reflection

New TC primitive: debugPrint.
```
  debugPrint : String → Nat → List ErrorPart → TC ⊤
```
This maps to the internal function reportSDoc. Debug output is enabled with the -v flag at the command line, or in an OPTIONS pragma. For instance, giving -v a.b.c:10 enables printing from debugPrint "a.b.c.d" 10 msg. In the Emacs mode, debug output ends up in the *Agda debug* buffer.

Built-ins

BUILTIN REFL is now superfluous, subsumed by BUILTIN EQUALITY [Issue #2389].

BUILTIN EQUALITY is now more liberal [Issue #2386]. It accepts, among others, the following new definitions of equality:

  -- Non-universe polymorphic:
  data _≡_ {A : Set} (x : A) : A → Set where
    refl : x ≡ x

  -- ... with explicit argument to refl;
  data _≡_ {A : Set} : (x y : A) → Set where
    refl : {x : A} → x ≡ x

  -- ... even visible
  data _≡_ {A : Set} : (x y : A) → Set where
    refl : (x : A) → x ≡ x

  -- Equality in a different universe than domain:
  -- (also with explicit argument to refl)
  data _≡_ {a} {A : Set a} (x : A) : A → Set where
    refl : x ≡ x

The standard definition is still:

  -- Equality in same universe as domain:
  data _≡_ {a} {A : Set a} (x : A) : A → Set a where
    refl : x ≡ x

Miscellaneous

Rule change for omitted top-level module headers. [Issue #1077]

If your file is named Bla.agda, then the following content is rejected.
```
  foo = Set
  module Bla where
    bar = Set
```
Before the fix of this issue, Agda would add the missing module header module Bla where at the top of the file. However, in this particular case it is more likely the user put the declaration foo = Set before the module start in error. Now you get the error
```
  Illegal declaration(s) before top-level module
```
if the following conditions are met:
1. There is at least one non-import declaration or non-toplevel pragma before the start of the first module.
2. The module has the same name as the file.
3. The module is the only module at this level (may have submodules, of course).
If you should see this error, insert a top-level module before the illegal declarations, or move them inside the existing module.

Emacs mode

New warnings:
- Unreachable clauses give rise to a simple warning. They are highlighted in gray.
- Incomplete patterns are non-fatal warnings: it is possible to keep interacting with the file (the reduction will simply be stuck on arguments not matching any pattern). The definition with incomplete patterns are highlighted in wheat.
Clauses which do not hold definitionally are now highlighted in white smoke.
Fewer commands have the side effect that the buffer is saved.
Aborting commands.

Now one can (try to) abort an Agda command by using C-c C-x C-a or a menu entry. The effect is similar to that of restarting Agda (C-c C-x C-r), but some state is preserved, which could mean that it takes less time to reload the module.

Warning: If a command is aborted while it is writing data to disk (for instance .agdai files or Haskell files generated by the GHC backend), then the resulting files may be corrupted. Note also that external commands (like GHC) are not aborted, and their output may continue to be sent to the Emacs mode.
New bindings for the Agda input method:
- All the bold digits are now available. The naming scheme is \Bx for digit x.
- Typing \: you can now get a whole slew of colons.
  
  (The Agda input method originally only bound the standard unicode colon, which looks deceptively like the normal colon.)

Case splitting now preserves underscores. [Issue #819]

  data ⊥ : Set where

  test : {A B : Set} → A → ⊥ → B
  test _ x = {! x !}

Splitting on x yields

  test _ ()

Interactively expanding ellipsis. [Issue #2589] An ellipsis in a with-clause can be expanded by splitting on “variable” “.” (dot).
```
  test0 : Nat → Nat
  test0 x with zero
  ... | q = {! . !}  -- C-c C-c
```
Splitting on dot here yields:
```
  test0 x | q = ?
```

New command to check an expression against the type of the hole it is in and see what it elaborates to. [Issue #2700] This is useful to determine e.g. what solution typeclass resolution yields. The command is bound to C-c C-; and respects the C-u modifier.

  record Pointed (A : Set) : Set where
    field point : A

  it : ∀ {A : Set} {{x : A}} → A
  it {{x}} = x

  instance _ = record { point = 3 - 4 }

  _ : Pointed Nat
  _ = {! it !} -- C-u C-u C-c C-;

yields

  Goal: Pointed Nat
  Elaborates to: record { point = 0 }

If agda2-give is called with a prefix, then giving is forced, i.e., the safety checks are skipped, including positivity, termination, and double type-checking. [Issue #2730]

Invoke forced giving with key sequence C-u C-c C-SPC.

Library management

The name field in an .agda-lib file is now optional. [Issue #2708]

This feature is convenient if you just want to specify the dependencies and include pathes for your local project in an .agda-lib file.

Naturally, libraries without names cannot be depended on.

Compiler backends

Unified compiler pragmas

The compiler pragmas (COMPILED, COMPILED_DATA, etc.) have been unified across backends into two new pragmas:

  {-# COMPILE <Backend> <Name> <Text> #-}
  {-# FOREIGN <Backend> <Text> #-}

The old pragmas still work, but will emit a warning if used. They will be removed completely in Agda 2.6.

The translation of old pragmas into new ones is as follows:

Old	New
`{-# COMPILED f e #-}`	`{-# COMPILE GHC f = e #-}`
`{-# COMPILED_TYPE A T #-}`	`{-# COMPILE GHC A = type T #-}`
`{-# COMPILED_DATA A D C1 .. CN #-}`	`{-# COMPILE GHC A = data D (C1 \| .. \| CN) #-}`
`{-# COMPILED_DECLARE_DATA #-}`	obsolete, removed
`{-# COMPILED_EXPORT f g #-}`	`{-# COMPILE GHC f as g #-}`
`{-# IMPORT M #-}`	`{-# FOREIGN GHC import qualified M #-}`
`{-# HASKELL code #-}`	`{-# FOREIGN GHC code #-}`
`{-# COMPILED_UHC f e #-}`	`{-# COMPILE UHC f = e #-}`
`{-# COMPILED_DATA_UHC A D C1 .. CN #-}`	`{-# COMPILE UHC A = data D (C1 \| .. \| CN) #-}`
`{-# IMPORT_UHC M #-}`	`{-# FOREIGN UHC __IMPORT__ M #-}`
`{-# COMPILED_JS f e #-}`	`{-# COMPILE JS f = e #-}`

GHC Haskell backend

The COMPILED pragma (and the corresponding COMPILE GHC pragma) is now also allowed for functions. This makes it possible to have both an Agda implementation and a native Haskell runtime implementation.

The GHC file header pragmas LANGUAGE, OPTIONS_GHC, and INCLUDE inside a FOREIGN GHC pragma are recognized and printed correctly at the top of the generated Haskell file. [Issue #2712]
UHC compiler backend

The UHC backend has been moved to its own repository [https://github.com/agda/agda-uhc] and is no longer part of the Agda distribution.
Haskell imports are no longer transitively inherited from imported modules.

The (now deprecated) IMPORT and IMPORT_UHC pragmas no longer cause import statements in modules importing the module containing the pragma.

The same is true for the corresponding FOREIGN pragmas.
Support for stand-alone backends.

There is a new API in Agda.Compiler.Backend for creating stand-alone backends using Agda as a library. This allows prospective backend writers to experiment with new backends without having to change the Agda code base.

HTML backend

Anchors for identifiers (excluding bound variables) are now the identifiers themselves rather than just the file position [Issue #2604].

Symbolic anchors look like

<a id="test1">
<a id="M.bla">

while other anchors just give the character position in the file:

<a id="42">

Top-level module names do not get a symbolic anchor, since the position of a top-level module is defined to be the beginning of the file.

Example:

module Issue2604 where   -- Character position anchor

test1 : Set₁             -- Issue2604.html#test1
test1 = bla
  where
  bla = Set              -- Character position anchor

test2 : Set₁             -- Issue2604.html#test2
test2 = bla
  where
  bla = Set              -- Character position anchor

test3 : Set₁             -- Issue2604.html#test3
test3 = bla
  module M where         -- Issue2604.html#M
  bla = Set              -- Issue2604.html#M.bla

module NamedModule where -- Issue2604.html#NamedModule
  test4 : Set₁           -- Issue2604.html#NamedModule.test4
  test4 = M.bla

module _ where           -- Character position anchor
  test5 : Set₁           -- Character position anchor
  test5 = M.bla

Some generated HTML files now have different file names [Issue #2725].

Agda now uses an encoding that amounts to first converting the module names to UTF-8, and then percent-encoding the resulting bytes. For instance, HTML for the module Σ is placed in %CE%A3.html.

LaTeX backend

The LaTeX backend now handles indentation in a different way [Issue #1832].

A constraint on the indentation of the first token t on a line is determined as follows:
- Let T be the set containing every previous token (in any code block) that is either the initial token on its line or preceded by at least one whitespace character.
- Let S be the set containing all tokens in T that are not shadowed by other tokens in T. A token t₁ is shadowed by t₂ if t₂ is further down than t₁ and does not start to the right of t₁.
- Let L be the set containing all tokens in S that start to the left of t, and E be the set containing all tokens in S that start in the same column as t.
- The constraint is that t must be indented further than every token in L, and aligned with every token in E.
Note that if any token in L or E belongs to a previous code block, then the constraint may not be satisfied unless (say) the AgdaAlign environment is used in an appropriate way.

If custom settings are used, for instance if \AgdaIndent is redefined, then the constraint discussed above may not be satisfied. (Note that the meaning of the \AgdaIndent command’s argument has changed, and that the command is now used in a different way in the generated LaTeX files.)

Examples:
- Here C is indented further than B:
```
postulate
  A  B
      C : Set
```
- Here C is not (necessarily) indented further than B, because X shadows B:
```
postulate
  A  B  : Set
  X
      C : Set
```
The new rule is inspired by, but not identical to, the one used by lhs2TeX’s poly mode (see Section 8.4 of the manual for lhs2TeX version 1.17).
Some spacing issues [#2353, #2441, #2733, #2740] have been fixed.
The user can now control the typesetting of (certain) individual tokens by redefining the \AgdaFormat command. Example:
```
\usepackage{ifthen}

% Insert extra space before some tokens.
\DeclareRobustCommand{\AgdaFormat}[2]{%
  \ifthenelse{
    \equal{#1}{≡⟨} \OR
    \equal{#1}{≡⟨⟩} \OR
    \equal{#1}{∎}
  }{\ }{}#2}
```
Note the use of \DeclareRobustCommand. The first argument to \AgdaFormat is the token, and the second argument the thing to be typeset.
One can now instruct the agda package not to select any fonts.

If the nofontsetup option is used, then some font packages are loaded, but specific fonts are not selected:
```
\usepackage[nofontsetup]{agda}
```
The height of empty lines is now configurable [#2734].

The height is controlled by the length \AgdaEmptySkip, which by default is \baselineskip.
The alignment feature regards the string +̲, containing + and a combining character, as having length two. However, it seems more reasonable to treat it as having length one, as it occupies a single column, if displayed “properly” using a monospace font. The new flag --count-clusters is an attempt at fixing this. When this flag is enabled the backend counts “extended grapheme clusters” rather than code points.

Note that this fix is not perfect: a single extended grapheme cluster might be displayed in different ways by different programs, and might, in some cases, occupy more than one column. Here are some examples of extended grapheme clusters, all of which are treated as a single character by the alignment algorithm:
```
│ │
│+̲│
│Ö̂│
│நி│
│ᄀힰᇹ│
│ᄀᄀᄀᄀᄀᄀힰᇹᇹᇹᇹᇹᇹ│
│ │
```
Note also that the layout machinery does not count extended grapheme clusters, but code points. The following code is syntactically correct, but if --count-clusters is used, then the LaTeX backend does not align the two field keywords:
```
  record +̲ : Set₁ where  field A : Set
                          field B : Set
```
The --count-clusters flag is not enabled in all builds of Agda, because the implementation depends on the ICU library, the installation of which could cause extra trouble for some users. The presence of this flag is controlled by the Cabal flag enable-cluster-counting.
A faster variant of the LaTeX backend: QuickLaTeX.

When this variant of the backend is used the top-level module is not type-checked, only scope-checked. This implies that some highlighting information is not available. For instance, overloaded constructors are not resolved.

QuickLaTeX can be invoked from the Emacs mode, or using agda --latex --only-scope-checking. If the module has already been type-checked successfully, then this information is reused; in this case QuickLaTeX behaves like the regular LaTeX backend.

The --only-scope-checking flag can also be used independently, but it is perhaps unclear what purpose that would serve. (The flag can currently not be combined with --html, --dependency-graph or --vim.) The flag is not allowed in safe mode.

Pragmas and options

The --safe option is now a valid pragma.

This makes it possible to declare a module as being part of the safe subset of the language by stating {-# OPTIONS --safe #-} at the top of the corresponding file. Incompatibilities between the --safe option and other options or language constructs are non-fatal errors.
The --no-main option is now a valid pragma.

One can now suppress the compiler warning about a missing main function by putting
```
  {-# OPTIONS --no-main #-}
```
on top of the file.
New command-line option and pragma --warning=MODE (or -W MODE) for setting the warning mode. Current options are
- warn for displaying warnings (default)
- error for turning warnings into errors
- ignore for not displaying warnings

List of fixed issues

For 2.5.3, the following issues have been fixed (see bug tracker):

#142: Inherited dot patterns in with functions are not checked
#623: Error message points to importing module rather than imported module
#657: Yet another display form problem
#668: Ability to stop, or restart, typechecking somehow
#705: confusing error message for ambiguous datatype module name
#719: Error message for duplicate module definition points to external module instead of internal module
#776: Unsolvable constraints should give error
#819: Case-splitting doesn’t preserve underscores
#883: Rewrite loses type information
#899: Instance search fails if there are several definitionally equal values in scope
#1077: problem with module syntax, with parametric module import
#1126: Port optimizations from the Epic backend
#1175: Internal Error in Auto
#1544: Positivity polymorphism needed for compositional positivity analysis
#1611: Interactive splitting instantiates meta
#1664: Add Reflection primitives to expose precedence and fixity
#1817: Solvable size constraints reported as unsolvable
#1832: Insufficient indentation in LaTeX-rendered Agda code
#1834: Copattern matching: order of clauses should not matter here
#1886: Second copies of telescopes not checked?
#1899: Positivity checker does not treat datatypes and record types in the same way
#1975: Type-incorrect instantiated overloaded constructor accepted in pattern
#1976: Type-incorrect instantiated projection accepted in pattern
#2035: Matching on string causes solver to fail with internal error
#2146: Unicode syntax for instance arguments
#2217: Abort Agda without losing state
#2229: Absence or presence of top-level module header affects scope
#2253: Wrong scope error for abstract constructors
#2261: Internal error in Auto/CaseSplit.hs:284
#2270: Printer does not use sections.
#2329: Size solver does not use type Size< i to gain the necessary information
#2354: Interaction between instance search, size solver, and ordinary constraint solver.
#2355: Literate Agda parser does not recognize TeX comments
#2360: With clause stripping chokes on ambiguous projection
#2362: Printing of parent patterns when with-clause does not match
#2363: Allow underscore in with-clause patterns
#2366: With-clause patterns renamed in error message
#2368: Internal error after refining a tactic @ MetaVars.hs:267
#2371: Shadowed module parameter crashes interaction
#2372: problems when instances are declared with inferred types
#2374: Ambiguous projection pattern could be disambiguated by visibility
#2376: Termination checking interacts badly with eta-contraction
#2377: open public is useless before module header
#2381: Search (C-c C-z) panics on pattern synonyms
#2386: Relax requirements of BUILTIN EQUALITY
#2389: BUILTIN REFL not needed
#2400: LaTeX backend error on LaTeX comments
#2402: Parameters not dropped when reporting incomplete patterns
#2403: Termination checker should reduce arguments in structural order check
#2405: instance search failing in parameterized module
#2408: DLub sorts are not serialized
#2412: Problem with checking with sized types
#2413: Agda crashes on x@y pattern
#2415: Size solver reports “inconsistent upper bound” even though there is a solution
#2416: Cannot give size as computed by solver
#2422: Overloaded inherited projections don’t resolve
#2423: Inherited projection on lhs
#2426: On just warning about missing cases
#2429: Irrelevant lambda should be accepted when relevant lambda is expected
#2430: Another regression related to parameter refinement?
#2433: rebindLocalRewriteRules re-adds global rewrite rules
#2434: Exact split analysis is too strict when matching on eta record constructor
#2441: Incorrect alignement in latex using the new ACM format
#2444: Generalising compiler pragmas
#2445: The LaTeX backend is slow
#2447: Cache loaded interfaces even if a type error is encountered
#2449: Agda depends on additional C library icu
#2451: Agda panics when attempting to rewrite a typeclass Eq
#2456: Internal error when postulating instance
#2458: Regression: Agda-2.5.3 loops where Agda-2.5.2 passes
#2462: Overloaded postfix projection does not resolve
#2464: Eta contraction for irrelevant functions breaks subject reduction
#2466: Case split to make hidden variable visible does not work
#2467: REWRITE without BUILTIN REWRITE crashes
#2469: “Partial” pattern match causes segfault at runtime
#2472: Regression related to the auto command
#2477: Sized data type analysis brittle, does not reduce size
#2478: Multiply defined labels on the user manual (pdf)
#2479: “Occurs check” error in generated Haskell code
#2480: Agda accepts incorrect (?) code, subject reduction broken
#2482: Wrong counting of data parameters with new-style mutual blocks
#2483: Files are sometimes truncated to a size of 201 bytes
#2486: Imports via FOREIGN are not transitively inherited anymore
#2488: Instance search inhibits holes for instance fields
#2493: Regression: Agda seems to loop when expression is given
#2494: Instance fields sometimes have incorrect goal types
#2495: Regression: termination checker of Agda-2.5.3 seemingly loops where Agda-2.5.2 passes
#2500: Adding fields to a record can cause Agda to reject previous definitions
#2510: Wrong error with –no-pattern-matching
#2517: “Not a variable error”
#2518: CopatternReductions in TreeLess
#2523: The documentation of --without-K is outdated
#2529: Unable to install Agda on Windows.
#2537: case splitting with ‘with’ creates {_} instead of replicating the arguments it found.
#2538: Internal error when parsing as-pattern
#2543: Case splitting with ellipsis produces spurious parentheses
#2545: Race condition in api tests
#2549: Rewrite rule for higher path constructor does not fire
#2550: Internal error in Agda.TypeChecking.Substitute
#2552: Let bindings in module telescopes crash Agda.Interaction.BasicOps
#2553: Internal error in Agda.TypeChecking.CheckInternal
#2554: More flexible size-assignment in successor style
#2555: Why does the positivity checker care about non-recursive occurrences?
#2558: Internal error in Warshall Solver
#2560: Internal Error in Reduce.Fast
#2564: Non-exact-split highlighting makes other highlighting disappear
#2568: agda2-infer-type-maybe-toplevel (in hole) does not respect “single-solution” requirement of instance resolution
#2571: Record pattern translation does not eta contract
#2573: Rewrite rules fail depending on unrelated changes
#2574: No link attached to module without toplevel name
#2575: Internal error, related to caching
#2577: deBruijn fail for higher order instance problem
#2578: Catch-all clause face used incorrectly for parent with pattern
#2579: Import statements with module instantiation should not trigger an error message
#2580: Implicit absurd match is NonVariant, explicit not
#2583: Wrong de Bruijn index introduced by absurd pattern
#2584: Duplicate warning printing
#2585: Definition by copatterns not modulo eta
#2586: “λ where” with single absurd clause not parsed
#2588: agda --latex produces invalid LaTeX when there are block comments
#2592: Internal Error in Agda/TypeChecking/Serialise/Instances/Common.hs
#2597: Inline record definitions confuse the reflection API
#2602: Debug output messes up AgdaInfo buffer
#2603: Internal error in MetaVars.hs
#2604: Use QNames as anchors in generated HTML
#2605: HTML backend generates anchors for whitespace
#2606: Check that LHS of a rewrite rule doesn’t reduce is too strict
#2612: exact-split documentation is outdated and incomplete
#2613: Parametrised modules, with-abstraction and termination
#2620: Internal error in auto.
#2621: Case splitting instantiates meta
#2626: triggered internal error with sized types in MetaVars module
#2629: Exact splitting should not complain about absurd clauses
#2631: docs for auto aren’t clear on how to use flags/options
#2632: some flags to auto dont seem to work in current agda 2.5.2
#2637: Internal error in Agda.TypeChecking.Pretty, possibly related to sized types
#2639: Performance regression, possibly related to the size solver
#2641: Required instance of FromNat when compiling imported files
#2642: Records with duplicate fields
#2644: Wrong substitution in expandRecordVar
#2645: Agda accepts postulated fields in a record
#2646: Only warn if fixities for undefined symbols are given
#2649: Empty list of “previous definition” in duplicate definition error
#2652: Added a new variant of the colon to the Agda input method
#2653: agda-mode: “cannot refine” inside instance argument even though term to be refined typechecks there
#2654: Internal error on result splitting without –postfix-projections
#2664: Segmentation fault with compiled programs using mutual record
#2665: Documentation: Record update syntax in wrong location
#2666: Internal error at Agda/Syntax/Abstract/Name.hs:113
#2667: Panic error on unbound variable.
#2669: Interaction: incorrect field variable name generation
#2671: Feature request: nullary pattern matching lambdas
#2679: Internal error at “Typechecking/Abstract.hs:133” and “TypeChecking/Telescope.hs:68”
#2682: What are the rules for projections of abstract records?
#2684: Bad error message for abstract constructor
#2686: Abstract constructors should be ignored when resolving overloading
#2690: [regression?] Agda engages in deep search instead of immediately failing
#2700: Add a command to check against goal type (and normalise)
#2703: Regression: Internal error for underapplied indexed constructor
#2705: The GHC backend might diverge in infinite file creation
#2708: Why is the name field in .agda-lib files mandatory?
#2710: Type checker hangs
#2712: Compiler Pragma for headers
#2714: Option –no-main should be allowed as file-local option
#2717: internal error at DisplayForm.hs:197
#2718: Interactive ‘give’ doesn’t insert enough parenthesis
#2721: Without-K doesn’t prevent heterogeneous conflict between literals
#2723: Unreachable clauses in definition by copattern matching trip clause compiler
#2725: File names for generated HTML files
#2726: Old regression related to with
#2727: Internal errors related to rewrite
#2729: Regression: case splitting uses variable name variants instead of the unused original names
#2730: Command to give in spite of termination errors
#2731: Agda fails to build with happy 1.19.6
#2733: Avoid some uses of \AgdaIndent?
#2734: Make height of empty lines configurable
#2736: Segfault using Alex 3.2.2 and cpphs
#2740: Indenting every line of code should be a no-op

Release notes for Agda version 2.5.2

Installation and infrastructure

Modular support for literate programming

Literate programming support has been moved out of the lexer and into the Agda.Syntax.Parser.Literate module.

Files ending in .lagda are still interpreted as literate TeX. The extension .lagda.tex may now also be used for literate TeX files.

Support for more literate code formats and extensions can be added modularly.

By default, .lagda.* files are opened in the Emacs mode corresponding to their last extension. One may switch to and from Agda mode manually.
reStructuredText

Literate Agda code can now be written in reStructuredText format, using the .lagda.rst extension.

As a general rule, Agda will parse code following a line ending in ::, as long as that line does not start with ... The module name must match the path of the file in the documentation, and must be given explicitly. Several files have been converted already, for instance:
- language/mixfix-operators.lagda.rst
- tools/compilers.lagda.rst
Note that:
- Code blocks inside an rST comment block will be type-checked by Agda, but not rendered in the documentation.
- Code blocks delimited by .. code-block:: agda will be rendered in the final documenation, but not type-checked by Agda.
- All lines inside a codeblock must be further indented than the first line of the code block.
- Indentation must be consistent between code blocks. In other words, the file as a whole must be a valid Agda file if all the literate text is replaced by white space.
Documentation testing

All documentation files in the doc/user-manual directory that end in .lagda.rst can be typechecked by running make user-manual-test, and also as part of the general test suite.
Support installation through Stack

The Agda sources now also include a configuration for the stack install tool (tested through continuous integration).

It should hence be possible to repeatably build any future Agda version (including unreleased commits) from source by checking out that version and running stack install from the checkout directory. By using repeatable builds, this should keep selecting the same dependencies in the face of new releases on Hackage.

For further motivation, see Issue #2005.
Removed the --test command-line option

This option ran the internal test-suite. This test-suite was implemented using Cabal supports for test-suites. [Issue #2083].
The --no-default-libraries flag has been split into two flags [Issue #1937]
- --no-default-libraries: Ignore the defaults file but still look for local .agda-lib files
- --no-libraries: Don’t use any .agda-lib files (the previous behaviour of --no-default-libraries).
If agda was built inside git repository, then the --version flag will display the hash of the commit used, and whether the tree was -dirty (i.e. there were uncommited changes in the working directory). Otherwise, only the version number is shown.

Language

Dot patterns are now optional

Consider the following program

data Vec (A : Set) : Nat → Set where
  []   : Vec A zero
  cons : ∀ n → A → Vec A n → Vec A (suc n)

vmap : ∀ {A B} n → (A → B) → Vec A n → Vec B n
vmap .zero    f []            = []
vmap .(suc m) f (cons m x xs) = cons m (f x) (vmap m f xs)

If we don’t care about the dot patterns they can (and could previously) be replaced by wildcards:

vmap : ∀ {A B} n → (A → B) → Vec A n → Vec B n
vmap _ f []            = []
vmap _ f (cons m x xs) = cons m (f x) (vmap m f xs)

Now it is also allowed to give a variable pattern in place of the dot pattern. In this case the variable will be bound to the value of the dot pattern. For our example:

vmap : ∀ {A B} n → (A → B) → Vec A n → Vec B n
vmap n f []            = []
vmap n f (cons m x xs) = cons m (f x) (vmap m f xs)

In the first clause n reduces to zero and in the second clause n reduces to suc m.

Module parameters can now be refined by pattern matching

Previously, pattern matches that would refine a variable outside the current left-hand side was disallowed. For instance, the following would give an error, since matching on the vector would instantiate n.
```
module _ {A : Set} {n : Nat} where
  f : Vec A n → Vec A n
  f []       = []
  f (x ∷ xs) = x ∷ xs
```
Now this is no longer disallowed. Instead n is bound to the appropriate value in each clause.
With-abstraction now abstracts also in module parameters

The change that allows pattern matching to refine module parameters also allows with-abstraction to abstract in them. For instance,
```
module _ (n : Nat) (xs : Vec Nat (n + n)) where
  f : Nat
  f with n + n
  f    | nn = ? -- xs : Vec Nat nn
```
Note: Any function argument or lambda-bound variable bound outside a given function counts as a module parameter.

To prevent abstraction in a parameter you can hide it inside a definition. In the above example,
```
module _ (n : Nat) (xs : Vec Nat (n + n)) where

  ys : Vec Nat (n + n)
  ys = xs

  f : Nat
  f with n + n
  f    | nn = ? -- xs : Vec Nat nn, ys : Vec Nat (n + n)
```
As-patterns [Issue #78].

As-patterns (@-patterns) are finally working and can be used to name a pattern. The name has the same scope as normal pattern variables (i.e. the right-hand side, where clause, and dot patterns). The name reduces to the value of the named pattern. For example::
```
module _ {A : Set} (_<_ : A → A → Bool) where
  merge : List A → List A → List A
  merge xs [] = xs
  merge [] ys = ys
  merge xs@(x ∷ xs₁) ys@(y ∷ ys₁) =
    if x < y then x ∷ merge xs₁ ys
             else y ∷ merge xs ys₁
```
Idiom brackets.

There is new syntactic sugar for idiom brackets:

(| e a1 .. an |) expands to

pure e <*> a1 <*> .. <*> an

The desugaring takes place before scope checking and only requires names pure and _<*>_ in scope. Idiom brackets work well with operators, for instance

(| if a then b else c |) desugars to

pure if_then_else_ <*> a <*> b <*> c

Limitations:
- The top-level application inside idiom brackets cannot include implicit applications, so (| foo {x = e} a b |) is illegal. In the case e is pure you can write (| (foo {x = e}) a b |) which desugars to
  
  pure (foo {x = e}) <*> a <*> b
- Binding syntax and operator sections cannot appear immediately inside idiom brackets.
Layout for pattern matching lambdas.

You can now write pattern matching lambdas using the syntax
```
λ where false → true
        true  → false
```
avoiding the need for explicit curly braces and semicolons.
Overloaded projections [Issue #1944].

Ambiguous projections are no longer a scope error. Instead they get resolved based on the type of the record value they are eliminating. This corresponds to constructors, which can be overloaded and get disambiguated based on the type they are introducing. Example:
```
module _ (A : Set) (a : A) where

record R B : Set where
  field f : B
open R public

record S B : Set where
  field f : B
open S public
```
Exporting f twice from both R and S is now allowed. Then,
```
r : R A
f r = a

s : S A
f s = f r
```
disambiguates to:
```
r : R A
R.f r = a

s : S A
S.f s = R.f r
```
If the type of the projection is known, it can also be disambiguated unapplied.
```
unapplied : R A -> A
unapplied = f
```
Postfix projections [Issue #1963].

Agda now supports a postfix syntax for projection application. This style is more in harmony with copatterns. For example:
```
record Stream (A : Set) : Set where
  coinductive
  field head : A
        tail : Stream A

open Stream

repeat : ∀{A} (a : A) → Stream A
repeat a .head = a
repeat a .tail = repeat a

zipWith : ∀{A B C} (f : A → B → C) (s : Stream A) (t : Stream B) → Stream C
zipWith f s t .head = f (s .head) (t .head)
zipWith f s t .tail = zipWith f (s .tail) (t .tail)

module Fib (Nat : Set) (zero one : Nat) (plus : Nat → Nat → Nat) where

  {-# TERMINATING #-}
  fib : Stream Nat
  fib .head = zero
  fib .tail .head = one
  fib .tail .tail = zipWith plus fib (fib .tail)
```
The thing we eliminate with projection now is visibly the head, i.e., the left-most expression of the sequence (e.g. repeat in repeat a .tail).

The syntax overlaps with dot patterns, but for type correct left hand sides there is no confusion: Dot patterns eliminate function types, while (postfix) projection patterns eliminate record types.

By default, Agda prints system-generated projections (such as by eta-expansion or case splitting) prefix. This can be changed with the new option:
```
{-# OPTIONS --postfix-projections #-}
```
Result splitting in extended lambdas (aka pattern lambdas) always produces postfix projections, as prefix projection pattern do not work here: a prefix projection needs to go left of the head, but the head is omitted in extended lambdas.
```
dup : ∀{A : Set}(a : A) → A × A
dup = λ{ a → ? }
```
Result splitting (C-c C-c RET) here will yield:
```
dup = λ{ a .proj₁ → ? ; a .proj₂ → ? }
```
Projection parameters [Issue #1954].

When copying a module, projection parameters will now stay hidden arguments, even if the module parameters are visible. This matches the situation we had for constructors since long. Example:
```
module P (A : Set) where
  record R : Set where
    field f : A

open module Q A = P A
```
Parameter A is now hidden in R.f:
```
test : ∀{A} → R A → A
test r = R.f r
```
Note that a module parameter that corresponds to the record value argument of a projection will not be hidden.
```
module M (A : Set) (r : R A) where
  open R A r public

test' : ∀{A} → R A → A
test' r = M.f r
```
Eager insertion of implicit arguments [Issue #2001]

Implicit arguments are now (again) eagerly inserted in left-hand sides. The previous behaviour of inserting implicits for where blocks, but not right-hand sides was not type safe.
Module applications can now be eta expanded/contracted without changing their behaviour [Issue #1985]

Previously definitions exported using open public got the incorrect type for underapplied module applications.

Example:
```
module A where
  postulate A : Set

module B (X : Set) where
  open A public

module C₁ = B
module C₂ (X : Set) = B X
```
Here both C₁.A and C₂.A have type (X : Set) → Set.
Polarity pragmas.

Polarity pragmas can be attached to postulates. The polarities express how the postulate’s arguments are used. The following polarities are available:

_: Unused.

++: Strictly positive.

+: Positive.

-: Negative.

*: Unknown/mixed.

Polarity pragmas have the form
```
{-# POLARITY name <zero or more polarities> #-}
```
and can be given wherever fixity declarations can be given. The listed polarities apply to the given postulate’s arguments (explicit/implicit/instance), from left to right. Polarities currently cannot be given for module parameters. If the postulate takes n arguments (excluding module parameters), then the number of polarities given must be between 0 and n (inclusive).

Polarity pragmas make it possible to use postulated type formers in recursive types in the following way:
```
postulate
  ∥_∥ : Set → Set

{-# POLARITY ∥_∥ ++ #-}

data D : Set where
  c : ∥ D ∥ → D
```
Note that one can use postulates that may seem benign, together with polarity pragmas, to prove that the empty type is inhabited:
```
postulate
  _⇒_    : Set → Set → Set
  lambda : {A B : Set} → (A → B) → A ⇒ B
  apply  : {A B : Set} → A ⇒ B → A → B

{-# POLARITY _⇒_ ++ #-}

data ⊥ : Set where

data D : Set where
  c : D ⇒ ⊥ → D

not-inhabited : D → ⊥
not-inhabited (c f) = apply f (c f)

inhabited : D
inhabited = c (lambda not-inhabited)

bad : ⊥
bad = not-inhabited inhabited
```
Polarity pragmas are not allowed in safe mode.
Declarations in a where-block are now private. [Issue #2101] This means that
```
f ps = body where
  decls
```
is now equivalent to
```
f ps = body where
  private
    decls
```
This changes little, since the decls were anyway not in scope outside body. However, it makes a difference for abstract definitions, because private type signatures can see through abstract definitions. Consider:
```
record Wrap (A : Set) : Set where
  field unwrap : A

postulate
  P : ∀{A : Set} → A → Set

abstract

  unnamedWhere : (A : Set) → Set
  unnamedWhere A = A
    where  -- the following definitions are private!
    B : Set
    B = Wrap A

    postulate
      b : B
      test : P (Wrap.unwrap b)  -- succeeds
```
The abstract is inherited in where-blocks from the parent (here: function unnamedWhere). Thus, the definition of B is opaque and the type equation B = Wrap A cannot be used to check type signatures, not even of abstract definitions. Thus, checking the type P (Wrap.unwrap b) would fail. However, if test is private, abstract definitions are translucent in its type, and checking succeeds. With the implemented change, all where-definitions are private, in this case B, b, and test, and the example succeeds.

Nothing changes for the named forms of where,
```
module M where
module _ where
```
For instance, this still fails:
```
abstract

  unnamedWhere : (A : Set) → Set
  unnamedWhere A = A
    module M where
    B : Set
    B = Wrap A

    postulate
      b : B
      test : P (Wrap.unwrap b)  -- fails
```
Private anonymous modules now work as expected [Issue #2199]

Previously the private was ignored for anonymous modules causing its definitions to be visible outside the module containing the anonymous module. This is no longer the case. For instance,
```
module M where
  private
    module _ (A : Set) where
      Id : Set
      Id = A

  foo : Set → Set
  foo = Id

open M

bar : Set → Set
bar = Id -- Id is no longer in scope here
```

Pattern synonyms are now expanded on left hand sides of DISPLAY pragmas [Issue #2132]. Example:

data D : Set where
  C c : D
  g : D → D

pattern C′ = C

{-# DISPLAY C′ = C′ #-}
{-# DISPLAY g C′ = c #-}

This now behaves as:

{-# DISPLAY C = C′ #-}
{-# DISPLAY g C = c #-}

Expected error for

test : C ≡ g C
test = refl

is thus:

C′ != c of type D

The built-in floats have new semantics to fix inconsistencies and to improve cross-platform portability.
- Float equality has been split into two primitives. primFloatEquality is designed to establish decidable propositional equality while primFloatNumericalEquality is intended for numerical computations. They behave as follows:
```
primFloatEquality NaN NaN = True
primFloatEquality 0.0 -0.0 = False

primFloatNumericalEquality NaN NaN = False
primFloatNumericalEquality 0.0 -0.0 = True
```
  This change fixes an inconsistency, see [Issue #2169]. For further detail see the user manual.
- Floats now have only one NaN value. This is necessary for proper Float support in the JavaScript backend, as JavaScript (and some other platforms) only support one NaN value.
- The primitive function primFloatLess was renamed primFloatNumericalLess.

Added new primitives to built-in floats:

primFloatNegate : Float → Float [Issue #2194]

Trigonometric primitives [Issue #2200]:

primCos   : Float → Float
primTan   : Float → Float
primASin  : Float → Float
primACos  : Float → Float
primATan  : Float → Float
primATan2 : Float → Float → Float

Anonymous declarations [Issue #1465].

A module can contain an arbitrary number of declarations named _ which will scoped-checked and type-checked but won’t be made available in the scope (nor exported). They cannot introduce arguments on the LHS (but one can use lambda-abstractions on the RHS) and they cannot be defined by recursion.
```
_ : Set → Set
_ = λ x → x
```

Rewriting

The REWRITE pragma can now handle several names. E.g.:
```
{-# REWRITE eq1 eq2 #-}
```

Reflection

You can now use macros in reflected terms [Issue #2130].

For instance, given a macro
```
macro
  some-tactic : Term → TC ⊤
  some-tactic = ...
```
the term def (quote some-tactic) [] represents a call to the macro. This makes it a lot easier to compose tactics.
The reflection machinery now uses normalisation less often:
- Macros no longer normalise the (automatically quoted) term arguments.
- The TC primitives inferType, checkType and quoteTC no longer normalise their arguments.
- The following deprecated constructions may also have been changed: quoteGoal, quoteTerm, quoteContext and tactic.
New TC primitive: withNormalisation.

To recover the old normalising behaviour of inferType, checkType, quoteTC and getContext, you can wrap them inside a call to withNormalisation true:
```
  withNormalisation : ∀ {a} {A : Set a} → Bool → TC A → TC A
```
New TC primitive: reduce.
```
reduce : Term → TC Term
```
Reduces its argument to weak head normal form.
Added new TC primitive: isMacro [Issue #2182]
```
isMacro : Name → TC Bool
```
Returns true if the name refers to a macro, otherwise false.

The record-type constructor now has an extra argument containing information about the record type’s fields:

  data Definition : Set where
    …
    record-type : (c : Name) (fs : List (Arg Name)) → Definition
    …

Type checking

Files with open metas can be imported now [Issue #964]. This should make simultaneous interactive development on several modules more pleasant.

Requires option: --allow-unsolved-metas

Internally, before serialization, open metas are turned into postulates named
```
  unsolved#meta.<nnn>
```
where <nnn> is the internal meta variable number.
The performance of the compile-time evaluator has been greatly improved.
- Fixed a memory leak in evaluator (Issue #2147).
- Reduction speed improved by an order of magnitude and is now comparable to the performance of GHCi. Still call-by-name though.
The detection of types that satisfy K added in Agda 2.5.1 has been rolled back (see Issue #2003).
Eta-equality for record types is now only on after the positivity checker has confirmed it is safe to have it. Eta-equality for unguarded inductive records previously lead to looping of the type checker. [See Issue #2197]
```
record R : Set where
  inductive
  field r : R

  loops : R
  loops = ?
```
As a consequence of this change, the following example does not type-check any more:
```
mutual
  record ⊤ : Set where

  test : ∀ {x y : ⊤} → x ≡ y
  test = refl
```
It fails because the positivity checker is only run after the mutual block, thus, eta-equality for ⊤ is not available when checking test.

One can declare eta-equality explicitly, though, to make this example work.
```
mutual
  record ⊤ : Set where
    eta-equality

  test : ∀ {x y : ⊤} → x ≡ y
  test = refl
```
Records with instance fields are now eta expanded before instance search.

For instance, assuming Eq and Ord with boolean functions _==_ and _<_ respectively,
```
  record EqAndOrd (A : Set) : Set where
    field {{eq}}  : Eq A
          {{ord}} : Ord A


  leq : {A : Set} {{_ : EqAndOrd A}} → A → A → Bool
  leq x y = x == y || x < y
```
Here the EqAndOrd record is automatically unpacked before instance search, revealing the component Eq and Ord instances.

This can be used to simulate superclass dependencies.
Overlappable record instance fields.

Instance fields in records can be marked as overlappable using the new overlap keyword:
```
  record Ord (A : Set) : Set where
    field
      _<_ : A → A → Bool
      overlap {{eqA}} : Eq A
```
When instance search finds multiple candidates for a given instance goal and they are all overlappable it will pick the left-most candidate instead of refusing to solve the instance goal.

This can be use to solve the problem arising from shared “superclass” dependencies. For instance, if you have, in addition to Ord above, a Num record that also has an Eq field and want to write a function requiring both Ord and Num, any Eq constraint will be solved by the Eq instance from whichever argument that comes first.
```
  record Num (A : Set) : Set where
    field
      fromNat : Nat → A
      overlap {{eqA}} : Eq A

  lessOrEqualFive : {A : Set} {{NumA : Num A}} {{OrdA : Ord A}} → A → Bool
  lessOrEqualFive x = x == fromNat 5 || x < fromNat 5
```
In this example the call to _==_ will use the eqA field from NumA rather than the one from OrdA. Note that these may well be different.
Instance fields can be left out of copattern matches [Issue #2288]

Missing cases for instance fields (marked {{ }}) in copattern matches will be solved using instance search. This makes defining instances with superclass fields much nicer. For instance, we can define Nat instances of Eq, Ord and Num from above as follows:
```
  instance
    EqNat : Eq Nat
    _==_ {{EqNat}} n m = eqNat n m

    OrdNat : Ord Nat
    _<_ {{OrdNat}} n m = lessNat n m

    NumNat : Num Nat
    fromNat {{NumNat}} n = n
```
The eqA fields of Ord and Num are filled in using instance search (with EqNat in this case).
Limited instance search depth [Issue #2269]

To prevent instance search from looping on bad instances (see Issue #1743) the search depth of instance search is now limited. The maximum depth can be set with the --instance-search-depth flag and the default value is 500.

Emacs mode

New command C-u C-u C-c C-n: Use show to display the result of normalisation.

Calling C-u C-u C-c C-n on an expression e (in a hole or at top level) normalises show e and prints the resulting string, or an error message if the expression does not normalise to a literal string.

This is useful when working with complex data structures for which you have defined a nice Show instance.

Note that the name show is hardwired into the command.

Changed feature: Interactively split result.

Make-case (C-c C-c) with no variables will now either introduce function arguments or do a copattern split (or fail).

This is as before:

test : {A B : Set} (a : A) (b : B) → A × B
test a b = ?

-- expected:
-- proj₁ (test a b) = {!!}
-- proj₂ (test a b) = {!!}

testFun : {A B : Set} (a : A) (b : B) → A × B
testFun = ?

-- expected:
-- testFun a b = {!!}

This is has changed:

record FunRec A : Set where
  field funField : A → A
open FunRec

testFunRec : ∀{A} → FunRec A
testFunRec = ?

-- expected (since 2016-05-03):
-- funField testFunRec = {!!}

-- used to be:
-- funField testFunRec x = {!!}

Changed feature: Split on hidden variables.

Make-case (C-c C-c) will no longer split on the given hidden variables, but only make them visible. (Splitting can then be performed in a second go.)
```
test : ∀{N M : Nat} → Nat → Nat → Nat
test N M = {!.N N .M!}
```
Invoking splitting will result in:
```
test {N} {M} zero M₁ = ?
test {N} {M} (suc N₁) M₁ = ?
```
The hidden .N and .M have been brought into scope, the visible N has been split upon.
Non-fatal errors/warnings.

Non-fatal errors and warnings are now displayed in the info buffer and do not interrupt the typechecking of the file.

Currently termination errors, unsolved metavariables, unsolved constraints, positivity errors, deprecated BUILTINs, and empty REWRITING pragmas are non-fatal errors.
Highlighting for positivity check failures

Negative occurences of a datatype in its definition are now highlighted in a way similar to termination errors.
The abbrev for codata was replaced by an abbrev for code environments.

If you type c C-x ' (on a suitably standard setup), then Emacs will insert the following text:
```
\begin{code}<newline>  <cursor><newline>\end{code}<newline>.
```
The LaTeX backend can now be invoked from the Emacs mode.

Using the compilation command (C-c C-x C-c).

The flag --latex-dir can be used to set the output directory (by default: latex). Note that if this directory is a relative path, then it is interpreted relative to the “project root”. (When the LaTeX backend is invoked from the command line the path is interpreted relative to the current working directory.) Example: If the module A.B.C is located in the file /foo/A/B/C.agda, then the project root is /foo/, and the default output directory is /foo/latex/.
The compilation command (C-c C-x C-c) now by default asks for a backend.

To avoid this question, set the customisation variable agda2-backend to an appropriate value.
The command agda2-measure-load-time no longer “touches” the file, and the optional argument DONT-TOUCH has been removed.
New command C-u (C-u) C-c C-s: Simplify or normalise the solution C-c C-s produces

When writing examples, it is nice to have the hole filled in with a normalised version of the solution. Calling C-c C-s on
```
_ : reverse (0 ∷ 1 ∷ []) ≡ ?
_ = refl
```
used to yield the non informative reverse (0 ∷ 1 ∷ []) when we would have hopped to get 1 ∷ 0 ∷ [] instead. We can now control finely the degree to which the solution is simplified.
Changed feature: Solving the hole at point

Calling C-c C-s inside a specific goal does not solve all the goals already instantiated internally anymore: it only solves the one at hand (if possible).
New bindings: All the blackboard bold letters are now available [Pull Request #2305]

The Agda input method only bound a handful of the blackboard bold letters but programmers were actually using more than these. They are now all available: lowercase and uppercase. Some previous bindings had to be modified for consistency. The naming scheme is as follows:
- \bx for lowercase blackboard bold
- \bX for uppercase blackboard bold
- \bGx for lowercase greek blackboard bold (similar to \Gx for greeks)
- \bGX for uppercase greek blackboard bold (similar to \GX for uppercase greeks)
Replaced binding for go back

Use M-, (instead of M-*) for go back in Emacs ≥ 25.1 (and continue using M-* with previous versions of Emacs).

Compiler backends

JS compiler backend

The JavaScript backend has been (partially) rewritten. The JavaScript backend now supports most Agda features, notably copatterns can now be compiled to JavaScript. Furthermore, the existing optimizations from the other backends now apply to the JavaScript backend as well.

GHC, JS and UHC compiler backends

Added new primitives to built-in floats [Issues #2194 and #2200]:

primFloatNegate : Float → Float
primCos         : Float → Float
primTan         : Float → Float
primASin        : Float → Float
primACos        : Float → Float
primATan        : Float → Float
primATan2       : Float → Float → Float

LaTeX backend

Code blocks are now (by default) surrounded by vertical space. [Issue #2198]

Use \AgdaNoSpaceAroundCode{} to avoid this vertical space, and \AgdaSpaceAroundCode{} to reenable it.

Note that, if \AgdaNoSpaceAroundCode{} is used, then empty lines before or after a code block will not necessarily lead to empty lines in the generated document. However, empty lines inside the code block do (by default) lead to empty lines in the output.

If you prefer the previous behaviour, then you can use the agda.sty file that came with the previous version of Agda.
\AgdaHide{...} now eats trailing spaces (using \ignorespaces).
New environments: AgdaAlign, AgdaSuppressSpace and AgdaMultiCode.

Sometimes one might want to break up a code block into multiple pieces, but keep code in different blocks aligned with respect to each other. Then one can use the AgdaAlign environment. Example usage:
```
  \begin{AgdaAlign}
  \begin{code}
    code
      code  (more code)
  \end{code}
  Explanation...
  \begin{code}
    aligned with "code"
      code  (aligned with (more code))
  \end{code}
  \end{AgdaAlign}
```
Note that AgdaAlign environments should not be nested.

Sometimes one might also want to hide code in the middle of a code block. This can be accomplished in the following way:
```
  \begin{AgdaAlign}
  \begin{code}
    visible
  \end{code}
  \AgdaHide{
  \begin{code}
    hidden
  \end{code}}
  \begin{code}
    visible
  \end{code}
  \end{AgdaAlign}
```
However, the result may be ugly: extra space is perhaps inserted around the code blocks.

The AgdaSuppressSpace environment ensures that extra space is only inserted before the first code block, and after the last one (but not if \AgdaNoSpaceAroundCode{} is used).

The environment takes one argument, the number of wrapped code blocks (excluding hidden ones). Example usage:
```
  \begin{AgdaAlign}
  \begin{code}
    code
      more code
  \end{code}
  Explanation...
  \begin{AgdaSuppressSpace}{2}
  \begin{code}
    aligned with "code"
      aligned with "more code"
  \end{code}
  \AgdaHide{
  \begin{code}
    hidden code
  \end{code}}
  \begin{code}
      also aligned with "more code"
  \end{code}
  \end{AgdaSuppressSpace}
  \end{AgdaAlign}
```
Note that AgdaSuppressSpace environments should not be nested.

There is also a combined environment, AgdaMultiCode, that combines the effects of AgdaAlign and AgdaSuppressSpace.

Tools

agda-ghc-names

The agda-ghc-names now has its own repository at

https://github.com/agda/agda-ghc-names

and is no longer distributed with Agda.

Release notes for Agda version 2.5.1.2

Fixed broken type signatures that were incorrectly accepted due to GHC #12784.

Release notes for Agda version 2.5.1.1

Installation and infrastructure

Added support for GHC 8.0.1.
Documentation is now built with Python >=3.3, as done by readthedocs.org.

Bug fixes

Fixed a serious performance problem with instance search

Issues #1952 and #1998. Also related: #1955 and #2025

Interactively splitting variable with C-c C-c no longer introduces new trailing patterns. This fixes Issue #1950.

data Ty : Set where
  _⇒_ : Ty → Ty → Ty

⟦_⟧ : Ty → Set
⟦ A ⇒ B ⟧ = ⟦ A ⟧ → ⟦ B ⟧

data Term : Ty → Set where
  K : (A B : Ty) → Term (A ⇒ (B ⇒ A))

test : (A : Ty) (a : Term A) → ⟦ A ⟧
test A a = {!a!}

Before change, case splitting on a would give

test .(A ⇒ (B ⇒ A)) (K A B) x x₁ = ?

Now, it yields

test .(A ⇒ (B ⇒ A)) (K A B) = ?

In literate TeX files, \begin{code} and \end{code} can be preceded (resp. followed) by TeX code on the same line. This fixes Issue #2077.
Other issues fixed (see bug tracker):

#1951 (mixfix binders not working in ‘syntax’)

#1967 (too eager insteance search error)

#1974 (lost constraint dependencies)

#1982 (internal error in unifier)

#2034 (function type instance goals)

Compiler backends

UHC compiler backend

Added support for UHC 1.1.9.4.

Release notes for Agda version 2.5.1

Documentation

There is now an official Agda User Manual: http://agda.readthedocs.org/en/stable/

Installation and infrastructure

Builtins and primitives are now defined in a new set of modules available to all users, independent of any particular library. The modules are

Agda.Builtin.Bool
Agda.Builtin.Char
Agda.Builtin.Coinduction
Agda.Builtin.Equality
Agda.Builtin.Float
Agda.Builtin.FromNat
Agda.Builtin.FromNeg
Agda.Builtin.FromString
Agda.Builtin.IO
Agda.Builtin.Int
Agda.Builtin.List
Agda.Builtin.Nat
Agda.Builtin.Reflection
Agda.Builtin.Size
Agda.Builtin.Strict
Agda.Builtin.String
Agda.Builtin.TrustMe
Agda.Builtin.Unit

The standard library reexports the primitives from the new modules.

The Agda.Builtin modules are installed in the same way as Agda.Primitive, but unlike Agda.Primitive they are not loaded automatically.

Pragmas and options

Library management

There is a new ‘library’ concept for managing include paths. A library consists of
- a name,
- a set of libraries it depends on, and
- a set of include paths.
A library is defined in a .agda-lib file using the following format:
```
name: LIBRARY-NAME  -- Comment
depend: LIB1 LIB2
  LIB3
  LIB4
include: PATH1
  PATH2
  PATH3
```
Dependencies are library names, not paths to .agda-lib files, and include paths are relative to the location of the library-file.

To be useable, a library file has to be listed (with its full path) in AGDA_DIR/libraries (or AGDA_DIR/libraries-VERSION, for a given Agda version). AGDA_DIR defaults to ~/.agda on Unix-like systems and C:/Users/USERNAME/AppData/Roaming/agda or similar on Windows, and can be overridden by setting the AGDA_DIR environment variable.

Environment variables in the paths (of the form $VAR or ${VAR}) are expanded. The location of the libraries file used can be overridden using the --library-file=FILE flag, although this is not expected to be very useful.

You can find out the precise location of the ‘libraries’ file by calling agda -l fjdsk Dummy.agda and looking at the error message (assuming you don’t have a library called fjdsk installed).

There are three ways a library gets used:
- You supply the --library=LIB (or -l LIB) option to Agda. This is equivalent to adding a -iPATH for each of the include paths of LIB and its (transitive) dependencies.
- No explicit --library flag is given, and the current project root (of the Agda file that is being loaded) or one of its parent directories contains a .agda-lib file defining a library LIB. This library is used as if a --librarary=LIB option had been given, except that it is not necessary for the library to be listed in the AGDA_DIR/libraries file.
- No explicit --library flag, and no .agda-lib file in the project root. In this case the file AGDA_DIR/defaults is read and all libraries listed are added to the path. The defaults file should contain a list of library names, each on a separate line. In this case the current directory is also added to the path.
  
  To disable default libraries, you can give the flag --no-default-libraries.
Library names can end with a version number (for instance, mylib-1.2.3). When resolving a library name (given in a --library flag, or listed as a default library or library dependency) the following rules are followed:
- If you don’t give a version number, any version will do.
- If you give a version number an exact match is required.
- When there are multiple matches an exact match is preferred, and otherwise the latest matching version is chosen.
For example, suppose you have the following libraries installed: mylib, mylib-1.0, otherlib-2.1, and otherlib-2.3. In this case, aside from the exact matches you can also say --library=otherlib to get otherlib-2.3.

New Pragma COMPILED_DECLARE_DATA for binding recursively defined Haskell data types to recursively defined Agda data types.

If you have a Haskell type like

{-# LANGUAGE GADTs #-}

module Issue223 where

data A where
  BA :: B -> A

data B where
  AB :: A -> B
  BB :: B

You can now bind it to corresponding mutual Agda inductive data types as follows:

{-# IMPORT Issue223 #-}

data A : Set
{-# COMPILED_DECLARE_DATA A Issue223.A #-}
data B : Set
{-# COMPILED_DECLARE_DATA B Issue223.B #-}

data A where
  BA : B → A

{-# COMPILED_DATA A Issue223.A Issue223.BA #-}
data B where
  AB : A → B
  BB : B

{-# COMPILED_DATA B Issue223.B Issue223.AB Issue223.BB #-}

This fixes Issue #223.

New pragma HASKELL for adding inline Haskell code (GHC backend only)

Arbitrary Haskell code can be added to a module using the HASKELL pragma. For instance,
```
{-# HASKELL
  echo :: IO ()
  echo = getLine >>= putStrLn
#-}

postulate echo : IO ⊤
{-# COMPILED echo echo #-}
```
New option --exact-split.

The --exact-split flag causes Agda to raise an error whenever a clause in a definition by pattern matching cannot be made to hold definitionally (i.e. as a reduction rule). Specific clauses can be excluded from this check by means of the {-# CATCHALL #-} pragma.

For instance, the following definition will be rejected as the second clause cannot be made to hold definitionally:
```
min : Nat → Nat → Nat
min zero    y       = zero
min x       zero    = zero
min (suc x) (suc y) = suc (min x y
```
Catchall clauses have to be marked as such, for instance:
```
eq : Nat → Nat → Bool
eq zero    zero    = true
eq (suc m) (suc n) = eq m n
{-# CATCHALL #-}
eq _       _       = false
```
New option: --no-exact-split.

This option can be used to override a global --exact-split in a file, by adding a pragma {-# OPTIONS --no-exact-split #-}.
New options: --sharing and --no-sharing.

These options are used to enable/disable sharing and call-by-need evaluation. The default is --no-sharing.

Note that they cannot appear in an OPTIONS pragma, but have to be given as command line arguments or added to the Agda Program Args from Emacs with M-x customize-group agda2.
New pragma DISPLAY.
```
{-# DISPLAY f e1 .. en = e #-}
```
This causes f e1 .. en to be printed in the same way as e, where ei can bind variables used in e. The expressions ei and e are scope checked, but not type checked.

For example this can be used to print overloaded (instance) functions with the overloaded name:
```
instance
  NumNat : Num Nat
  NumNat = record { ..; _+_ = natPlus }

{-# DISPLAY natPlus a b = a + b #-}
```
Limitations
- Left-hand sides are restricted to variables, constructors, defined functions or types, and literals. In particular, lambdas are not allowed in left-hand sides.
- Since DISPLAY pragmas are not type checked implicit argument insertion may not work properly if the type of f computes to an implicit function space after pattern matching.
Removed pragma {-# ETA R #-}

The pragma {-# ETA R #-} is replaced by the eta-equality directive inside record declarations.
New option --no-eta-equality.

The --no-eta-equality flag disables eta rules for declared record types. It has the same effect as no-eta-equality inside each declaration of a record type R.

If used with the OPTIONS pragma it will not affect records defined in other modules.
The semantics of {-# REWRITE r #-} pragmas in parametrized modules has changed (see Issue #1652).

Rewrite rules are no longer lifted to the top context. Instead, they now only apply to terms in (extensions of) the module context. If you want the old behaviour, you should put the {-# REWRITE r #-} pragma outside of the module (i.e. unindent it).
New pragma {-# INLINE f #-} causes f to be inlined during compilation.
The STATIC pragma is now taken into account during compilation.

Calls to a function marked STATIC are normalised before compilation. The typical use case for this is to mark the interpreter of an embedded language as STATIC.
Option --type-in-type no longer implies --no-universe-polymorphism, thus, it can be used with explicit universe levels. [Issue #1764] It simply turns off error reporting for any level mismatch now. Examples:
```
{-# OPTIONS --type-in-type #-}

Type : Set
Type = Set

data D {α} (A : Set α) : Set where
  d : A → D A

data E α β : Set β where
  e : Set α → E α β
```
New NO_POSITIVITY_CHECK pragma to switch off the positivity checker for data/record definitions and mutual blocks.

The pragma must precede a data/record definition or a mutual block.

The pragma cannot be used in --safe mode.

Examples (see Issue1614*.agda and Issue1760*.agda in test/Succeed/):
1. Skipping a single data definition.
```
{-# NO_POSITIVITY_CHECK #-}
data D : Set where
  lam : (D → D) → D
```
2. Skipping a single record definition.
```
{-# NO_POSITIVITY_CHECK #-}
record U : Set where
  field ap : U → U
```
3. Skipping an old-style mutual block: Somewhere within a mutual block before a data/record definition.
```
mutual
  data D : Set where
    lam : (D → D) → D

  {-# NO_POSITIVITY_CHECK #-}
  record U : Set where
    field ap : U → U
```
4. Skipping an old-style mutual block: Before the mutual keyword.
```
{-# NO_POSITIVITY_CHECK #-}
mutual
  data D : Set where
    lam : (D → D) → D

  record U : Set where
    field ap : U → U
```
5. Skipping a new-style mutual block: Anywhere before the declaration or the definition of data/record in the block.
```
record U : Set
data D   : Set

record U where
  field ap : U → U

{-# NO_POSITIVITY_CHECK #-}
data D where
  lam : (D → D) → D
```
Removed --no-coverage-check option. [Issue #1918]

Language

Operator syntax

The default fixity for syntax declarations has changed from -666 to 20.
Sections.

Operators can be sectioned by replacing arguments with underscores. There must not be any whitespace between these underscores and the adjacent nameparts. Examples:
```
pred : ℕ → ℕ
pred = _∸ 1

T : Bool → Set
T = if_then ⊤ else ⊥

if : {A : Set} (b : Bool) → A → A → A
if b = if b then_else_
```
Sections are translated into lambda expressions. Examples:
```
_∸ 1              ↦  λ section → section ∸ 1

if_then ⊤ else ⊥  ↦  λ section → if section then ⊤ else ⊥

if b then_else_   ↦  λ section section₁ →
                         if b then section else section₁
```
Operator sections have the same fixity as the underlying operator (except in cases like if b then_else_, in which the section is “closed”, but the operator is not).

Operator sections are not supported in patterns (with the exception of dot patterns), and notations coming from syntax declarations cannot be sectioned.
A long-standing operator fixity bug has been fixed. As a consequence some programs that used to parse no longer do.

Previously each precedence level was (incorrectly) split up into five separate ones, ordered as follows, with the earlier ones binding less tightly than the later ones:
- Non-associative operators.
- Left associative operators.
- Right associative operators.
- Prefix operators.
- Postfix operators.
Now this problem has been addressed. It is no longer possible to mix operators of a given precedence level but different associativity. However, prefix and right associative operators are seen as having the same associativity, and similarly for postfix and left associative operators.

Examples

The following code is no longer accepted:
```
infixl 6 _+_
infix  6 _∸_

rejected : ℕ
rejected = 1 + 0 ∸ 1
```
However, the following previously rejected code is accepted:
```
infixr 4 _,_
infix  4 ,_

,_ : {A : Set} {B : A → Set} {x : A} → B x → Σ A B
, y = _ , y

accepted : Σ ℕ λ i → Σ ℕ λ j → Σ (i ≡ j) λ _ → Σ ℕ λ k → j ≡ k
accepted = 5 , , refl , , refl
```
The classification of notations with binders into the categories infix, prefix, postfix or closed has changed. [Issue #1450]

The difference is that, when classifying the notation, only regular holes are taken into account, not binding ones.

Example: The notation
```
syntax m >>= (λ x → f) = x <- m , f
```
was previously treated as infix, but is now treated as prefix.
Notation can now include wildcard binders.

Example: syntax Σ A (λ _ → B) = A × B
If an overloaded operator is in scope with several distinct precedence levels, then several instances of this operator will be included in the operator grammar, possibly leading to ambiguity. Previously the operator was given the default fixity [Issue #1436].

There is an exception to this rule: If there are multiple precedences, but at most one is explicitly declared, then only one instance will be included in the grammar. If there are no explicitly declared precedences, then this instance will get the default precedence, and otherwise it will get the declared precedence.

If multiple occurrences of an operator are “merged” in the grammar, and they have distinct associativities, then they are treated as being non-associative.

The three paragraphs above also apply to identical notations (coming from syntax declarations) for a given overloaded name.

Examples:
```
module A where

  infixr 5 _∷_
  infixr 5 _∙_
  infixl 3 _+_
  infix  1 bind

  syntax bind c (λ x → d) = x ← c , d

module B where

  infix  5 _∷_
  infixr 4 _∙_
  -- No fixity declaration for _+_.
  infixl 2 bind

  syntax bind c d = c ∙ d

module C where

  infixr 2 bind

  syntax bind c d = c ∙ d

open A
open B
open C

-- _∷_ is infix 5.
-- _∙_ has two fixities: infixr 4 and infixr 5.
-- _+_ is infixl 3.
-- A.bind's notation is infix 1.
-- B.bind and C.bind's notations are infix 2.

-- There is one instance of "_ ∷ _" in the grammar, and one
-- instance of "_ + _".

-- There are three instances of "_ ∙ _" in the grammar, one
-- corresponding to A._∙_, one corresponding to B._∙_, and one
-- corresponding to both B.bind and C.bind.
```

Reflection

The reflection framework has received a massive overhaul.

A new type of reflected type checking computations supplants most of the old reflection primitives. The quoteGoal, quoteContext and tactic primitives are deprecated and will be removed in the future, and the unquoteDecl and unquote primitives have changed behaviour. Furthermore the following primitive functions have been replaced by builtin type checking computations:
```
- primQNameType              --> AGDATCMGETTYPE
- primQNameDefinition        --> AGDATCMGETDEFINITION
- primDataConstructors       --> subsumed by AGDATCMGETDEFINITION
- primDataNumberOfParameters --> subsumed by AGDATCMGETDEFINITION
```
See below for details.
Types are no longer packaged with a sort.

The AGDATYPE and AGDATYPEEL built-ins have been removed. Reflected types are now simply terms.

Reflected definitions have more information.

The type for reflected definitions has changed to

data Definition : Set where
  fun-def     : List Clause  → Definition
  data-type   : Nat → List Name → Definition -- parameters and constructors
  record-type : Name → Definition            -- name of the data/record type
  data-con    : Name → Definition            -- name of the constructor
  axiom       : Definition
  prim-fun    : Definition

Correspondingly the built-ins for function, data and record definitions (AGDAFUNDEF, AGDAFUNDEFCON, AGDADATADEF, AGDARECORDDEF) have been removed.

Reflected type checking computations.

There is a primitive TC monad representing type checking computations. The unquote, unquoteDecl, and the new unquoteDef all expect computations in this monad (see below). The interface to the monad is the following

-- Error messages can contain embedded names and terms.
data ErrorPart : Set where
  strErr  : String → ErrorPart
  termErr : Term → ErrorPart
  nameErr : Name → ErrorPart

{-# BUILTIN AGDAERRORPART       ErrorPart #-}
{-# BUILTIN AGDAERRORPARTSTRING strErr    #-}
{-# BUILTIN AGDAERRORPARTTERM   termErr   #-}
{-# BUILTIN AGDAERRORPARTNAME   nameErr   #-}

postulate
  TC         : ∀ {a} → Set a → Set a
  returnTC   : ∀ {a} {A : Set a} → A → TC A
  bindTC     : ∀ {a b} {A : Set a} {B : Set b} → TC A → (A → TC B) → TC B

  -- Unify two terms, potentially solving metavariables in the process.
  unify      : Term → Term → TC ⊤

  -- Throw a type error. Can be caught by catchTC.
  typeError  : ∀ {a} {A : Set a} → List ErrorPart → TC A

  -- Block a type checking computation on a metavariable. This will abort
  -- the computation and restart it (from the beginning) when the
  -- metavariable is solved.
  blockOnMeta : ∀ {a} {A : Set a} → Meta → TC A

  -- Backtrack and try the second argument if the first argument throws a
  -- type error.
  catchTC    : ∀ {a} {A : Set a} → TC A → TC A → TC A

  -- Infer the type of a given term
  inferType  : Term → TC Type

  -- Check a term against a given type. This may resolve implicit arguments
  -- in the term, so a new refined term is returned. Can be used to create
  -- new metavariables: newMeta t = checkType unknown t
  checkType  : Term → Type → TC Term

  -- Compute the normal form of a term.
  normalise  : Term → TC Term

  -- Get the current context.
  getContext : TC (List (Arg Type))

  -- Extend the current context with a variable of the given type.
  extendContext : ∀ {a} {A : Set a} → Arg Type → TC A → TC A

  -- Set the current context.
  inContext     : ∀ {a} {A : Set a} → List (Arg Type) → TC A → TC A

  -- Quote a value, returning the corresponding Term.
  quoteTC : ∀ {a} {A : Set a} → A → TC Term

  -- Unquote a Term, returning the corresponding value.
  unquoteTC : ∀ {a} {A : Set a} → Term → TC A

  -- Create a fresh name.
  freshName  : String → TC QName

  -- Declare a new function of the given type. The function must be defined
  -- later using 'defineFun'. Takes an Arg Name to allow declaring instances
  -- and irrelevant functions. The Visibility of the Arg must not be hidden.
  declareDef : Arg QName → Type → TC ⊤

  -- Define a declared function. The function may have been declared using
  -- 'declareDef' or with an explicit type signature in the program.
  defineFun  : QName → List Clause → TC ⊤

  -- Get the type of a defined name. Replaces 'primQNameType'.
  getType    : QName → TC Type

  -- Get the definition of a defined name. Replaces 'primQNameDefinition'.
  getDefinition : QName → TC Definition

{-# BUILTIN AGDATCM                   TC                 #-}
{-# BUILTIN AGDATCMRETURN             returnTC           #-}
{-# BUILTIN AGDATCMBIND               bindTC             #-}
{-# BUILTIN AGDATCMUNIFY              unify              #-}
{-# BUILTIN AGDATCMNEWMETA            newMeta            #-}
{-# BUILTIN AGDATCMTYPEERROR          typeError          #-}
{-# BUILTIN AGDATCMBLOCKONMETA        blockOnMeta        #-}
{-# BUILTIN AGDATCMCATCHERROR         catchTC            #-}
{-# BUILTIN AGDATCMINFERTYPE          inferType          #-}
{-# BUILTIN AGDATCMCHECKTYPE          checkType          #-}
{-# BUILTIN AGDATCMNORMALISE          normalise          #-}
{-# BUILTIN AGDATCMGETCONTEXT         getContext         #-}
{-# BUILTIN AGDATCMEXTENDCONTEXT      extendContext      #-}
{-# BUILTIN AGDATCMINCONTEXT          inContext          #-}
{-# BUILTIN AGDATCMQUOTETERM          quoteTC            #-}
{-# BUILTIN AGDATCMUNQUOTETERM        unquoteTC          #-}
{-# BUILTIN AGDATCMFRESHNAME          freshName          #-}
{-# BUILTIN AGDATCMDECLAREDEF         declareDef         #-}
{-# BUILTIN AGDATCMDEFINEFUN          defineFun          #-}
{-# BUILTIN AGDATCMGETTYPE            getType            #-}
{-# BUILTIN AGDATCMGETDEFINITION      getDefinition      #-}

Builtin type for metavariables

There is a new builtin type for metavariables used by the new reflection framework. It is declared as follows and comes with primitive equality, ordering and show.

postulate Meta : Set
{-# BUILTIN AGDAMETA Meta #-}
primitive primMetaEquality : Meta → Meta → Bool
primitive primMetaLess : Meta → Meta → Bool
primitive primShowMeta : Meta → String

There are corresponding new constructors in the Term and Literal data types:

data Term : Set where
  ...
  meta : Meta → List (Arg Term) → Term

{-# BUILTIN AGDATERMMETA meta #-}

data Literal : Set where
  ...
  meta : Meta → Literal

{-# BUILTIN AGDALITMETA meta #-}

Builtin unit type

The type checker needs to know about the unit type, which you can allow by
```
record ⊤ : Set where
{-# BUILTIN UNIT ⊤ #-}
```
Changed behaviour of unquote

The unquote primitive now expects a type checking computation instead of a pure term. In particular unquote e requires
```
e : Term → TC ⊤
```
where the argument is the representation of the hole in which the result should go. The old unquote behaviour (where unquote expected a Term argument) can be recovered by
```
OLD: unquote v
NEW: unquote λ hole → unify hole v
```
Changed behaviour of unquoteDecl

The unquoteDecl primitive now expects a type checking computation instead of a pure function definition. It is possible to define multiple (mutually recursive) functions at the same time. More specifically
```
unquoteDecl x₁ .. xₙ = m
```
requires m : TC ⊤ and that x₁ .. xₙ are defined (using declareDef and defineFun) after executing m. As before x₁ .. xₙ : QName in m, but have their declared types outside the unquoteDecl.
New primitive unquoteDef

There is a new declaration
```
unquoteDef x₁ .. xₙ = m
```
This works exactly as unquoteDecl (see above) with the exception that x₁ .. xₙ are required to already be declared.

The main advantage of unquoteDef over unquoteDecl is that unquoteDef is allowed in mutual blocks, allowing mutually recursion between generated definitions and hand-written definitions.
The reflection interface now exposes the name hint (as a string) for variables. As before, the actual binding structure is with de Bruijn indices. The String value is just a hint used as a prefix to help display the variable. The type Abs is a new builtin type used for the constructors Term.lam, Term.pi, Pattern.var (bultins AGDATERMLAM, AGDATERMPI and AGDAPATVAR).
```
data Abs (A : Set) : Set where
  abs : (s : String) (x : A) → Abs A
{-# BUILTIN ABS    Abs #-}
{-# BUILTIN ABSABS abs #-}
```
Updated constructor types:
```
Term.lam    : Hiding   → Abs Term → Term
Term.pi     : Arg Type → Abs Type → Term
Pattern.var : String   → Pattern
```
Reflection-based macros

Macros are functions of type t1 → t2 → .. → Term → TC ⊤ that are defined in a macro block. Macro application is guided by the type of the macro, where Term arguments desugar into the quoteTerm syntax and Name arguments into the quote syntax. Arguments of any other type are preserved as-is. The last Term argument is the hole term given to unquote computation (see above).

For example, the macro application f u v w where the macro f has the type Term → Name → Bool → Term → TC ⊤ desugars into unquote (f (quoteTerm u) (quote v) w)

Limitations:
- Macros cannot be recursive. This can be worked around by defining the recursive function outside the macro block and have the macro call the recursive function.
Silly example:
```
macro
  plus-to-times : Term → Term → TC ⊤
  plus-to-times (def (quote _+_) (a ∷ b ∷ [])) hole = unify hole (def (quote _*_) (a ∷ b ∷ []))
  plus-to-times v hole = unify hole v

thm : (a b : Nat) → plus-to-times (a + b) ≡ a * b
thm a b = refl
```
Macros are most useful when writing tactics, since they let you hide the reflection machinery. For instance, suppose you have a solver
```
magic : Type → Term
```
that takes a reflected goal and outputs a proof (when successful). You can then define the following macro
```
macro
  by-magic : Term → TC ⊤
  by-magic hole =
    bindTC (inferType hole) λ goal →
    unify hole (magic goal)
```
This lets you apply the magic tactic without any syntactic noise at all:
```
thm : ¬ P ≡ NP
thm = by-magic
```

Literals and built-ins

Overloaded number literals.

You can now overload natural number literals using the new builtin FROMNAT:
```
{-# BUILTIN FROMNAT fromNat #-}
```
The target of the builtin should be a defined name. Typically you would do something like
```
record Number (A : Set) : Set where
  field fromNat : Nat → A

open Number {{...}} public

{-# BUILTIN FROMNAT fromNat #-}
```
This will cause number literals n to be desugared to fromNat n before type checking.
Negative number literals.

Number literals can now be negative. For floating point literals it works as expected. For integer literals there is a new builtin FROMNEG that enables negative integer literals:
```
{-# BUILTIN FROMNEG fromNeg #-}
```
This causes negative literals -n to be desugared to fromNeg n.
Overloaded string literals.

String literals can be overladed using the FROMSTRING builtin:
```
{-# BUILTIN FROMSTRING fromString #-}
```
The will cause string literals s to be desugared to fromString s before type checking.
Change to builtin integers.

The INTEGER builtin now needs to be bound to a datatype with two constructors that should be bound to the new builtins INTEGERPOS and INTEGERNEGSUC as follows:
```
data Int : Set where
  pos    : Nat -> Int
  negsuc : Nat -> Int
{-# BUILTIN INTEGER       Int    #-}
{-# BUILTIN INTEGERPOS    pos    #-}
{-# BUILTIN INTEGERNEGSUC negsuc #-}
```
where negsuc n represents the integer -n - 1. For instance, -5 is represented as negsuc 4. All primitive functions on integers except primShowInteger have been removed, since these can be defined without too much trouble on the above representation using the corresponding functions on natural numbers.

The primitives that have been removed are
```
primIntegerPlus
primIntegerMinus
primIntegerTimes
primIntegerDiv
primIntegerMod
primIntegerEquality
primIntegerLess
primIntegerAbs
primNatToInteger
```
New primitives for strict evaluation
```
primitive
  primForce      : ∀ {a b} {A : Set a} {B : A → Set b} (x : A) → (∀ x → B x) → B x
  primForceLemma : ∀ {a b} {A : Set a} {B : A → Set b} (x : A) (f : ∀ x → B x) → primForce x f ≡ f x
```
primForce x f evaluates to f x if x is in weak head normal form, and primForceLemma x f evaluates to refl in the same situation. The following values are considered to be in weak head normal form:
- constructor applications
- literals
- lambda abstractions
- type constructor (data/record types) applications
- function types
- Set a

Modules

Modules in import directives

When you use using/hiding/renaming on a name it now automatically applies to any module of the same name, unless you explicitly mention the module. For instance,
```
open M using (D)
```
is equivalent to
```
open M using (D; module D)
```
if M defines a module D. This is most useful for record and data types where you always get a module of the same name as the type.

With this feature there is no longer useful to be able to qualify a constructor (or field) by the name of the data type even when it differs from the name of the corresponding module. The follow (weird) code used to work, but doesn’t work anymore:
```
module M where
  data D where
    c : D
open M using (D) renaming (module D to MD)
foo : D
foo = D.c
```
If you want to import only the type name and not the module you have to hide it explicitly:
```
open M using (D) hiding (module D)
```
See discussion on Issue #836.
Private definitions of a module are no longer in scope at the Emacs mode top-level.

The reason for this change is that .agdai-files are stripped of unused private definitions (which can yield significant performance improvements for module-heavy code).

To test private definitions you can create a hole at the bottom of the module, in which private definitions will be visible.

Records

New record directives eta-equality/no-eta-equality

The keywords eta-equality/no-eta-equality enable/disable eta rules for the (inductive) record type being declared.

record Σ (A : Set) (B : A -> Set) : Set where
  no-eta-equality
  constructor _,_
  field
    fst : A
    snd : B fst
open Σ

-- fail : ∀ {A : Set}{B : A -> Set} → (x : Σ A B) → x ≡ (fst x , snd x)
-- fail x = refl
--
-- x != fst x , snd x of type Σ .A .B
-- when checking that the expression refl has type x ≡ (fst x , snd x)

Building records from modules.

The record { <fields> } syntax is now extended to accept module names as well. Fields are thus defined using the corresponding definitions from the given module.

For instance assuming this record type R and module M:
```
record R : Set where
  field
    x : X
    y : Y
    z : Z

module M where
  x = {! ... !}
  y = {! ... !}

r : R
r = record { M; z = {! ... !} }
```
Previously one had to write record { x = M.x; y = M.y; z = {! ... !} }.

More precisely this construction now supports any combination of explicit field definitions and applied modules.

If a field is both given explicitly and available in one of the modules, then the explicit one takes precedence.

If a field is available in more than one module then this is ambiguous and therefore rejected. As a consequence the order of assignments does not matter.

The modules can be both applied to arguments and have import directives such as hiding, using, and renaming. In particular this construct subsumes the record update construction.

Here is an example of record update:
```
-- Record update. Same as: record r { y = {! ... !} }
r2 : R
r2 = record { R r; y = {! ... !} }
```
A contrived example showing the use of hiding/renaming:
```
module M2 (a : A) where
  w = {! ... !}
  z = {! ... !}

r3 : A → R
r3 a = record { M hiding (y); M2 a renaming (w to y) }
```

Record patterns are now accepted.

Examples:

swap : {A B : Set} (p : A × B) → B × A
swap record{ proj₁ = a; proj₂ = b } = record{ proj₁ = b; proj₂ = a }

thd3 : ...
thd3 record{ proj₂ = record { proj₂ = c }} = c

Record modules now properly hide all their parameters [Issue #1759]

Previously parameters to parent modules were not hidden in the record module, resulting in different behaviour between
```
module M (A : Set) where
  record R (B : Set) : Set where
```
and
```
module M where
  record R (A B : Set) : Set where
```
where in the former case, A would be an explicit argument to the module M.R, but implicit in the latter case. Now A is implicit in both cases.

Instance search

Performance has been improved, recursive instance search which was previously exponential in the depth is now only quadratic.

Constructors of records and datatypes are not anymore automatically considered as instances, you have to do so explicitely, for instance:

-- only [b] is an instance of D
data D : Set where
  a : D
  instance
    b : D
  c : D

-- the constructor is now an instance
record tt : Set where
  instance constructor tt

Lambda-bound variables are no longer automatically considered instances.

Lambda-bound variables need to be bound as instance arguments to be considered for instance search. For example,
```
_==_ : {A : Set} {{_ : Eq A}} → A → A → Bool

fails : {A : Set} → Eq A → A → Bool
fails eqA x = x == x

works : {A : Set} {{_ : Eq A}} → A → Bool
works x = x == x
```

Let-bound variables are no longer automatically considered instances.

To make a let-bound variable available as an instance it needs to be declared with the instance keyword, just like top-level instances. For example,

mkEq : {A : Set} → (A → A → Bool) → Eq A

fails : {A : Set} → (A → A → Bool) → A → Bool
fails eq x = let eqA = mkEq eq in x == x

works : {A : Set} → (A → A → Bool) → A → Bool
works eq x = let instance eqA = mkEq eq in x == x

Record fields can be declared instances.

For example,
```
record EqSet : Set₁ where
  field
    set : Set
    instance eq : Eq set
```
This causes the projection function eq : (E : EqSet) → Eq (set E) to be considered for instance search.

Instance search can now find arguments in variable types (but such candidates can only be lambda-bound variables, they can’t be declared as instances)

module _ {A : Set} (P : A → Set) where

  postulate
    bla : {x : A} {{_ : P x}} → Set → Set

  -- Works, the instance argument is found in the context
  test :  {x : A} {{_ : P x}} → Set → Set
  test B = bla B

  -- Still forbidden, because [P] could be instantiated later to anything
  instance
   postulate
    forbidden : {x : A} → P x

Instance search now refuses to solve constraints with unconstrained metavariables, since this can lead to non-termination.

See [Issue #1532] for an example.
Top-level instances are now only considered if they are in scope. [Issue #1913]

Note that lambda-bound instances need not be in scope.

Other changes

Unicode ellipsis character is allowed for the ellipsis token ... in with expressions.
Prop is no longer a reserved word.

Type checking

Large indices.

Force constructor arguments no longer count towards the size of a datatype. For instance, the definition of equality below is accepted.
```
data _≡_ {a} {A : Set a} : A → A → Set where
  refl : ∀ x → x ≡ x
```
This gets rid of the asymmetry that the version of equality which indexes only on the second argument could be small, but not the version above which indexes on both arguments.
Detection of datatypes that satisfy K (i.e. sets)

Agda will now try to detect datatypes that satisfy K when --without-K is enabled. A datatype satisfies K when it follows these three rules:
- The types of all non-recursive constructor arguments should satisfy K.
- All recursive constructor arguments should be first-order.
- The types of all indices should satisfy K.
For example, the types Nat, List Nat, and x ≡ x (where x : Nat) are all recognized by Agda as satisfying K.
New unifier for case splitting

The unifier used by Agda for case splitting has been completely rewritten. The new unifier takes a much more type-directed approach in order to avoid the problems in issues #1406, #1408, #1427, and #1435.

The new unifier also has eta-equality for record types built-in. This should avoid unnecessary case splitting on record constructors and improve the performance of Agda on code that contains deeply nested record patterns (see issues #473, #635, #1575, #1603, #1613, and #1645).

In some cases, the locations of the dot patterns computed by the unifier did not correspond to the locations given by the user (see Issue #1608). This has now been fixed by adding an extra step after case splitting that checks whether the user-written patterns are compatible with the computed ones.

In some rare cases, the new unifier is still too restrictive when --without-K is enabled because it cannot generalize over the datatype indices (yet). For example, the following code is rejected:
```
data Bar : Set₁ where
  bar : Bar
  baz : (A : Set) → Bar

data Foo : Bar → Set where
  foo : Foo bar

test : foo ≡ foo → Set₁
test refl = Set
```
The aggressive behaviour of with introduced in 2.4.2.5 has been rolled back [Issue #1692]. With no longer abstracts in the types of variables appearing in the with-expressions. [Issue #745]

This means that the following example no longer works:
```
fails : (f : (x : A) → a ≡ x) (b : A) → b ≡ a
fails f b with a | f b
fails f b | .b | refl = f b
```
The with no longer abstracts the type of f over a, since f appears in the second with-expression f b. You can use a nested with to make this example work.

This example does work again:
```
test : ∀{A : Set}{a : A}{f : A → A} (p : f a ≡ a) → f (f a) ≡ a
test p rewrite p = p
```
After rewrite p the goal has changed to f a ≡ a, but the type of p has not been rewritten, thus, the final p solves the goal.

The following, which worked in 2.4.2.5, no longer works:
```
fails : (f : (x : A) → a ≡ x) (b : A) → b ≡ a
fails f b rewrite f b = f b
```
The rewrite with f b : a ≡ b is not applied to f as the latter is part of the rewrite expression f b. Thus, the type of f remains untouched, and the changed goal b ≡ b is not solved by f b.
When using rewrite on a term eq of type lhs ≡ rhs, the lhs is no longer abstracted in rhs [Issue #520]. This means that
```
f pats rewrite eq = body
```
is more than syntactic sugar for
```
f pats with lhs | eq
f pats | _ | refl = body
```
In particular, the following application of rewrite is now possible
```
id : Bool → Bool
id true  = true
id false = false

is-id : ∀ x → x ≡ id x
is-id true  = refl
is-id false = refl

postulate
  P : Bool → Set
  b : Bool
  p : P (id b)

proof : P b
proof rewrite is-id b = p
```
Previously, this was desugared to
```
proof with b | is-id b
proof | _ | refl = p
```
which did not type check as refl does not have type b ≡ id b. Now, Agda gets the task of checking refl : _ ≡ id b leading to instantiation of _ to id b.

Compiler backends

Major Bug Fixes:
- Function clauses with different arities are now always compiled correctly by the GHC/UHC backends. (Issue #727)
Co-patterns
- The GHC/UHC backends now support co-patterns. (Issues #1567, #1632)
Optimizations
- Builtin naturals are now represented as arbitrary-precision Integers. See the user manual, section “Agda Compilers -> Optimizations” for details.
GHC Haskell backend (MAlonzo)
- Pragmas
  
  Since builtin naturals are compiled to Integer you can no longer give a {-# COMPILED_DATA #-} pragma for Nat. The same goes for builtin booleans, integers, floats, characters and strings which are now hard-wired to appropriate Haskell types.
UHC compiler backend

A new backend targeting the Utrecht Haskell Compiler (UHC) is available. It targets the UHC Core language, and it’s design is inspired by the Epic backend. See the user manual, section “Agda Compilers -> UHC Backend” for installation instructions.
- FFI
  
  The UHC backend has a FFI to Haskell similar to MAlonzo’s. The target Haskell code also needs to be compilable using UHC, which does not support the Haskell base library version 4.*.
  
  FFI pragmas for the UHC backend are not checked in any way. If the pragmas are wrong, bad things will happen.
- Imports
  
  Additional Haskell modules can be brought into scope with the IMPORT_UHC pragma:
```
{-# IMPORT_UHC Data.Char #-}
```
  The Haskell modules UHC.Base and UHC.Agda.Builtins are always in scope and don’t need to be imported explicitly.
- Datatypes
  
  Agda datatypes can be bound to Haskell datatypes as follows:
  
  Haskell:
```
data HsData a = HsCon1 | HsCon2 (HsData a)
```
  Agda:
```
data AgdaData (A : Set) : Set where
  AgdaCon1 : AgdaData A
  AgdaCon2 : AgdaData A -> AgdaData A
{-# COMPILED_DATA_UHC AgdaData HsData HsCon1 HsCon2 #-}
```
  The mapping has to cover all constructors of the used Haskell datatype, else runtime behavior is undefined!
  
  There are special reserved names to bind Agda datatypes to certain Haskell datatypes. For example, this binds an Agda datatype to Haskell’s list datatype:
  
  Agda:
```
data AgdaList (A : Set) : Set where
  Nil : AgdaList A
  Cons : A -> AgdaList A -> AgdaList A
{-# COMPILED_DATA_UHC AgdaList __LIST__ __NIL__ __CONS__ #-}
```
  The following “magic” datatypes are available:
```
HS Datatype | Datatype Pragma | HS Constructor | Constructor Pragma
()            __UNIT__          ()               __UNIT__
List          __LIST__          (:)              __CONS__
                                []               __NIL__
Bool          __BOOL__          True             __TRUE__
                                False            __FALSE__
```
- Functions
  
  Agda postulates can be bound to Haskell functions. Similar as in MAlonzo, all arguments of type Set need to be dropped before calling Haskell functions. An example calling the return function:
  
  Agda:
```
postulate hs-return : {A : Set} -> A -> IO A
{-# COMPILED_UHC hs-return (\_ -> UHC.Agda.Builtins.primReturn) #-}
```

Emacs mode and interaction

Module contents (C-c C-o) now also works for records. [See Issue #1926 ] If you have an inferable expression of record type in an interaction point, you can invoke C-c C-o to see its fields and types. Example
```
record R : Set where
  field f : A

test : R → R
test r = {!r!}  -- C-c C-o here
```
Less aggressive error notification.

Previously Emacs could jump to the position of an error even if the type-checking process was not initiated in the current buffer. Now this no longer happens: If the type-checking process was initiated in another buffer, then the cursor is moved to the position of the error in the buffer visiting the file (if any) and in every window displaying the file, but focus should not change from one file to another.

In the cases where focus does change from one file to another, one can now use the go-back functionality to return to the previous position.
Removed the agda-include-dirs customization parameter.

Use agda-program-args with -iDIR or -lLIB instead, or add libraries to ~/.agda/defaults (C:/Users/USERNAME/AppData/Roaming/agda/defaults or similar on Windows). See Library management, above, for more information.

Tools

LaTeX-backend

The default font has been changed to XITS (which is part of TeX Live):

http://www.ctan.org/tex-archive/fonts/xits/

This font is more complete with respect to Unicode.

agda-ghc-names

New tool: The command
```
agda-ghc-names fixprof <compile-dir> <ProgName>.prof
```
converts *.prof files obtained from profiling runs of MAlonzo-compiled code to *.agdaIdents.prof, with the original Agda identifiers replacing the MAlonzo-generated Haskell identifiers.

For usage and more details, see src/agda-ghc-names/README.txt.

Highlighting and textual backends

Names in import directives are now highlighted and are clickable. [Issue #1714] This leads also to nicer printing in the LaTeX and html backends.

Fixed issues

See bug tracker (milestone 2.5.1)

Release notes for Agda version 2.4.2.5

Installation and infrastructure

Added support for GHC 7.10.3.
Added cpphs Cabal flag

Turn on/off this flag to choose cpphs/cpp as the C preprocessor.

This flag is turn on by default.

(This flag was added in Agda 2.4.2.1 but it was not documented)

Pragmas and options

Termination pragmas are no longer allowed inside where clauses [Issue #1137].

Type checking

with-abstraction is more aggressive, abstracts also in types of variables that are used in the with-expressions, unless they are also used in the types of the with-expressions. [Issue #1692]

Example:
```
test : (f : (x : A) → a ≡ x) (b : A) → b ≡ a
test f b with a | f b
test f b | .b | refl = f b
```
Previously, with would not abstract in types of variables that appear in the with-expressions, in this case, both f and b, leaving their types unchanged. Now, it tries to abstract in f, as only b appears in the types of the with-expressions which are A (of a) and a ≡ b (of f b). As a result, the type of f changes to (x : A) → b ≡ x and the type of the goal to b ≡ b (as previously).

This also affects rewrite, which is implemented in terms of with.
```
test : (f : (x : A) → a ≡ x) (b : A) → b ≡ a
test f b rewrite f b = f b
```
As the new with is not fully backwards-compatible, some parts of your Agda developments using with or rewrite might need maintenance.

Fixed issues

See bug tracker

Release notes for Agda version 2.4.2.4

Installation and infrastructure

Removed support for GHC 7.4.2.

Pragmas and options

Option --copatterns is now on by default. To switch off parsing of copatterns, use:
```
{-# OPTIONS --no-copatterns #-}
```
Option --rewriting is now needed to use REWRITE pragmas and rewriting during reduction. Rewriting is not --safe.

To use rewriting, first specify a relation symbol R that will later be used to add rewrite rules. A canonical candidate would be propositional equality
```
{-# BUILTIN REWRITE _≡_ #-}
```
but any symbol R of type Δ → A → A → Set i for some A and i is accepted. Then symbols q can be added to rewriting provided their type is of the form Γ → R ds l r. This will add a rewrite rule
```
Γ ⊢ l ↦ r : A[ds/Δ]
```
to the signature, which fires whenever a term is an instance of l. For example, if
```
plus0 : ∀ x → x + 0 ≡ x
```
(ideally, there is a proof for plus0, but it could be a postulate), then
```
{-# REWRITE plus0 #-}
```
will prompt Agda to rewrite any well-typed term of the form t + 0 to t.

Some caveats: Agda accepts and applies rewrite rules naively, it is very easy to break consistency and termination of type checking. Some examples of rewrite rules that should not be added:
```
refl     : ∀ x → x ≡ x             -- Agda loops
plus-sym : ∀ x y → x + y ≡ y + x   -- Agda loops
absurd   : true ≡ false            -- Breaks consistency
```
Adding only proven equations should at least preserve consistency, but this is only a conjecture, so know what you are doing! Using rewriting, you are entering into the wilderness, where you are on your own!

Language

forall / ∀ now parses like λ, i.e., the following parses now [Issue #1583]:
```
⊤ × ∀ (B : Set) → B → B
```
The underscore pattern _ can now also stand for an inaccessible pattern (dot pattern). This alleviates the need for writing ._. [Issue #1605] Instead of
```
transVOld : ∀{A : Set} (a b c : A) → a ≡ b → b ≡ c → a ≡ c
transVOld _ ._ ._ refl refl = refl
```
one can now write
```
  transVNew : ∀{A : Set} (a b c : A) → a ≡ b → b ≡ c → a ≡ c
  transVNew _ _ _ refl refl = refl
```
and let Agda decide where to put the dots. This was always possible by using hidden arguments
```
transH : ∀{A : Set}{a b c : A} → a ≡ b → b ≡ c → a ≡ c
transH refl refl = refl
```
which is now equivalent to
```
transHNew : ∀{A : Set}{a b c : A} → a ≡ b → b ≡ c → a ≡ c
transHNew {a = _}{b = _}{c = _} refl refl = refl
```
Before, underscore _ stood for an unnamed variable that could not be instantiated by an inaccessible pattern. If one no wants to prevent Agda from instantiating, one needs to use a variable name other than underscore (however, in practice this situation seems unlikely).

Type checking

Polarity of phantom arguments to data and record types has changed. [Issue #1596] Polarity of size arguments is Nonvariant (both monotone and antitone). Polarity of other arguments is Covariant (monotone). Both were Invariant before (neither monotone nor antitone).

The following example type-checks now:

open import Common.Size

-- List should be monotone in both arguments
-- (even when `cons' is missing).

data List (i : Size) (A : Set) : Set where
  [] : List i A

castLL : ∀{i A} → List i (List i A) → List ∞ (List ∞ A)
castLL x = x

-- Stream should be antitone in the first and monotone in the second argument
-- (even with field `tail' missing).

record Stream (i : Size) (A : Set) : Set where
  coinductive
  field
    head : A

castSS : ∀{i A} → Stream ∞ (Stream ∞ A) → Stream i (Stream i A)
castSS x = x

SIZELT lambdas must be consistent [Issue #1523, see Abel and Pientka, ICFP 2013]. When lambda-abstracting over type (Size< size) then size must be non-zero, for any valid instantiation of size variables.

The good:

data Nat (i : Size) : Set where
  zero : ∀ (j : Size< i) → Nat i
  suc  : ∀ (j : Size< i) → Nat j → Nat i

{-# TERMINATING #-}
-- This definition is fine, the termination checker is too strict at the moment.
fix : ∀ {C : Size → Set}
   → (∀ i → (∀ (j : Size< i) → Nat j -> C j) → Nat i → C i)
   → ∀ i → Nat i → C i
fix t i (zero j)  = t i (λ (k : Size< i) → fix t k) (zero j)
fix t i (suc j n) = t i (λ (k : Size< i) → fix t k) (suc j n)

The λ (k : Size< i) is fine in both cases, as context

i : Size, j : Size< i

guarantees that i is non-zero.

The bad:

record Stream {i : Size} (A : Set) : Set where
  coinductive
  constructor _∷ˢ_
  field
    head  : A
    tail  : ∀ {j : Size< i} → Stream {j} A
open Stream public

_++ˢ_ : ∀ {i A} → List A → Stream {i} A → Stream {i} A
[]        ++ˢ s = s
(a ∷ as)  ++ˢ s = a ∷ˢ (as ++ˢ s)

This fails, maybe unjustified, at

i : Size, s : Stream {i} A
  ⊢
    a ∷ˢ (λ {j : Size< i} → as ++ˢ s)

Fixed by defining the constructor by copattern matching:

record Stream {i : Size} (A : Set) : Set where
  coinductive
  field
    head  : A
    tail  : ∀ {j : Size< i} → Stream {j} A
open Stream public

_∷ˢ_ : ∀ {i A} → A → Stream {i} A → Stream {↑ i} A
head  (a ∷ˢ as) = a
tail  (a ∷ˢ as) = as

_++ˢ_ : ∀ {i A} → List A → Stream {i} A → Stream {i} A
[]        ++ˢ s = s
(a ∷ as)  ++ˢ s = a ∷ˢ (as ++ˢ s)

The ugly:

fix : ∀ {C : Size → Set}
   → (∀ i → (∀ (j : Size< i) → C j) → C i)
   → ∀ i → C i
fix t i = t i λ (j : Size< i) → fix t j

For i=0, there is no such j at runtime, leading to looping behavior.

Interaction

Issue #635 has been fixed. Case splitting does not spit out implicit record patterns any more.

record Cont : Set₁ where
  constructor _◃_
  field
    Sh  : Set
    Pos : Sh → Set

open Cont

data W (C : Cont) : Set where
  sup : (s : Sh C) (k : Pos C s → W C) → W C

bogus : {C : Cont} → W C → Set
bogus w = {!w!}

Case splitting on w yielded, since the fix of Issue #473,

bogus {Sh ◃ Pos} (sup s k) = ?

Now it gives, as expected,

bogus (sup s k) = ?

Performance

As one result of the 21st Agda Implementor’s Meeting (AIM XXI), serialization of the standard library is 50% faster (time reduced by a third), without using additional disk space for the interface files.

Bug fixes

Issues fixed (see bug tracker):

#1546 (copattern matching and with-clauses)

#1560 (positivity checker inefficiency)

#1584 (let pattern with trailing implicit)

Release notes for Agda version 2.4.2.3

Installation and infrastructure

Added support for GHC 7.10.1.
Removed support for GHC 7.0.4.

Language

_is no longer a valid name for a definition. The following fails now: [Issue #1465]
```
postulate _ : Set
```
Typed bindings can now contain hiding information [Issue #1391]. This means you can now write
```
assoc : (xs {ys zs} : List A) → ((xs ++ ys) ++ zs) ≡ (xs ++ (ys ++ zs))
```
instead of the longer
```
assoc : (xs : List A) {ys zs : List A} → ...
```
It also works with irrelevance
```
.(xs {ys zs} : List A) → ...
```
but of course does not make sense if there is hiding information already. Thus, this is (still) a parse error:
```
{xs {ys zs} : List A} → ...
```

The builtins for sized types no longer need accompanying postulates. The BUILTIN pragmas for size stuff now also declare the identifiers they bind to.

{-# BUILTIN SIZEUNIV SizeUniv #-}  --  SizeUniv : SizeUniv
{-# BUILTIN SIZE     Size     #-}  --  Size     : SizeUniv
{-# BUILTIN SIZELT   Size<_   #-}  --  Size<_   : ..Size → SizeUniv
{-# BUILTIN SIZESUC  ↑_       #-}  --  ↑_       : Size → Size
{-# BUILTIN SIZEINF  ∞        #-}  --  ∞       : Size

Size and Size< now live in the new universe SizeUniv. It is forbidden to build function spaces in this universe, in order to prevent the malicious assumption of a size predecessor

pred : (i : Size) → Size< i

[Issue #1428].

Unambiguous notations (coming from syntax declarations) that resolve to ambiguous names are now parsed unambiguously [Issue #1194].
If only some instances of an overloaded name have a given associated notation (coming from syntax declarations), then this name can only be resolved to the given instances of the name, not to other instances [Issue #1194].

Previously, if different instances of an overloaded name had different associated notations, then none of the notations could be used. Now all of them can be used.

Note that notation identity does not only involve the right-hand side of the syntax declaration. For instance, the following notations are not seen as identical, because the implicit argument names are different:
```
module A where

  data D : Set where
    c : {x y : D} → D

  syntax c {x = a} {y = b} = a ∙ b

module B where

  data D : Set where
    c : {y x : D} → D

  syntax c {y = a} {x = b} = a ∙ b
```
If an overloaded operator is in scope with at least two distinct fixities, then it gets the default fixity [Issue #1436].

Similarly, if two or more identical notations for a given overloaded name are in scope, and these notations do not all have the same fixity, then they get the default fixity.

Type checking

Functions of varying arity can now have with-clauses and use rewrite.

Example:

NPred : Nat → Set
NPred 0       = Bool
NPred (suc n) = Nat → NPred n

const : Bool → ∀{n} → NPred n
const b {0}       = b
const b {suc n} m = const b {n}

allOdd : ∀ n → NPred n
allOdd 0 = true
allOdd (suc n) m with even m
... | true  = const false
... | false = allOdd n

Function defined by copattern matching can now have with-clauses and use rewrite.

Example:

{-# OPTIONS --copatterns #-}

record Stream (A : Set) : Set where
  coinductive
  constructor delay
  field
    force : A × Stream A
open Stream

map : ∀{A B} → (A → B) → Stream A → Stream B
force (map f s) with force s
... | a , as = f a , map f as

record Bisim {A B} (R : A → B → Set) (s : Stream A) (t : Stream B) : Set where
  coinductive
  constructor ~delay
  field
    ~force : let a , as = force s
                 b , bs = force t
             in  R a b × Bisim R as bs
open Bisim

SEq : ∀{A} (s t : Stream A) → Set
SEq = Bisim (_≡_)

-- Slightly weird definition of symmetry to demonstrate rewrite.

~sym' : ∀{A} {s t : Stream A} → SEq s t → SEq t s
~force (~sym' {s = s} {t} p) with force s | force t | ~force p
... | a , as | b , bs | r , q rewrite r = refl , ~sym' q

Instances can now be defined by copattern matching. [Issue #1413] The following example extends the one in [Abel, Pientka, Thibodeau, Setzer, POPL 2013, Section 2.2]:

{-# OPTIONS --copatterns #-}

-- The Monad type class

record Monad (M : Set → Set) : Set1 where
  field
    return : {A : Set}   → A → M A
    _>>=_  : {A B : Set} → M A → (A → M B) → M B
open Monad {{...}}

-- The State newtype

record State (S A : Set) : Set where
  field
    runState : S → A × S
open State

-- State is an instance of Monad

instance
  stateMonad : {S : Set} → Monad (State S)
  runState (return {{stateMonad}} a  ) s  = a , s    -- NEW
  runState (_>>=_  {{stateMonad}} m k) s₀ =          -- NEW
    let a , s₁ = runState m s₀
    in  runState (k a) s₁

-- stateMonad fulfills the monad laws

leftId : {A B S : Set}(a : A)(k : A → State S B) →
  (return a >>= k) ≡ k a
leftId a k = refl

rightId : {A B S : Set}(m : State S A) →
  (m >>= return) ≡ m
rightId m = refl

assoc : {A B C S : Set}(m : State S A)(k : A → State S B)(l : B → State S C) →
   ((m >>= k) >>= l) ≡ (m >>= λ a → k a >>= l)
assoc m k l = refl

Emacs mode

The new menu option Switch to another version of Agda tries to do what it says.
Changed feature: Interactively split result.

[ This is as before: ] Make-case (C-c C-c) with no variables given tries to split on the result to introduce projection patterns. The hole needs to be of record type, of course.
```
test : {A B : Set} (a : A) (b : B) → A × B
test a b = ?
```
Result-splitting ? will produce the new clauses:
```
proj₁ (test a b) = ?
proj₂ (test a b) = ?
```
[ This has changed: ] If hole is of function type, make-case will introduce only pattern variables (as much as it can).
```
testFun : {A B : Set} (a : A) (b : B) → A × B
testFun = ?
```
Result-splitting ? will produce the new clause:
```
testFun a b = ?
```
A second invocation of make-case will then introduce projection patterns.

Error messages

Agda now suggests corrections of misspelled options, e.g.

{-# OPTIONS
  --dont-termination-check
  --without-k
  --senf-gurke
  #-}

Unrecognized options:

--dont-termination-check (did you mean --no-termination-check ?)
--without-k (did you mean --without-K ?)
--senf-gurke

Nothing close to --senf-gurke, I am afraid.

Compiler backends

The Epic backend has been removed [Issue #1481].

Bug fixes

Fixed bug with unquoteDecl not working in instance blocks [Issue #1491].
Other issues fixed (see bug tracker

#1497

#1500

Release notes for Agda version 2.4.2.2

Bug fixes

Compilation on Windows fixed.
Other issues fixed (see bug tracker)

#1332

#1353

#1360

#1366

#1369

Release notes for Agda version 2.4.2.1

Pragmas and options

New pragma {-# TERMINATING #-} replacing {-# NO_TERMINATION_CHECK #-}

Complements the existing pragma {-# NON_TERMINATING #-}. Skips termination check for the associated definitions and marks them as terminating. Thus, it is a replacement for {-# NO_TERMINATION_CHECK #-} with the same semantics.

You can no longer use pragma {-# NO_TERMINATION_CHECK #-} to skip the termination check, but must label your definitions as either {-# TERMINATING #-} or {-# NON_TERMINATING #-} instead.

Note: {-# OPTION --no-termination-check #-} labels all your definitions as {-# TERMINATING #-}, putting you in the danger zone of a loop in the type checker.

Language

Referring to a local variable shadowed by module opening is now an error. Previous behavior was preferring the local over the imported definitions. [Issue #1266]

Note that module parameters are locals as well as variables bound by λ, dependent function type, patterns, and let.

Example:
```
module M where
  A = Set1

test : (A : Set) → let open M in A
```
The last A produces an error, since it could refer to the local variable A or to the definition imported from module M.
with on a variable bound by a module telescope or a pattern of a parent function is now forbidden. [Issue #1342]
```
data Unit : Set where
  unit : Unit

id : (A : Set) → A → A
id A a = a

module M (x : Unit) where

  dx : Unit → Unit
  dx unit = x

  g : ∀ u → x ≡ dx u
  g with x
  g | unit  = id (∀ u → unit ≡ dx u) ?
```
Even though this code looks right, Agda complains about the type expression ∀ u → unit ≡ dx u. If you ask Agda what should go there instead, it happily tells you that it wants ∀ u → unit ≡ dx u. In fact what you do not see and Agda will never show you is that the two expressions actually differ in the invisible first argument to dx, which is visible only outside module M. What Agda wants is an invisible unit after dx, but all you can write is an invisible x (which is inserted behind the scenes).

To avoid those kinds of paradoxes, with is now outlawed on module parameters. This should ensure that the invisible arguments are always exactly the module parameters.

Since a where block is desugared as module with pattern variables of the parent clause as module parameters, the same strikes you for uses of with on pattern variables of the parent function.
```
f : Unit → Unit
f x = unit
  where
    dx : Unit → Unit
    dx unit = x

    g : ∀ u → x ≡ dx u
    g with x
    g | unit  = id ((u : Unit) → unit ≡ dx u) ?
```
The with on pattern variable x of the parent clause f x = unit is outlawed now.

Type checking

Termination check failure is now a proper error.

We no longer continue type checking after termination check failures. Use pragmas {-# NON_TERMINATING #-} and {-# NO_TERMINATION_CHECK #-} near the offending definitions if you want to do so. Or switch off the termination checker altogether with {-# OPTIONS --no-termination-check #-} (at your own risk!).
(Since Agda 2.4.2): Termination checking --without-K restricts structural descent to arguments ending in data types or Size. Likewise, guardedness is only tracked when result type is data or record type.
```
mutual
  data WOne : Set where wrap : FOne → WOne
  FOne = ⊥ → WOne

noo : (X : Set) → (WOne ≡ X) → X → ⊥
noo .WOne refl (wrap f) = noo FOne iso f
```
noo is rejected since at type X the structural descent f < wrap f is discounted --without-K.
```
data Pandora : Set where
  C : ∞ ⊥ → Pandora

loop : (A : Set) → A ≡ Pandora → A
loop .Pandora refl = C (♯ (loop ⊥ foo))
```
loop is rejected since guardedness is not tracked at type A --without-K.

See issues #1023, #1264, #1292.

Termination checking

The termination checker can now recognize simple subterms in dot patterns.

data Subst : (d : Nat) → Set where
  c₁ : ∀ {d} → Subst d → Subst d
  c₂ : ∀ {d₁ d₂} → Subst d₁ → Subst d₂ → Subst (suc d₁ + d₂)

postulate
  comp : ∀ {d₁ d₂} → Subst d₁ → Subst d₂ → Subst (d₁ + d₂)

lookup : ∀ d → Nat → Subst d → Set₁
lookup d             zero    (c₁ ρ)             = Set
lookup d             (suc v) (c₁ ρ)             = lookup d v ρ
lookup .(suc d₁ + d₂) v      (c₂ {d₁} {d₂} ρ σ) = lookup (d₁ + d₂) v (comp ρ σ)

The dot pattern here is actually normalized, so it is

suc (d₁ + d₂)

and the corresponding recursive call argument is (d₁ + d₂). In such simple cases, Agda can now recognize that the pattern is constructor applied to call argument, which is valid descent.

Note however, that Agda only looks for syntactic equality when identifying subterms, since it is not allowed to normalize terms on the rhs during termination checking.

Actually writing the dot pattern has no effect, this works as well, and looks pretty magical… ;-)

hidden : ∀{d} → Nat → Subst d → Set₁
hidden zero    (c₁ ρ)   = Set
hidden (suc v) (c₁ ρ)   = hidden v ρ
hidden v       (c₂ ρ σ) = hidden v (comp ρ σ)

Tools

LaTeX-backend

Fixed the issue of identifiers containing operators being typeset with excessive math spacing.

Bug fixes

Issue #1194
Issue #836: Fields and constructors can be qualified by the record/data type as well as by their record/data module. This now works also for record/data type imported from parametrized modules:
```
module M (_ : Set₁) where

  record R : Set₁ where
    field
      X : Set

open M Set using (R)  -- rather than using (module R)

X : R → Set
X = R.X
```

Release notes for Agda version 2.4.2

Pragmas and options

New option: --with-K

This can be used to override a global --without-K in a file, by adding a pragma {-# OPTIONS --with-K #-}.
New pragma {-# NON_TERMINATING #-}

This is a safer version of NO_TERMINATION_CHECK which doesn’t treat the affected functions as terminating. This means that NON_TERMINATING functions do not reduce during type checking. They do reduce at run-time and when invoking C-c C-n at top-level (but not in a hole).

Language

Instance search is now more efficient and recursive (see Issue #938) (but without termination check yet).

A new keyword instance has been introduced (in the style of abstract and private) which must now be used for every definition/postulate that has to be taken into account during instance resolution. For example:
```
record RawMonoid (A : Set) : Set where
  field
    nil  : A
    _++_ : A -> A -> A

open RawMonoid {{...}}

instance
  rawMonoidList : {A : Set} -> RawMonoid (List A)
  rawMonoidList = record { nil = []; _++_ = List._++_ }

  rawMonoidMaybe : {A : Set} {{m : RawMonoid A}} -> RawMonoid (Maybe A)
  rawMonoidMaybe {A} = record { nil = nothing ; _++_ = catMaybe }
    where
      catMaybe : Maybe A -> Maybe A -> Maybe A
      catMaybe nothing mb = mb
      catMaybe ma nothing = ma
      catMaybe (just a) (just b) = just (a ++ b)
```
Moreover, each type of an instance must end in (something that reduces to) a named type (e.g. a record, a datatype or a postulate). This allows us to build a simple index structure
```
data/record name  -->  possible instances
```
that speeds up instance search.

Instance search takes into account all local bindings and all global instance bindings and the search is recursive. For instance, searching for
```
? : RawMonoid (Maybe (List A))
```
will consider the candidates {rawMonoidList, rawMonoidMaybe}, fail to unify the first one, succeeding with the second one
```
? = rawMonoidMaybe {A = List A} {{m = ?m}} : RawMonoid (Maybe (List A))
```
and continue with goal
```
?m : RawMonoid (List A)
```
This will then find
```
?m = rawMonoidList {A = A}
```
and putting together we have the solution.

Be careful that there is no termination check for now, you can easily make Agda loop by declaring the identity function as an instance. But it shouldn’t be possible to make Agda loop by only declaring structurally recursive instances (whatever that means).

Additionally:
- Uniqueness of instances is up to definitional equality (see Issue #899).
- Instances of the following form are allowed:
```
EqSigma : {A : Set} {B : A → Set} {{EqA : Eq A}}
          {{EqB : {a : A} → Eq (B a)}}
          → Eq (Σ A B)
```
  When searching recursively for an instance of type {a : A} → Eq (B a), a lambda will automatically be introduced and instance search will search for something of type Eq (B a) in the context extended by a : A. When searching for an instance, the a argument does not have to be implicit, but in the definition of EqSigma, instance search will only be able to use EqB if a is implicit.
- There is no longer any attempt to solve irrelevant metas by instance search.
- Constructors of records and datatypes are automatically added to the instance table.

You can now use quote in patterns.

For instance, here is a function that unquotes a (closed) natural number term.

unquoteNat : Term → Maybe Nat
unquoteNat (con (quote Nat.zero) [])            = just zero
unquoteNat (con (quote Nat.suc) (arg _ n ∷ [])) = fmap suc (unquoteNat n)
unquoteNat _                                    = nothing

The builtin constructors AGDATERMUNSUPPORTED and AGDASORTUNSUPPORTED are now translated to meta variables when unquoting.
New syntactic sugar tactic e and tactic e | e1 | .. | en.

It desugars as follows and makes it less unwieldy to call reflection-based tactics.
```
tactic e                --> quoteGoal g in unquote (e g)
tactic e | e1 | .. | en --> quoteGoal g in unquote (e g) e1 .. en
```
Note that in the second form the tactic function should generate a function from a number of new subgoals to the original goal. The type of e should be Term -> Term in both cases.
New reflection builtins for literals.

The term data type AGDATERM now needs an additional constructor AGDATERMLIT taking a reflected literal defined as follows (with appropriate builtin bindings for the types Nat, Float, etc).
```
data Literal : Set where
  nat    : Nat    → Literal
  float  : Float  → Literal
  char   : Char   → Literal
  string : String → Literal
  qname  : QName  → Literal

{-# BUILTIN AGDALITERAL   Literal #-}
{-# BUILTIN AGDALITNAT    nat     #-}
{-# BUILTIN AGDALITFLOAT  float   #-}
{-# BUILTIN AGDALITCHAR   char    #-}
{-# BUILTIN AGDALITSTRING string  #-}
{-# BUILTIN AGDALITQNAME  qname   #-}
```
When quoting (quoteGoal or quoteTerm) literals will be mapped to the AGDATERMLIT constructor. Previously natural number literals were quoted to suc/zero application and other literals were quoted to AGDATERMUNSUPPORTED.

New reflection builtins for function definitions.

AGDAFUNDEF should now map to a data type defined as follows

(with

{-# BUILTIN QNAME       QName   #-}
{-# BUILTIN ARG         Arg     #-}
{-# BUILTIN AGDATERM    Term    #-}
{-# BUILTIN AGDATYPE    Type    #-}
{-# BUILTIN AGDALITERAL Literal #-}

data Pattern : Set where
  con    : QName → List (Arg Pattern) → Pattern
  dot    : Pattern
  var    : Pattern
  lit    : Literal → Pattern
  proj   : QName → Pattern
  absurd : Pattern

{-# BUILTIN AGDAPATTERN   Pattern #-}
{-# BUILTIN AGDAPATCON    con     #-}
{-# BUILTIN AGDAPATDOT    dot     #-}
{-# BUILTIN AGDAPATVAR    var     #-}
{-# BUILTIN AGDAPATLIT    lit     #-}
{-# BUILTIN AGDAPATPROJ   proj    #-}
{-# BUILTIN AGDAPATABSURD absurd  #-}

data Clause : Set where
  clause        : List (Arg Pattern) → Term → Clause
  absurd-clause : List (Arg Pattern) → Clause

{-# BUILTIN AGDACLAUSE       Clause        #-}
{-# BUILTIN AGDACLAUSECLAUSE clause        #-}
{-# BUILTIN AGDACLAUSEABSURD absurd-clause #-}

data FunDef : Set where
  fun-def : Type → List Clause → FunDef

{-# BUILTIN AGDAFUNDEF    FunDef  #-}
{-# BUILTIN AGDAFUNDEFCON fun-def #-}

New reflection builtins for extended (pattern-matching) lambda.

The AGDATERM data type has been augmented with a constructor
```
AGDATERMEXTLAM : List AGDACLAUSE → List (ARG AGDATERM) → AGDATERM
```
Absurd lambdas (λ ()) are quoted to extended lambdas with an absurd clause.
Unquoting declarations.

You can now define (recursive) functions by reflection using the new unquoteDecl declaration
```
unquoteDecl x = e
```
Here e should have type AGDAFUNDEF and evaluate to a closed value. This value is then spliced in as the definition of x. In the body e, x has type QNAME which lets you splice in recursive definitions.

Standard modifiers, such as fixity declarations, can be applied to x as expected.
Quoted levels

Universe levels are now quoted properly instead of being quoted to AGDASORTUNSUPPORTED. Setω still gets an unsupported sort, however.

Module applicants can now be operator applications.

Example:

postulate
  [_] : A -> B

module M (b : B) where

module N (a : A) = M [ a ]

[See Issue #1245]

Minor change in module application semantics. [Issue #892]

Previously re-exported functions were not redefined when instantiating a module. For instance
```
module A where f = ...
module B (X : Set) where
  open A public
module C = B Nat
```
In this example C.f would be an alias for A.f, so if both A and C were opened f would not be ambiguous. However, this behaviour is not correct when A and B share some module parameters (Issue #892). To fix this C now defines its own copy of f (which evaluates to A.f), which means that opening A and C results in an ambiguous f.

Type checking

Recursive records need to be declared as either inductive or coinductive. inductive is no longer default for recursive records. Examples:

record _×_ (A B : Set) : Set where
  constructor _,_
  field
    fst : A
    snd : B

record Tree (A : Set) : Set where
  inductive
  constructor tree
  field
    elem     : A
    subtrees : List (Tree A)

record Stream (A : Set) : Set where
  coinductive
  constructor _::_
  field
    head : A
    tail : Stream A

If you are using old-style (musical) coinduction, a record may have to be declared as inductive, paradoxically.

record Stream (A : Set) : Set where
  inductive -- YES, THIS IS INTENDED !
  constructor _∷_
  field
    head : A
    tail : ∞ (Stream A)

This is because the “coinduction” happens in the use of ∞ and not in the use of record.

Tools

Emacs mode

A new menu option Display can be used to display the version of the running Agda process.

LaTeX-backend

New experimental option references has been added. When specified, i.e.:
```
\usepackage[references]{agda}
```
a new command called \AgdaRef is provided, which lets you reference previously typeset commands, e.g.:

Let us postulate \AgdaRef{apa}.
```
\begin{code}
postulate
  apa : Set
\end{code}
```
Above apa will be typeset (highlighted) the same in the text as in the code, provided that the LaTeX output is post-processed using src/data/postprocess-latex.pl, e.g.:
```
cp $(dirname $(dirname $(agda-mode locate)))/postprocess-latex.pl .
agda -i. --latex Example.lagda
cd latex/
perl ../postprocess-latex.pl Example.tex > Example.processed
mv Example.processed Example.tex
xelatex Example.tex
```
Mix-fix and Unicode should work as expected (Unicode requires XeLaTeX/LuaLaTeX), but there are limitations:
- Overloading identifiers should be avoided, if multiples exist \AgdaRef will typeset according to the first it finds.
- Only the current module is used, should you need to reference identifiers in other modules then you need to specify which other module manually, i.e. \AgdaRef[module]{identifier}.

Release notes for Agda 2 version 2.4.0.2

The Agda input mode now supports alphabetical super and subscripts, in addition to the numerical ones that were already present. [Issue #1240]
New feature: Interactively split result.

Make case (C-c C-c) with no variables given tries to split on the result to introduce projection patterns. The hole needs to be of record type, of course.
```
test : {A B : Set} (a : A) (b : B) → A × B
test a b = ?
```
Result-splitting ? will produce the new clauses:
```
proj₁ (test a b) = ?
proj₂ (test a b) = ?
```
If hole is of function type ending in a record type, the necessary pattern variables will be introduced before the split. Thus, the same result can be obtained by starting from:
```
test : {A B : Set} (a : A) (b : B) → A × B
test = ?
```

The so far undocumented ETA pragma now throws an error if applied to definitions that are not records.

ETA can be used to force eta-equality at recursive record types, for which eta is not enabled automatically by Agda. Here is such an example:

mutual
  data Colist (A : Set) : Set where
    [] : Colist A
    _∷_ : A → ∞Colist A → Colist A

  record ∞Colist (A : Set) : Set where
    coinductive
    constructor delay
    field       force : Colist A

open ∞Colist

{-# ETA ∞Colist #-}

test : {A : Set} (x : ∞Colist A) → x ≡ delay (force x)
test x = refl

Note: Unsafe use of ETA can make Agda loop, e.g. by triggering infinite eta expansion!

Bugs fixed (see bug tracker):

#1203

#1205

#1209

#1213

#1214

#1216

#1225

#1226

#1231

#1233

#1239

#1241

#1243

Release notes for Agda 2 version 2.4.0.1

The option --compile-no-main has been renamed to --no-main.
COMPILED_DATA pragmas can now be given for records.
Various bug fixes.

Release notes for Agda 2 version 2.4.0

Installation and infrastructure

A new module called Agda.Primitive has been introduced. This module is available to all users, even if the standard library is not used. Currently the module contains level primitives and their representation in Haskell when compiling with MAlonzo:
```
infixl 6 _⊔_

postulate
  Level : Set
  lzero : Level
  lsuc  : (ℓ : Level) → Level
  _⊔_   : (ℓ₁ ℓ₂ : Level) → Level

{-# COMPILED_TYPE Level ()      #-}
{-# COMPILED lzero ()           #-}
{-# COMPILED lsuc  (\_ -> ())   #-}
{-# COMPILED _⊔_   (\_ _ -> ()) #-}

{-# BUILTIN LEVEL     Level  #-}
{-# BUILTIN LEVELZERO lzero  #-}
{-# BUILTIN LEVELSUC  lsuc   #-}
{-# BUILTIN LEVELMAX  _⊔_    #-}
```
To bring these declarations into scope you can use a declaration like the following one:
```
open import Agda.Primitive using (Level; lzero; lsuc; _⊔_)
```
The standard library reexports these primitives (using the names zero and suc instead of lzero and lsuc) from the Level module.

Existing developments using universe polymorphism might now trigger the following error message:
```
Duplicate binding for built-in thing LEVEL, previous binding to
  .Agda.Primitive.Level
```
To fix this problem, please remove the duplicate bindings.

Technical details (perhaps relevant to those who build Agda packages):

The include path now always contains a directory <DATADIR>/lib/prim, and this directory is supposed to contain a subdirectory Agda containing a file Primitive.agda.

The standard location of <DATADIR> is system- and installation-specific. E.g., in a Cabal --user installation of Agda-2.3.4 on a standard single-ghc Linux system it would be $HOME/.cabal/share/Agda-2.3.4 or something similar.

The location of the <DATADIR> directory can be configured at compile-time using Cabal flags (--datadir and --datasubdir). The location can also be set at run-time, using the Agda_datadir environment variable.

Pragmas and options

Pragma NO_TERMINATION_CHECK placed within a mutual block is now applied to the whole mutual block (rather than being discarded silently). Adding to the uses 1.-4. outlined in the release notes for 2.3.2 we allow:

3a. Skipping an old-style mutual block: Somewhere within mutual block before a type signature or first function clause.
```
mutual
  {-# NO_TERMINATION_CHECK #-}
  c : A
  c = d

  d : A
  d = c
```
New option --no-pattern-matching

Disables all forms of pattern matching (for the current file). You can still import files that use pattern matching.
New option -v profile:7

Prints some stats on which phases Agda spends how much time. (Number might not be very reliable, due to garbage collection interruptions, and maybe due to laziness of Haskell.)
New option --no-sized-types

Option --sized-types is now default. --no-sized-types will turn off an extra (inexpensive) analysis on data types used for subtyping of sized types.

Language

Experimental feature: quoteContext

There is a new keyword quoteContext that gives users access to the list of names in the current local context. For instance:
```
open import Data.Nat
open import Data.List
open import Reflection

foo : ℕ → ℕ → ℕ
foo 0 m = 0
foo (suc n) m = quoteContext xs in ?
```
In the remaining goal, the list xs will consist of two names, n and m, corresponding to the two local variables. At the moment it is not possible to access let bound variables (this feature may be added in the future).

Experimental feature: Varying arity. Function clauses may now have different arity, e.g.,

Sum : ℕ → Set
Sum 0       = ℕ
Sum (suc n) = ℕ → Sum n

sum : (n : ℕ) → ℕ → Sum n
sum 0       acc   = acc
sum (suc n) acc m = sum n (m + acc)

or,

T : Bool → Set
T true  = Bool
T false = Bool → Bool

f : (b : Bool) → T b
f false true  = false
f false false = true
f true = true

This feature is experimental. Yet unsupported:

Varying arity and with.
Compilation of functions with varying arity to Haskell, JS, or Epic.

Experimental feature: copatterns. (Activated with option --copatterns)

We can now define a record by explaining what happens if you project the record. For instance:

{-# OPTIONS --copatterns #-}

record _×_ (A B : Set) : Set where
  constructor _,_
  field
    fst : A
    snd : B
open _×_

pair : {A B : Set} → A → B → A × B
fst (pair a b) = a
snd (pair a b) = b

swap : {A B : Set} → A × B → B × A
fst (swap p) = snd p
snd (swap p) = fst p

swap3 : {A B C : Set} → A × (B × C) → C × (B × A)
fst (swap3 t)       = snd (snd t)
fst (snd (swap3 t)) = fst (snd t)
snd (snd (swap3 t)) = fst t

Taking a projection on the left hand side (lhs) is called a projection pattern, applying to a pattern is called an application pattern. (Alternative terms: projection/application copattern.)

In the first example, the symbol pair, if applied to variable patterns a and b and then projected via fst, reduces to a. pair by itself does not reduce.

A typical application are coinductive records such as streams:

record Stream (A : Set) : Set where
  coinductive
  field
    head : A
    tail : Stream A
open Stream

repeat : {A : Set} (a : A) -> Stream A
head (repeat a) = a
tail (repeat a) = repeat a

Again, repeat a by itself will not reduce, but you can take a projection (head or tail) and then it will reduce to the respective rhs. This way, we get the lazy reduction behavior necessary to avoid looping corecursive programs.

Application patterns do not need to be trivial (i.e., variable patterns), if we mix with projection patterns. E.g., we can have

nats : Nat -> Stream Nat
head (nats zero) = zero
tail (nats zero) = nats zero
head (nats (suc x)) = x
tail (nats (suc x)) = nats x

Here is an example (not involving coinduction) which demostrates records with fields of function type:

-- The State monad

record State (S A : Set) : Set where
  constructor state
  field
    runState : S → A × S
open State

-- The Monad type class

record Monad (M : Set → Set) : Set1 where
  constructor monad
  field
    return : {A : Set}   → A → M A
    _>>=_  : {A B : Set} → M A → (A → M B) → M B


-- State is an instance of Monad
-- Demonstrates the interleaving of projection and application patterns

stateMonad : {S : Set} → Monad (State S)
runState (Monad.return stateMonad a  ) s  = a , s
runState (Monad._>>=_  stateMonad m k) s₀ =
  let a , s₁ = runState m s₀
  in  runState (k a) s₁

module MonadLawsForState {S : Set} where

  open Monad (stateMonad {S})

  leftId : {A B : Set}(a : A)(k : A → State S B) →
    (return a >>= k) ≡ k a
  leftId a k = refl

  rightId : {A B : Set}(m : State S A) →
    (m >>= return) ≡ m
  rightId m = refl

  assoc : {A B C : Set}(m : State S A)(k : A → State S B)(l : B → State S C) →
    ((m >>= k) >>= l) ≡ (m >>= λ a → (k a >>= l))
  assoc m k l = refl

Copatterns are yet experimental and the following does not work:

Copatterns and with clauses.
Compilation of copatterns to Haskell, JS, or Epic.
Projections generated by
```
open R {{...}}
```
are not handled properly on lhss yet.
Conversion checking is slower in the presence of copatterns, since stuck definitions of record type do no longer count as neutral, since they can become unstuck by applying a projection. Thus, comparing two neutrals currently requires comparing all they projections, which repeats a lot of work.

Top-level module no longer required.

The top-level module can be omitted from an Agda file. The module name is then inferred from the file name by dropping the path and the .agda extension. So, a module defined in /A/B/C.agda would get the name C.

You can also suppress only the module name of the top-level module by writing
```
module _ where
```
This works also for parameterised modules.

Module parameters are now always hidden arguments in projections. For instance:

module M (A : Set) where

  record Prod (B : Set) : Set where
    constructor _,_
    field
      fst : A
      snd : B
  open Prod public

open M

Now, the types of fst and snd are

fst : {A : Set}{B : Set} → Prod A B → A
snd : {A : Set}{B : Set} → Prod A B → B

Until 2.3.2, they were

fst : (A : Set){B : Set} → Prod A B → A
snd : (A : Set){B : Set} → Prod A B → B

This change is a step towards symmetry of constructors and projections. (Constructors always took the module parameters as hidden arguments).

Telescoping lets: Local bindings are now accepted in telescopes of modules, function types, and lambda-abstractions.

The syntax of telescopes as been extended to support let:
```
id : (let ★ = Set) (A : ★) → A → A
id A x = x
```
In particular one can now open modules inside telescopes:
```
module Star where
  ★ : Set₁
  ★ = Set


module MEndo (let open Star) (A : ★) where
  Endo : ★
  Endo = A → A
```
Finally a shortcut is provided for opening modules:
```
module N (open Star) (A : ★) (open MEndo A) (f : Endo) where
  ...
```
The semantics of the latter is
```
module _ where
  open Star
  module _ (A : ★) where
    open MEndo A
    module N (f : Endo) where
      ...
```
The semantics of telescoping lets in function types and lambda abstractions is just expanding them into ordinary lets.
More liberal left-hand sides in lets [Issue #1028]:

You can now write left-hand sides with arguments also for let bindings without a type signature. For instance,
```
let f x = suc x in f zero
```
Let bound functions still can’t do pattern matching though.
Ambiguous names in patterns are now optimistically resolved in favor of constructors. [Issue #822] In particular, the following succeeds now:
```
module M where

  data D : Set₁ where
    [_] : Set → D

postulate [_] : Set → Set

open M

Foo : _ → Set
Foo [ A ] = A
```

Anonymous where-modules are opened public. [Issue #848]

<clauses>
f args = rhs
  module _ telescope where
    body
<more clauses>

means the following (not proper Agda code, since you cannot put a module in-between clauses)

<clauses>
module _ {arg-telescope} telescope where
  body

f args = rhs
<more clauses>

Example:

A : Set1
A = B module _ where
  B : Set1
  B = Set

C : Set1
C = B

Builtin ZERO and SUC have been merged with NATURAL.

When binding the NATURAL builtin, ZERO and SUC are bound to the appropriate constructors automatically. This means that instead of writing
```
{-# BUILTIN NATURAL Nat #-}
{-# BUILTIN ZERO zero #-}
{-# BUILTIN SUC suc #-}
```
you just write
```
{-# BUILTIN NATURAL Nat #-}
```

Pattern synonym can now have implicit arguments. [Issue #860]

For example,

pattern tail=_ {x} xs = x ∷ xs

len : ∀ {A} → List A → Nat
len []         = 0
len (tail= xs) = 1 + len xs

Syntax declarations can now have implicit arguments. [Issue #400]

For example
```
id : ∀ {a}{A : Set a} -> A -> A
id x = x

syntax id {A} x = x ∈ A
```
Minor syntax changes
- -} is now parsed as end-comment even if no comment was begun. As a consequence, the following definition gives a parse error
```
f : {A- : Set} -> Set
f {A-} = A-
```
  because Agda now sees ID(f) LBRACE ID(A) END-COMMENT, and no longer ID(f) LBRACE ID(A-) RBRACE.
  
  The rational is that the previous lexing was to context-sensitive, attempting to comment-out f using {- and -} lead to a parse error.
- Fixities (binding strengths) can now be negative numbers as well. [Issue #1109]
```
infix -1 _myop_
```
- Postulates are now allowed in mutual blocks. [Issue #977]
- Empty where blocks are now allowed. [Issue #947]
- Pattern synonyms are now allowed in parameterised modules. [Issue #941]
- Empty hiding and renaming lists in module directives are now allowed.
- Module directives using, hiding, renaming and public can now appear in arbitrary order. Multiple using/hiding/renaming directives are allowed, but you still cannot have both using and hiding (because that doesn’t make sense). [Issue #493]

Goal and error display

The error message Refuse to construct infinite term has been removed, instead one gets unsolved meta variables. Reason: the error was thrown over-eagerly. [Issue #795]
If an interactive case split fails with message
```
  Since goal is solved, further case distinction is not supported;
  try `Solve constraints' instead
```
then the associated interaction meta is assigned to a solution. Press C-c C-= (Show constraints) to view the solution and C-c C-s (Solve constraints) to apply it. [Issue #289]

Type checking

[ Issue #376 ] Implemented expansion of bound record variables during meta assignment. Now Agda can solve for metas X that are applied to projected variables, e.g.:

X (fst z) (snd z) = z

X (fst z)         = fst z

Technically, this is realized by substituting (x , y) for z with fresh bound variables x and y. Here the full code for the examples:

record Sigma (A : Set)(B : A -> Set) : Set where
  constructor _,_
  field
    fst : A
    snd : B fst
open Sigma

test : (A : Set) (B : A -> Set) ->
  let X : (x : A) (y : B x) -> Sigma A B
      X = _
  in  (z : Sigma A B) -> X (fst z) (snd z) ≡ z
test A B z = refl

test' : (A : Set) (B : A -> Set) ->
  let X : A -> A
      X = _
  in  (z : Sigma A B) -> X (fst z) ≡ fst z
test' A B z = refl

The fresh bound variables are named fst(z) and snd(z) and can appear in error messages, e.g.:

fail : (A : Set) (B : A -> Set) ->
  let X : A -> Sigma A B
      X = _
  in  (z : Sigma A B) -> X (fst z) ≡ z
fail A B z = refl

results in error:

Cannot instantiate the metavariable _7 to solution fst(z) , snd(z)
since it contains the variable snd(z) which is not in scope of the
metavariable or irrelevant in the metavariable but relevant in the
solution
when checking that the expression refl has type _7 A B (fst z) ≡ z

Dependent record types and definitions by copatterns require reduction with previous function clauses while checking the current clause. [Issue #907]

For a simple example, consider
```
test : ∀ {A} → Σ Nat λ n → Vec A n
proj₁ test = zero
proj₂ test = []
```
For the second clause, the lhs and rhs are typed as
```
proj₂ test : Vec A (proj₁ test)
[]         : Vec A zero
```
In order for these types to match, we have to reduce the lhs type with the first function clause.

Note that termination checking comes after type checking, so be careful to avoid non-termination! Otherwise, the type checker might get into an infinite loop.
The implementation of the primitive primTrustMe has changed. It now only reduces to REFL if the two arguments x and y have the same computational normal form. Before, it reduced when x and y were definitionally equal, which included type-directed equality laws such as eta-equality. Yet because reduction is untyped, calling conversion from reduction lead to Agda crashes [Issue #882].

The amended description of primTrustMe is (cf. release notes for 2.2.6):
```
primTrustMe : {A : Set} {x y : A} → x ≡ y
```
Here _≡_ is the builtin equality (see BUILTIN hooks for equality, above).

If x and y have the same computational normal form, then primTrustMe {x = x} {y = y} reduces to refl.

A note on primTrustMe’s runtime behavior: The MAlonzo compiler replaces all uses of primTrustMe with the REFL builtin, without any check for definitional equality. Incorrect uses of primTrustMe can potentially lead to segfaults or similar problems of the compiled code.
Implicit patterns of record type are now only eta-expanded if there is a record constructor. [Issues #473, #635]
```
data D : Set where
  d : D

data P : D → Set where
  p : P d

record Rc : Set where
  constructor c
  field f : D

works : {r : Rc} → P (Rc.f r) → Set
works p = D
```
This works since the implicit pattern r is eta-expanded to c x which allows the type of p to reduce to P x and x to be unified with d. The corresponding explicit version is:
```
works' : (r : Rc) → P (Rc.f r) → Set
works' (c .d) p = D
```
However, if the record constructor is removed, the same example will fail:
```
record R : Set where
  field f : D

fails : {r : R} → P (R.f r) → Set
fails p = D

-- d != R.f r of type D
-- when checking that the pattern p has type P (R.f r)
```
The error is justified since there is no pattern we could write down for r. It would have to look like
```
record { f = .d }
```
but anonymous record patterns are not part of the language.
Absurd lambdas at different source locations are no longer different. [Issue #857] In particular, the following code type-checks now:
```
absurd-equality : _≡_ {A = ⊥ → ⊥} (λ()) λ()
absurd-equality = refl
```
Which is a good thing!
Printing of named implicit function types.

When printing terms in a context with bound variables Agda renames new bindings to avoid clashes with the previously bound names. For instance, if A is in scope, the type (A : Set) → A is printed as (A₁ : Set) → A₁. However, for implicit function types the name of the binding matters, since it can be used when giving implicit arguments.

For this situation, the following new syntax has been introduced: {x = y : A} → B is an implicit function type whose bound variable (in scope in B) is y, but where the name of the argument is x for the purposes of giving it explicitly. For instance, with A in scope, the type {A : Set} → A is now printed as {A = A₁ : Set} → A₁.

This syntax is only used when printing and is currently not being parsed.

Changed the semantics of --without-K. [Issue #712, Issue #865, Issue #1025]

New specification of --without-K:

When --without-K is enabled, the unification of indices for pattern matching is restricted in two ways:

Reflexive equations of the form x == x are no longer solved, instead Agda gives an error when such an equation is encountered.
When unifying two same-headed constructor forms c us and c vs of type D pars ixs, the datatype indices ixs (but not the parameters) have to be self-unifiable, i.e. unification of ixs with itself should succeed positively. This is a nontrivial requirement because of point 1.

Examples:

The J rule is accepted.

J : {A : Set} (P : {x y : A} → x ≡ y → Set) →
    (∀ x → P (refl x)) →
    ∀ {x y} (x≡y : x ≡ y) → P x≡y
J P p (refl x) = p x
```agda

This definition is accepted since unification of `x` with `y`
doesn't require deletion or injectivity.

The K rule is rejected.

K : {A : Set} (P : {x : A} → x ≡ x → Set) →
    (∀ x → P (refl {x = x})) →
   ∀ {x} (x≡x : x ≡ x) → P x≡x
K P p refl = p _

Definition is rejected with the following error:

Cannot eliminate reflexive equation x = x of type A because K has
been disabled.
when checking that the pattern refl has type x ≡ x

Symmetry of the new criterion.

test₁ : {k l m : ℕ} → k + l ≡ m → ℕ
test₁ refl = zero

test₂ : {k l m : ℕ} → k ≡ l + m → ℕ
test₂ refl = zero

Both versions are now accepted (previously only the first one was).

Handling of parameters.

cons-injective : {A : Set} (x y : A) → (x ∷ []) ≡ (y ∷ []) → x ≡ y
cons-injective x .x refl = refl

Parameters are not unified, so they are ignored by the new criterion.

A larger example: antisymmetry of ≤.

data _≤_ : ℕ → ℕ → Set where
  lz : (n : ℕ) → zero ≤ n
  ls : (m n : ℕ) → m ≤ n → suc m ≤ suc n

≤-antisym : (m n : ℕ) → m ≤ n → n ≤ m → m ≡ n
≤-antisym .zero    .zero    (lz .zero) (lz .zero)   = refl
≤-antisym .(suc m) .(suc n) (ls m n p) (ls .n .m q) =
             cong suc (≤-antisym m n p q)

[ Issue #1025 ]

postulate mySpace : Set
postulate myPoint : mySpace

data Foo : myPoint ≡ myPoint → Set where
  foo : Foo refl

test : (i : foo ≡ foo) → i ≡ refl
test refl = {!!}

When applying injectivity to the equation foo ≡ foo of type Foo refl, it is checked that the index refl of type myPoint ≡ myPoint is self-unifiable. The equation refl ≡ refl again requires injectivity, so now the index myPoint is checked for self-unifiability, hence the error:

Cannot eliminate reflexive equation myPoint = myPoint of type
mySpace because K has been disabled.
when checking that the pattern refl has type foo ≡ foo

Termination checking

A buggy facility coined “matrix-shaped orders” that supported uncurried functions (which take tuples of arguments instead of one argument after another) has been removed from the termination checker. [Issue #787]
Definitions which fail the termination checker are not unfolded any longer to avoid loops or stack overflows in Agda. However, the termination checker for a mutual block is only invoked after type-checking, so there can still be loops if you define a non-terminating function. But termination checking now happens before the other supplementary checks: positivity, polarity, injectivity and projection-likeness. Note that with the pragma {-# NO_TERMINATION_CHECK #-} you can make Agda treat any function as terminating.
Termination checking of functions defined by with has been improved.

Cases which previously required --termination-depth to pass the termination checker (due to use of with) no longer need the flag. For example
```
merge : List A → List A → List A
merge [] ys = ys
merge xs [] = xs
merge (x ∷ xs) (y ∷ ys) with x ≤ y
merge (x ∷ xs) (y ∷ ys)    | false = y ∷ merge (x ∷ xs) ys
merge (x ∷ xs) (y ∷ ys)    | true  = x ∷ merge xs (y ∷ ys)
```
This failed to termination check previously, since the with expands to an auxiliary function merge-aux:
```
merge-aux x y xs ys false = y ∷ merge (x ∷ xs) ys
merge-aux x y xs ys true  = x ∷ merge xs (y ∷ ys)
```
This function makes a call to merge in which the size of one of the arguments is increasing. To make this pass the termination checker now inlines the definition of merge-aux before checking, thus effectively termination checking the original source program.

As a result of this transformation doing with on a variable no longer preserves termination. For instance, this does not termination check:
```
bad : Nat → Nat
bad n with n
... | zero  = zero
... | suc m = bad m
```
The performance of the termination checker has been improved. For higher --termination-depth the improvement is significant. While the default --termination-depth is still 1, checking with higher --termination-depth should now be feasible.

Compiler backends

The MAlonzo compiler backend now has support for compiling modules that are not full programs (i.e. don’t have a main function). The goal is that you can write part of a program in Agda and the rest in Haskell, and invoke the Agda functions from the Haskell code. The following features were added for this reason:
- A new command-line option --compile-no-main: the command
```
agda --compile-no-main Test.agda
```
  will compile Test.agda and all its dependencies to Haskell and compile the resulting Haskell files with --make, but (unlike --compile) not tell GHC to treat Test.hs as the main module. This type of compilation can be invoked from Emacs by customizing the agda2-backend variable to value MAlonzoNoMain and then calling C-c C-x C-c as before.
- A new pragma COMPILED_EXPORT was added as part of the MAlonzo FFI. If we have an Agda file containing the following:
```
 module A.B where

 test : SomeType
 test = someImplementation

 {-# COMPILED_EXPORT test someHaskellId #-}
```
  then test will be compiled to a Haskell function called someHaskellId in module MAlonzo.Code.A.B that can be invoked from other Haskell code. Its type will be translated according to the normal MAlonzo rules.

Tools

Emacs mode

A new goal command Helper Function Type (C-c C-h) has been added.

If you write an application of an undefined function in a goal, the Helper Function Type command will print the type that the function needs to have in order for it to fit the goal. The type is also added to the Emacs kill-ring and can be pasted into the buffer using C-y.

The application must be of the form f args where f is the name of the helper function you want to create. The arguments can use all the normal features like named implicits or instance arguments.

Example:

Here’s a start on a naive reverse on vectors:
```
reverse : ∀ {A n} → Vec A n → Vec A n
reverse [] = []
reverse (x ∷ xs) = {!snoc (reverse xs) x!}
```
Calling C-c C-h in the goal prints
```
snoc : ∀ {A} {n} → Vec A n → A → Vec A (suc n)
```
A new command Explain why a particular name is in scope (C-c C-w) has been added. [Issue #207]

This command can be called from a goal or from the top-level and will as the name suggests explain why a particular name is in scope.

For each definition or module that the given name can refer to a trace is printed of all open statements and module applications leading back to the original definition of the name.

For example, given
```
module A (X : Set₁) where
  data Foo : Set where
    mkFoo : Foo
module B (Y : Set₁) where
  open A Y public
module C = B Set
open C
```
Calling C-c C-w on mkFoo at the top-level prints
```
mkFoo is in scope as
* a constructor Issue207.C._.Foo.mkFoo brought into scope by
  - the opening of C at Issue207.agda:13,6-7
  - the application of B at Issue207.agda:11,12-13
  - the application of A at Issue207.agda:9,8-9
  - its definition at Issue207.agda:6,5-10
```
This command is useful if Agda complains about an ambiguous name and you need to figure out how to hide the undesired interpretations.
Improvements to the make case command (C-c C-c)
- One can now also split on hidden variables, using the name (starting with .) with which they are printed. Use C-c C-, to see all variables in context.
- Concerning the printing of generated clauses:
  - Uses named implicit arguments to improve readability.
  - Picks explicit occurrences over implicit ones when there is a choice of binding site for a variable.
  - Avoids binding variables in implicit positions by replacing dot patterns that uses them by wildcards (._).
Key bindings for lots of “mathematical” characters (examples: 𝐴𝑨𝒜𝓐𝔄) have been added to the Agda input method. Example: type \MiA\MIA\McA\MCA\MfA to get 𝐴𝑨𝒜𝓐𝔄.

Note: \McB does not exist in Unicode (as well as others in that style), but the \MC (bold) alphabet is complete.
Key bindings for “blackboard bold” B (𝔹) and 0-9 (𝟘-𝟡) have been added to the Agda input method (\bb and \b[0-9]).
Key bindings for controlling simplification/normalisation:

Commands like Goal type and context (C-c C-,) could previously be invoked in two ways. By default the output was normalised, but if a prefix argument was used (for instance via C-u C-c C-,), then no explicit normalisation was performed. Now there are three options:
- By default (C-c C-,) the output is simplified.
- If C-u is used exactly once (C-u C-c C-,), then the result is neither (explicitly) normalised nor simplified.
- If C-u is used twice (C-u C-u C-c C-,), then the result is normalised.

LaTeX-backend

Two new color scheme options were added to agda.sty:

\usepackage[bw]{agda}, which highlights in black and white; \usepackage[conor]{agda}, which highlights using Conor’s colors.

The default (no options passed) is to use the standard colors.
If agda.sty cannot be found by the LateX environment, it is now copied into the LateX output directory (latex by default) instead of the working directory. This means that the commands needed to produce a PDF now is
```
agda --latex -i . <file>.lagda
cd latex
pdflatex <file>.tex
```
The LaTeX-backend has been made more tool agnostic, in particular XeLaTeX and LuaLaTeX should now work. Here is a small example (test/LaTeXAndHTML/succeed/UnicodeInput.lagda):
```
\documentclass{article}
\usepackage{agda}
\begin{document}

\begin{code}
data αβγδεζθικλμνξρστυφχψω : Set₁ where

postulate
  →⇒⇛⇉⇄↦⇨↠⇀⇁ : Set
\end{code}

\[
∀X [ ∅ ∉ X ⇒ ∃f:X ⟶  ⋃ X\ ∀A ∈ X (f(A) ∈ A) ]
\]
\end{document}
```
Compiled as follows, it should produce a nice looking PDF (tested with TeX Live 2012):
```
agda --latex <file>.lagda
cd latex
xelatex <file>.tex (or lualatex <file>.tex)
```
If symbols are missing or XeLaTeX/LuaLaTeX complains about the font missing, try setting a different font using:
```
\setmathfont{<math-font>}
```
Use the fc-list tool to list available fonts.

Add experimental support for hyperlinks to identifiers

If the hyperref LateX package is loaded before the Agda package and the links option is passed to the Agda package, then the Agda package provides a function called \AgdaTarget. Identifiers which have been declared targets, by the user, will become clickable hyperlinks in the rest of the document. Here is a small example (test/LaTeXAndHTML/succeed/Links.lagda):

\documentclass{article}
\usepackage{hyperref}
\usepackage[links]{agda}
\begin{document}

\AgdaTarget{ℕ}
\AgdaTarget{zero}
\begin{code}
data ℕ : Set where
  zero  : ℕ
  suc   : ℕ → ℕ
\end{code}

See next page for how to define \AgdaFunction{two} (doesn't turn into a
link because the target hasn't been defined yet). We could do it
manually though; \hyperlink{two}{\AgdaDatatype{two}}.

\newpage

\AgdaTarget{two}
\hypertarget{two}{}
\begin{code}
two : ℕ
two = suc (suc zero)
\end{code}

\AgdaInductiveConstructor{zero} is of type
\AgdaDatatype{ℕ}. \AgdaInductiveConstructor{suc} has not been defined to
be a target so it doesn't turn into a link.

\newpage

Now that the target for \AgdaFunction{two} has been defined the link
works automatically.

\begin{code}
data Bool : Set where
  true false : Bool
\end{code}

The AgdaTarget command takes a list as input, enabling several
targets to be specified as follows:

\AgdaTarget{if, then, else, if\_then\_else\_}
\begin{code}
if_then_else_ : {A : Set} → Bool → A → A → A
if true  then t else f = t
if false then t else f = f
\end{code}

\newpage

Mixfix identifier need their underscores escaped:
\AgdaFunction{if\_then\_else\_}.

\end{document}

The boarders around the links can be suppressed using hyperref’s hidelinks option:

  \usepackage[hidelinks]{hyperref}

Note that the current approach to links does not keep track of scoping or types, and hence overloaded names might create links which point to the wrong place. Therefore it is recommended to not overload names when using the links option at the moment, this might get fixed in the future.

Release notes for Agda 2 version 2.3.2.2

Fixed a bug that sometimes made it tricky to use the Emacs mode on Windows [Issue #757].
Made Agda build with newer versions of some libraries.
Fixed a bug that caused ambiguous parse error messages [Issue #147].

Release notes for Agda 2 version 2.3.2.1

Installation

Made it possible to compile Agda with more recent versions of hashable, QuickCheck and Win32.
Excluded mtl-2.1.

Type checking

Fixed bug in the termination checker (Issue #754).

Release notes for Agda 2 version 2.3.2

Installation

The Agda-executable package has been removed.

The executable is now provided as part of the Agda package.
The Emacs mode no longer depends on haskell-mode or GHCi.
Compilation of Emacs mode Lisp files.

You can now compile the Emacs mode Lisp files by running agda-mode compile. This command is run by make install.

Compilation can, in some cases, give a noticeable speedup.

WARNING: If you reinstall the Agda mode without recompiling the Emacs Lisp files, then Emacs may continue using the old, compiled files.

Pragmas and options

The --without-K check now reconstructs constructor parameters.

New specification of --without-K:

If the flag is activated, then Agda only accepts certain case-splits. If the type of the variable to be split is D pars ixs, where D is a data (or record) type, pars stands for the parameters, and ixs the indices, then the following requirements must be satisfied:
- The indices ixs must be applications of constructors (or literals) to distinct variables. Constructors are usually not applied to parameters, but for the purposes of this check constructor parameters are treated as other arguments.
- These distinct variables must not be free in pars.
Irrelevant arguments are printed as _ by default now. To turn on printing of irrelevant arguments, use option
```
--show-irrelevant
```
New: Pragma NO_TERMINATION_CHECK to switch off termination checker for individual function definitions and mutual blocks.

The pragma must precede a function definition or a mutual block. Examples (see test/Succeed/NoTerminationCheck.agda):
1. Skipping a single definition: before type signature.
```
{-# NO_TERMINATION_CHECK #-}
a : A
a = a
```
2. Skipping a single definition: before first clause.
```
b : A
{-# NO_TERMINATION_CHECK #-}
b = b
```
3. Skipping an old-style mutual block: Before mutual keyword.
```
{-# NO_TERMINATION_CHECK #-}
mutual
  c : A
  c = d

  d : A
  d = c
```
4. Skipping a new-style mutual block: Anywhere before a type signature or first function clause in the block
```
i : A
j : A

i = j
{-# NO_TERMINATION_CHECK #-}
j = i
```
The pragma cannot be used in --safe mode.

Language

Let binding record patterns

record _×_ (A B : Set) : Set where
  constructor _,_
  field
    fst : A
    snd : B
open _×_

let (x , (y , z)) = t
in  u

will now be interpreted as

let x = fst t
    y = fst (snd t)
    z = snd (snd t)
in  u

Note that the type of t needs to be inferable. If you need to provide a type signature, you can write the following:

let a : ...
    a = t
    (x , (y , z)) = a
in  u

Pattern synonyms

A pattern synonym is a declaration that can be used on the left hand side (when pattern matching) as well as the right hand side (in expressions). For example:
```
pattern z    = zero
pattern ss x = suc (suc x)

f : ℕ -> ℕ
f z       = z
f (suc z) = ss z
f (ss n)  = n
```
Pattern synonyms are implemented by substitution on the abstract syntax, so definitions are scope-checked but not type-checked. They are particularly useful for universe constructions.
Qualified mixfix operators

It is now possible to use a qualified mixfix operator by qualifying the first part of the name. For instance
```
import Data.Nat as Nat
import Data.Bool as Bool

two = Bool.if true then 1 Nat.+ 1 else 0
```

Sections [Issue #735]. Agda now parses anonymous modules as sections:

module _ {a} (A : Set a) where

  data List : Set a where
    []  : List
    _∷_ : (x : A) (xs : List) → List

module _ {a} {A : Set a} where

  _++_ : List A → List A → List A
  []       ++ ys = ys
  (x ∷ xs) ++ ys = x ∷ (xs ++ ys)

test : List Nat
test = (5 ∷ []) ++ (3 ∷ [])

In general, now the syntax

module _ parameters where
  declarations

is accepted and has the same effect as

private
  module M parameters where
    declarations
open M public

for a fresh name M.

Instantiating a module in an open import statement [Issue #481]. Now accepted:
```
open import Path.Module args [using/hiding/renaming (...)]
```
This only brings the imported identifiers from Path.Module into scope, not the module itself! Consequently, the following is pointless, and raises an error:
```
  import Path.Module args [using/hiding/renaming (...)]
```
You can give a private name M to the instantiated module via
```
import Path.Module args as M [using/hiding/renaming (...)]
open import Path.Module args as M [using/hiding/renaming (...)]
```
Try to avoid as as part of the arguments. as is not a keyword; the following can be legal, although slightly obfuscated Agda code:
```
open import as as as as as as
```
Implicit module parameters can be given by name. E.g.
```
open M {namedArg = bla}
```
This feature has been introduced in Agda 2.3.0 already.
Multiple type signatures sharing a same type can now be written as a single type signature.
```
one two : ℕ
one = suc zero
two = suc one
```

Goal and error display

Meta-variables that were introduced by hidden argument arg are now printed as _arg_number instead of just _number. [Issue #526]
Agda expands identifiers in anonymous modules when printing. Should make some goals nicer to read. [Issue #721]
When a module identifier is ambiguous, Agda tells you if one of them is a data type module. [Issues #318, #705]

Type checking

Improved coverage checker. The coverage checker splits on arguments that have constructor or literal pattern, committing to the left-most split that makes progress. Consider the lookup function for vectors:
```
data Fin : Nat → Set where
  zero : {n : Nat} → Fin (suc n)
  suc  : {n : Nat} → Fin n → Fin (suc n)

data Vec (A : Set) : Nat → Set where
  []  : Vec A zero
  _∷_ : {n : Nat} → A → Vec A n → Vec A (suc n)

_!!_ : {A : Set}{n : Nat} → Vec A n → Fin n → A
(x ∷ xs) !! zero  = x
(x ∷ xs) !! suc i = xs !! i
```
In Agda up to 2.3.0, this definition is rejected unless we add an absurd clause
```
[] !! ()
```
This is because the coverage checker committed on splitting on the vector argument, even though this inevitably lead to failed coverage, because a case for the empty vector [] is missing.

The improvement to the coverage checker consists on committing only on splits that have a chance of covering, since all possible constructor patterns are present. Thus, Agda will now split first on the Fin argument, since cases for both zero and suc are present. Then, it can split on the Vec argument, since the empty vector is already ruled out by instantiating n to a suc _.
Instance arguments resolution will now consider candidates which still expect hidden arguments. For example:
```
record Eq (A : Set) : Set where
  field eq : A → A → Bool

open Eq {{...}}

eqFin : {n : ℕ} → Eq (Fin n)
eqFin = record { eq = primEqFin }

testFin : Bool
testFin = eq fin1 fin2
```
The type-checker will now resolve the instance argument of the eq function to eqFin {_}. This is only done for hidden arguments, not instance arguments, so that the instance search stays non-recursive.

Constraint solving: Upgraded Miller patterns to record patterns. [Issue #456]

Agda now solves meta-variables that are applied to record patterns. A typical (but here, artificial) case is:

record Sigma (A : Set)(B : A -> Set) : Set where
  constructor _,_
  field
    fst : A
    snd : B fst

test : (A : Set)(B : A -> Set) ->
  let X : Sigma A B -> Sigma A B
      X = _
  in  (x : A)(y : B x) -> X (x , y) ≡ (x , y)
test A B x y = refl

This yields a constraint of the form

_X A B (x , y) := t[x,y]

(with t[x,y] = (x, y)) which is not a Miller pattern. However, Agda now solves this as

_X A B z := t[fst z,snd z].

Changed: solving recursive constraints. [Issue #585]

Until 2.3.0, Agda sometimes inferred values that did not pass the termination checker later, or would even make Agda loop. To prevent this, the occurs check now also looks into the definitions of the current mutual block, to avoid constructing recursive solutions. As a consequence, also terminating recursive solutions are no longer found automatically.

This effects a programming pattern where the recursively computed type of a recursive function is left to Agda to solve.
```
mutual

  T : D -> Set
  T pattern1 = _
  T pattern2 = _

  f : (d : D) -> T d
  f pattern1 = rhs1
  f pattern2 = rhs2
```
This might no longer work from now on. See examples test/Fail/Issue585*.agda.
Less eager introduction of implicit parameters. [Issue #679]

Until Agda 2.3.0, trailing hidden parameters were introduced eagerly on the left hand side of a definition. For instance, one could not write
```
test : {A : Set} -> Set
test = \ {A} -> A
```
because internally, the hidden argument {A : Set} was added to the left-hand side, yielding
```
test {_} = \ {A} -> A
```
which raised a type error. Now, Agda only introduces the trailing implicit parameters it has to, in order to maintain uniform function arity. For instance, in
```
test : Bool -> {A B C : Set} -> Set
test true {A}      = A
test false {B = B} = B
```
Agda will introduce parameters A and B in all clauses, but not C, resulting in
```
test : Bool -> {A B C : Set} -> Set
test true  {A} {_}     = A
test false {_} {B = B} = B
```
Note that for checking where-clauses, still all hidden trailing parameters are in scope. For instance:
```
id : {i : Level}{A : Set i} -> A -> A
id = myId
  where myId : forall {A} -> A -> A
        myId x = x
```
To be able to fill in the meta variable _1 in
```
myId : {A : Set _1} -> A -> A
```
the hidden parameter {i : Level} needs to be in scope.

As a result of this more lazy introduction of implicit parameters, the following code now passes.
```
data Unit : Set where
  unit : Unit

T : Unit → Set
T unit = {u : Unit} → Unit

test : (u : Unit) → T u
test unit with unit
... | _ = λ {v} → v
```
Before, Agda would eagerly introduce the hidden parameter {v} as unnamed left-hand side parameter, leaving no way to refer to it.

The related Issue #655 has also been addressed. It is now possible to make `synonym’ definitions
```
name = expression
```
even when the type of expression begins with a hidden quantifier. Simple example:
```
id2 = id
```
That resulted in unsolved metas until 2.3.0.
Agda detects unused arguments and ignores them during equality checking. [Issue #691, solves also Issue #44]

Agda’s polarity checker now assigns ‘Nonvariant’ to arguments that are not actually used (except for absurd matches). If f’s first argument is Nonvariant, then f x is definitionally equal to f y regardless of x and y. It is similar to irrelevance, but does not require user annotation.

For instance, unused module parameters do no longer get in the way:
```
module M (x : Bool) where

  not : Bool → Bool
  not true  = false
  not false = true

open M true
open M false renaming (not to not′)

test : (y : Bool) → not y ≡ not′ y
test y = refl
```
Matching against record or absurd patterns does not count as `use’, so we get some form of proof irrelevance:
```
data ⊥ : Set where
record ⊤ : Set where
  constructor trivial

data Bool : Set where
  true false : Bool

True : Bool → Set
True true  = ⊤
True false = ⊥

fun : (b : Bool) → True b → Bool
fun true  trivial = true
fun false ()

test : (b : Bool) → (x y : True b) → fun b x ≡ fun b y
test b x y = refl
```
More examples in test/Succeed/NonvariantPolarity.agda.

Phantom arguments: Parameters of record and data types are considered `used’ even if they are not actually used. Consider:
```
False : Nat → Set
False zero    = ⊥
False (suc n) = False n

module Invariant where
  record Bla (n : Nat)(p : False n) : Set where

module Nonvariant where
  Bla : (n : Nat) → False n → Set
  Bla n p = ⊤
```
Even though record Bla does not use its parameters n and p, they are considered as used, allowing “phantom type” techniques.

In contrast, the arguments of function Bla are recognized as unused. The following code type-checks if we open Invariant but leaves unsolved metas if we open Nonvariant.
```
drop-suc : {n : Nat}{p : False n} → Bla (suc n) p → Bla n p
drop-suc _ = _

bla : (n : Nat) → {p : False n} → Bla n p → ⊥
bla zero {()} b
bla (suc n) b = bla n (drop-suc b)
```
If Bla is considered invariant, the hidden argument in the recursive call can be inferred to be p. If it is considered non-variant, then Bla n X = Bla n p does not entail X = p and the hidden argument remains unsolved. Since bla does not actually use its hidden argument, its value is not important and it could be searched for. Unfortunately, polarity analysis of bla happens only after type checking, thus, the information that bla is non-variant in p is not available yet when meta-variables are solved. (See test/Fail/BrokenInferenceDueToNonvariantPolarity.agda)
Agda now expands simple definitions (one clause, terminating) to check whether a function is constructor headed. [Issue #747] For instance, the following now also works:
```
MyPair : Set -> Set -> Set
MyPair A B = Pair A B

Vec : Set -> Nat -> Set
Vec A zero    = Unit
Vec A (suc n) = MyPair A (Vec A n)
```
Here, Unit and Pair are data or record types.

Compiler backends

-Werror is now overridable.

To enable compilation of Haskell modules containing warnings, the -Werror flag for the MAlonzo backend has been made overridable. If, for example, --ghc-flag=-Wwarn is passed when compiling, one can get away with things like:
```
data PartialBool : Set where
  true : PartialBool

{-# COMPILED_DATA PartialBool Bool True #-}
```
The default behavior remains as it used to be and rejects the above program.

Tools

Emacs mode

Asynchronous Emacs mode.

One can now use Emacs while a buffer is type-checked. If the buffer is edited while the type-checker runs, then syntax highlighting will not be updated when type-checking is complete.
Interactive syntax highlighting.

The syntax highlighting is updated while a buffer is type-checked:
- At first the buffer is highlighted in a somewhat crude way (without go-to-definition information for overloaded constructors).
- If the highlighting level is “interactive”, then the piece of code that is currently being type-checked is highlighted as such. (The default is “non-interactive”.)
- When a mutual block has been type-checked it is highlighted properly (this highlighting includes warnings for potential non-termination).
The highlighting level can be controlled via the new configuration variable agda2-highlight-level.

Multiple case-splits can now be performed in one go.

Consider the following example:

_==_ : Bool → Bool → Bool
b₁ == b₂ = {!!}

If you split on b₁ b₂, then you get the following code:

_==_ : Bool → Bool → Bool
true == true = {!!}
true == false = {!!}
false == true = {!!}
false == false = {!!}

The order of the variables matters. Consider the following code:

lookup : ∀ {a n} {A : Set a} → Vec A n → Fin n → A
lookup xs i = {!!}

If you split on xs i, then you get the following code:

lookup : ∀ {a n} {A : Set a} → Vec A n → Fin n → A
lookup [] ()
lookup (x ∷ xs) zero = {!!}
lookup (x ∷ xs) (suc i) = {!!}

However, if you split on i xs, then you get the following code instead:

lookup : ∀ {a n} {A : Set a} → Vec A n → Fin n → A
lookup (x ∷ xs) zero = ?
lookup (x ∷ xs) (suc i) = ?

This code is rejected by Agda 2.3.0, but accepted by 2.3.2 thanks to improved coverage checking (see above).

The Emacs mode now presents information about which module is currently being type-checked.
New global menu entry: Information about the character at point.

If this entry is selected, then information about the character at point is displayed, including (in many cases) information about how to type the character.
Commenting/uncommenting the rest of the buffer.

One can now comment or uncomment the rest of the buffer by typing C-c C-x M-; or by selecting the menu entry Comment/uncomment the rest of the buffer”.
The Emacs mode now uses the Agda executable instead of GHCi.

The *ghci* buffer has been renamed to *agda2*.

A new configuration variable has been introduced: agda2-program-name, the name of the Agda executable (by default agda).

The variable agda2-ghci-options has been replaced by agda2-program-args: extra arguments given to the Agda executable (by default none).

If you want to limit Agda’s memory consumption you can add some arguments to agda2-program-args, for instance +RTS -M1.5G -RTS.
The Emacs mode no longer depends on haskell-mode.

Users who have customised certain haskell-mode variables (such as haskell-ghci-program-args) may want to update their configuration.

LaTeX-backend

An experimental LaTeX-backend which does precise highlighting a la the HTML-backend and code alignment a la lhs2TeX has been added.

Here is a sample input literate Agda file:

\documentclass{article}

\usepackage{agda}

\begin{document}

The following module declaration will be hidden in the output.

\AgdaHide{
\begin{code}
module M where
\end{code}
}

Two or more spaces can be used to make the backend align stuff.

\begin{code}
data ℕ : Set where
  zero  : ℕ
  suc   : ℕ → ℕ

_+_ : ℕ → ℕ → ℕ
zero   + n = n
suc m  + n = suc (m + n)
\end{code}

\end{document}

To produce an output PDF issue the following commands:

agda --latex -i . <file>.lagda
pdflatex latex/<file>.tex

Only the top-most module is processed, like with lhs2tex and unlike with the HTML-backend. If you want to process imported modules you have to call agda --latex manually on each of those modules.

There are still issues related to formatting, see the bug tracker for more information:

https://code.google.com/p/agda/issues/detail?id=697

The default agda.sty might therefore change in backwards-incompatible ways, as work proceeds in trying to resolve those problems.

Implemented features:

Two or more spaces can be used to force alignment of things, like with lhs2tex. See example above.
The highlighting information produced by the type checker is used to generate the output. For example, the data declaration in the example above, produces:
```
\AgdaKeyword{data} \AgdaDatatype{ℕ} \AgdaSymbol{:}
    \AgdaPrimitiveType{Set} \AgdaKeyword{where}
```
These LaTeX commands are defined in agda.sty (which is imported by \usepackage{agda}) and cause the highlighting.
The LaTeX-backend checks if agda.sty is found by the LaTeX environment, if it isn’t a default agda.sty is copied from Agda’s data-dir into the working directory (and thus made available to the LaTeX environment).

If the default agda.sty isn’t satisfactory (colors, fonts, spacing, etc) then the user can modify it and make put it somewhere where the LaTeX environment can find it. Hopefully most aspects should be modifiable via agda.sty rather than having to tweak the implementation.
--latex-dir can be used to change the default output directory.

Release notes for Agda 2 version 2.3.0

Language

New more liberal syntax for mutually recursive definitions.

It is no longer necessary to use the mutual keyword to define mutually recursive functions or datatypes. Instead, it is enough to declare things before they are used. Instead of
```
mutual
  f : A
  f = a[f, g]

  g : B[f]
  g = b[f, g]
```
you can now write
```
f : A
g : B[f]
f = a[f, g]
g = b[f, g].
```
With the new style you have more freedom in choosing the order in which things are type checked (previously type signatures were always checked before definitions). Furthermore you can mix arbitrary declarations, such as modules and postulates, with mutually recursive definitions.

For data types and records the following new syntax is used to separate the declaration from the definition:
```
-- Declaration.
data Vec (A : Set) : Nat → Set  -- Note the absence of 'where'.

-- Definition.
data Vec A where
  []   : Vec A zero
  _::_ : {n : Nat} → A → Vec A n → Vec A (suc n)

-- Declaration.
record Sigma (A : Set) (B : A → Set) : Set

-- Definition.
record Sigma A B where
  constructor _,_
  field fst : A
        snd : B fst
```
When making separated declarations/definitions private or abstract you should attach the private keyword to the declaration and the abstract keyword to the definition. For instance, a private, abstract function can be defined as
```
private
  f : A
abstract
  f = e
```
Finally it may be worth noting that the old style of mutually recursive definitions is still supported (it basically desugars into the new style).
Pattern matching lambdas.

Anonymous pattern matching functions can be defined using the syntax
```
\ { p11 .. p1n -> e1 ; ... ; pm1 .. pmn -> em }
```
(where, as usual, \ and -> can be replaced by λ and →). Internally this is translated into a function definition of the following form:
```
.extlam p11 .. p1n = e1
...
.extlam pm1 .. pmn = em
```
This means that anonymous pattern matching functions are generative. For instance, refl will not be accepted as an inhabitant of the type
```
(λ { true → true ; false → false }) ≡
(λ { true → true ; false → false }),
```
because this is equivalent to extlam1 ≡ extlam2 for some distinct fresh names extlam1 and extlam2.

Currently the where and with constructions are not allowed in (the top-level clauses of) anonymous pattern matching functions.

Examples:
```
and : Bool → Bool → Bool
and = λ { true x → x ; false _ → false }

xor : Bool → Bool → Bool
xor = λ { true  true  → false
        ; false false → false
        ; _     _     → true
        }

fst : {A : Set} {B : A → Set} → Σ A B → A
fst = λ { (a , b) → a }

snd : {A : Set} {B : A → Set} (p : Σ A B) → B (fst p)
snd = λ { (a , b) → b }
```
Record update syntax.

Assume that we have a record type and a corresponding value:
```
record MyRecord : Set where
  field
    a b c : ℕ

old : MyRecord
old = record { a = 1; b = 2; c = 3 }
```
Then we can update (some of) the record value’s fields in the following way:
```
new : MyRecord
new = record old { a = 0; c = 5 }
```
Here new normalises to record { a = 0; b = 2; c = 5 }. Any expression yielding a value of type MyRecord can be used instead of old.

Record updating is not allowed to change types: the resulting value must have the same type as the original one, including the record parameters. Thus, the type of a record update can be inferred if the type of the original record can be inferred.

The record update syntax is expanded before type checking. When the expression
```
record old { upd-fields }
```
is checked against a record type R, it is expanded to
```
let r = old in record { new-fields },
```
where old is required to have type R and new-fields is defined as follows: for each field x in R,
- if x = e is contained in upd-fields then x = e is included in new-fields, and otherwise
- if x is an explicit field then x = R.x r is included in new-fields, and
- if x is an implicit or instance field, then it is omitted from new-fields.
(Instance arguments are explained below.) The reason for treating implicit and instance fields specially is to allow code like the following:
```
record R : Set where
  field
    {length} : ℕ
    vec      : Vec ℕ length
    -- More fields…

xs : R
xs = record { vec = 0 ∷ 1 ∷ 2 ∷ [] }

ys = record xs { vec = 0 ∷ [] }
```
Without the special treatment the last expression would need to include a new binding for length (for instance length = _).
Record patterns which do not contain data type patterns, but which do contain dot patterns, are no longer rejected.
When the --without-K flag is used literals are now treated as constructors.
Under-applied functions can now reduce.

Consider the following definition:
```
id : {A : Set} → A → A
id x = x
```
Previously the expression id would not reduce. This has been changed so that it now reduces to λ x → x. Usually this makes little difference, but it can be important in conjunction with with. See Issue #365 for an example.
Unused AgdaLight legacy syntax (x y : A; z v : B) for telescopes has been removed.

Universe polymorphism

Universe polymorphism is now enabled by default. Use --no-universe-polymorphism to disable it.

Universe levels are no longer defined as a data type.

The basic level combinators can be introduced in the following way:

postulate
  Level : Set
  zero  : Level
  suc   : Level → Level
  max   : Level → Level → Level

{-# BUILTIN LEVEL     Level #-}
{-# BUILTIN LEVELZERO zero  #-}
{-# BUILTIN LEVELSUC  suc   #-}
{-# BUILTIN LEVELMAX  max   #-}

The BUILTIN equality is now required to be universe-polymorphic.
trustMe is now universe-polymorphic.

Meta-variables and unification

Unsolved meta-variables are now frozen after every mutual block. This means that they cannot be instantiated by subsequent code. For instance,
```
one : Nat
one = _

bla : one ≡ suc zero
bla = refl
```
leads to an error now, whereas previously it lead to the instantiation of _ with suc zero. If you want to make use of the old behaviour, put the two definitions in a mutual block.

All meta-variables are unfrozen during interactive editing, so that the user can fill holes interactively. Note that type-checking of interactively given terms is not perfect: Agda sometimes refuses to load a file, even though no complaints were raised during the interactive construction of the file. This is because certain checks (for instance, positivity) are only invoked when a file is loaded.
Record types can now be inferred.

If there is a unique known record type with fields matching the fields in a record expression, then the type of the expression will be inferred to be the record type applied to unknown parameters.

If there is no known record type with the given fields the type checker will give an error instead of producing lots of unsolved meta-variables.

Note that “known record type” refers to any record type in any imported module, not just types which are in scope.
The occurrence checker distinguishes rigid and strongly rigid occurrences [Reed, LFMTP 2009; Abel & Pientka, TLCA 2011].

The completeness checker now accepts the following code:
```
h : (n : Nat) → n ≡ suc n → Nat
h n ()
```
Internally this generates a constraint _n = suc _n where the meta-variable _n occurs strongly rigidly, i.e. on a constructor path from the root, in its own defining term tree. This is never solvable.

Weakly rigid recursive occurrences may have a solution [Jason Reed’s PhD thesis, page 106]:
```
test : (k : Nat) →
       let X : (Nat → Nat) → Nat
           X = _
       in
       (f : Nat → Nat) → X f ≡ suc (f (X (λ x → k)))
test k f = refl
```
The constraint _X k f = suc (f (_X k (λ x → k))) has the solution _X k f = suc (f (suc k)), despite the recursive occurrence of _X. Here _X is not strongly rigid, because it occurs under the bound variable f. Previously Agda rejected this code; now it instead complains about an unsolved meta-variable.
Equation constraints involving the same meta-variable in the head now trigger pruning [Pientka, PhD, Sec. 3.1.2; Abel & Pientka, TLCA 2011]. Example:
```
same : let X : A → A → A → A × A
           X = _
       in {x y z : A} → X x y y ≡ (x , y)
                      × X x x y ≡ X x y y
same = refl , refl
```
The second equation implies that X cannot depend on its second argument. After pruning the first equation is linear and can be solved.
Instance arguments.

A new type of hidden function arguments has been added: instance arguments. This new feature is based on influences from Scala’s implicits and Agda’s existing implicit arguments.

Plain implicit arguments are marked by single braces: {…}. Instance arguments are instead marked by double braces: {{…}}. Example:
```
postulate
  A : Set
  B : A → Set
  a : A
  f : {{a : A}} → B a
```
Instead of the double braces you can use the symbols ⦃ and ⦄, but these symbols must in many cases be surrounded by whitespace. (If you are using Emacs and the Agda input method, then you can conjure up the symbols by typing \{{ and \}}, respectively.)

Instance arguments behave as ordinary implicit arguments, except for one important aspect: resolution of arguments which are not provided explicitly. For instance, consider the following code:
```
  test = f
```
Here Agda will notice that f’s instance argument was not provided explicitly, and try to infer it. All definitions in scope at f’s call site, as well as all variables in the context, are considered. If exactly one of these names has the required type A, then the instance argument will be instantiated to this name.

This feature can be used as an alternative to Haskell type classes. If we define
```
record Eq (A : Set) : Set where
  field equal : A → A → Bool,
```
then we can define the following projection:
```
equal : {A : Set} {{eq : Eq A}} → A → A → Bool
equal {{eq}} = Eq.equal eq
```
Now consider the following expression:
```
equal false false ∨ equal 3 4
```
If the following Eq “instances” for Bool and ℕ are in scope, and no others, then the expression is accepted:
```
eq-Bool : Eq Bool
eq-Bool = record { equal = … }

eq-ℕ : Eq ℕ
eq-ℕ = record { equal = … }
```
A shorthand notation is provided to avoid the need to define projection functions manually:
```
module Eq-with-implicits = Eq {{...}}
```
This notation creates a variant of Eq’s record module, where the main Eq argument is an instance argument instead of an explicit one. It is equivalent to the following definition:
```
module Eq-with-implicits {A : Set} {{eq : Eq A}} = Eq eq
```
Note that the short-hand notation allows you to avoid naming the “-with-implicits” module:
```
open Eq {{...}}
```
Instance argument resolution is not recursive. As an example, consider the following “parametrised instance”:
```
eq-List : {A : Set} → Eq A → Eq (List A)
eq-List {A} eq = record { equal = eq-List-A }
  where
  eq-List-A : List A → List A → Bool
  eq-List-A []       []       = true
  eq-List-A (a ∷ as) (b ∷ bs) = equal a b ∧ eq-List-A as bs
  eq-List-A _        _        = false
```
Assume that the only Eq instances in scope are eq-List and eq-ℕ. Then the following code does not type-check:
```
test = equal (1 ∷ 2 ∷ []) (3 ∷ 4 ∷ [])
```
However, we can make the code work by constructing a suitable instance manually:
```
test′ = equal (1 ∷ 2 ∷ []) (3 ∷ 4 ∷ [])
  where eq-List-ℕ = eq-List eq-ℕ
```
By restricting the “instance search” to be non-recursive we avoid introducing a new, compile-time-only evaluation model to Agda.

For more information about instance arguments, see Devriese & Piessens [ICFP 2011]. Some examples are also available in the examples/instance-arguments subdirectory of the Agda distribution.

Irrelevance

Dependent irrelevant function types.

Some examples illustrating the syntax of dependent irrelevant function types:

.(x y : A) → B    .{x y z : A} → B
∀ x .y → B        ∀ x .{y} {z} .v → B

The declaration

f : .(x : A) → B[x]
f x = t[x]

requires that x is irrelevant both in t[x] and in B[x]. This is possible if, for instance, B[x] = B′ x, with B′ : .A → Set.

Dependent irrelevance allows us to define the eliminator for the Squash type:

record Squash (A : Set) : Set where
  constructor squash
  field
    .proof : A

elim-Squash : {A : Set} (P : Squash A → Set)
              (ih : .(a : A) → P (squash a)) →
              (a⁻ : Squash A) → P a⁻
elim-Squash P ih (squash a) = ih a

Note that this would not type-check with

(ih : (a : A) -> P (squash a)).

Records with only irrelevant fields.

The following now works:

record IsEquivalence {A : Set} (_≈_ : A → A → Set) : Set where
  field
    .refl  : Reflexive _≈_
    .sym   : Symmetric _≈_
    .trans : Transitive _≈_

record Setoid : Set₁ where
  infix 4 _≈_
  field
    Carrier        : Set
    _≈_            : Carrier → Carrier → Set
    .isEquivalence : IsEquivalence _≈_

  open IsEquivalence isEquivalence public

Previously Agda complained about the application IsEquivalence isEquivalence, because isEquivalence is irrelevant and the IsEquivalence module expected a relevant argument. Now, when record modules are generated for records consisting solely of irrelevant arguments, the record parameter is made irrelevant:

module IsEquivalence {A : Set} {_≈_ : A → A → Set}
                    .(r : IsEquivalence {A = A} _≈_) where
 …

Irrelevant things are no longer erased internally. This means that they are printed as ordinary terms, not as _ as before.
The new flag --experimental-irrelevance enables irrelevant universe levels and matching on irrelevant data when only one constructor is available. These features are very experimental and likely to change or disappear.

Reflection

The reflection API has been extended to mirror features like irrelevance, instance arguments and universe polymorphism, and to give (limited) access to definitions. For completeness all the builtins and primitives are listed below:

-- Names.

postulate Name : Set

{-# BUILTIN QNAME Name #-}

primitive
  -- Equality of names.
  primQNameEquality : Name → Name → Bool

-- Is the argument visible (explicit), hidden (implicit), or an
-- instance argument?

data Visibility : Set where
  visible hidden instance : Visibility

{-# BUILTIN HIDING   Visibility #-}
{-# BUILTIN VISIBLE  visible    #-}
{-# BUILTIN HIDDEN   hidden     #-}
{-# BUILTIN INSTANCE instance   #-}

-- Arguments can be relevant or irrelevant.

data Relevance : Set where
  relevant irrelevant : Relevance

{-# BUILTIN RELEVANCE  Relevance  #-}
{-# BUILTIN RELEVANT   relevant   #-}
{-# BUILTIN IRRELEVANT irrelevant #-}

-- Arguments.

data Arg A : Set where
  arg : (v : Visibility) (r : Relevance) (x : A) → Arg A

{-# BUILTIN ARG    Arg #-}
{-# BUILTIN ARGARG arg #-}

-- Terms.

mutual
  data Term : Set where
    -- Variable applied to arguments.
    var     : (x : ℕ) (args : List (Arg Term)) → Term
    -- Constructor applied to arguments.
    con     : (c : Name) (args : List (Arg Term)) → Term
    -- Identifier applied to arguments.
    def     : (f : Name) (args : List (Arg Term)) → Term
    -- Different kinds of λ-abstraction.
    lam     : (v : Visibility) (t : Term) → Term
    -- Pi-type.
    pi      : (t₁ : Arg Type) (t₂ : Type) → Term
    -- A sort.
    sort    : Sort → Term
    -- Anything else.
    unknown : Term

  data Type : Set where
    el : (s : Sort) (t : Term) → Type

  data Sort : Set where
    -- A Set of a given (possibly neutral) level.
    set     : (t : Term) → Sort
    -- A Set of a given concrete level.
    lit     : (n : ℕ) → Sort
    -- Anything else.
    unknown : Sort

{-# BUILTIN AGDASORT            Sort    #-}
{-# BUILTIN AGDATYPE            Type    #-}
{-# BUILTIN AGDATERM            Term    #-}
{-# BUILTIN AGDATERMVAR         var     #-}
{-# BUILTIN AGDATERMCON         con     #-}
{-# BUILTIN AGDATERMDEF         def     #-}
{-# BUILTIN AGDATERMLAM         lam     #-}
{-# BUILTIN AGDATERMPI          pi      #-}
{-# BUILTIN AGDATERMSORT        sort    #-}
{-# BUILTIN AGDATERMUNSUPPORTED unknown #-}
{-# BUILTIN AGDATYPEEL          el      #-}
{-# BUILTIN AGDASORTSET         set     #-}
{-# BUILTIN AGDASORTLIT         lit     #-}
{-# BUILTIN AGDASORTUNSUPPORTED unknown #-}

postulate
  -- Function definition.
  Function  : Set
  -- Data type definition.
  Data-type : Set
  -- Record type definition.
  Record    : Set

{-# BUILTIN AGDAFUNDEF    Function  #-}
{-# BUILTIN AGDADATADEF   Data-type #-}
{-# BUILTIN AGDARECORDDEF Record    #-}

-- Definitions.

data Definition : Set where
  function     : Function  → Definition
  data-type    : Data-type → Definition
  record′      : Record    → Definition
  constructor′ : Definition
  axiom        : Definition
  primitive′   : Definition

{-# BUILTIN AGDADEFINITION                Definition   #-}
{-# BUILTIN AGDADEFINITIONFUNDEF          function     #-}
{-# BUILTIN AGDADEFINITIONDATADEF         data-type    #-}
{-# BUILTIN AGDADEFINITIONRECORDDEF       record′      #-}
{-# BUILTIN AGDADEFINITIONDATACONSTRUCTOR constructor′ #-}
{-# BUILTIN AGDADEFINITIONPOSTULATE       axiom        #-}
{-# BUILTIN AGDADEFINITIONPRIMITIVE       primitive′   #-}

primitive
  -- The type of the thing with the given name.
  primQNameType        : Name → Type
  -- The definition of the thing with the given name.
  primQNameDefinition  : Name → Definition
  -- The constructors of the given data type.
  primDataConstructors : Data-type → List Name

As an example the expression

primQNameType (quote zero)

is definitionally equal to

el (lit 0) (def (quote ℕ) [])

(if zero is a constructor of the data type ℕ).

New keyword: unquote.

The construction unquote t converts a representation of an Agda term to actual Agda code in the following way:
1. The argument t must have type Term (see the reflection API above).
2. The argument is normalised.
3. The entire construction is replaced by the normal form, which is treated as syntax written by the user and type-checked in the usual way.
Examples:
```
test : unquote (def (quote ℕ) []) ≡ ℕ
test = refl

id : (A : Set) → A → A
id = unquote (lam visible (lam visible (var 0 [])))

id-ok : id ≡ (λ A (x : A) → x)
id-ok = refl
```
New keyword: quoteTerm.

The construction quoteTerm t is similar to quote n, but whereas quote is restricted to names n, quoteTerm accepts terms t. The construction is handled in the following way:
1. The type of t is inferred. The term t must be type-correct.
2. The term t is normalised.
3. The construction is replaced by the Term representation (see the reflection API above) of the normal form. Any unsolved metavariables in the term are represented by the unknown term constructor.
Examples:
```
test₁ : quoteTerm (λ {A : Set} (x : A) → x) ≡
        lam hidden (lam visible (var 0 []))
test₁ = refl

-- Local variables are represented as de Bruijn indices.
test₂ : (λ {A : Set} (x : A) → quoteTerm x) ≡ (λ x → var 0 [])
test₂ = refl

-- Terms are normalised before being quoted.
test₃ : quoteTerm (0 + 0) ≡ con (quote zero) []
test₃ = refl
```

Compiler backends

MAlonzo

The MAlonzo backend’s FFI now handles universe polymorphism in a better way.

The translation of Agda types and kinds into Haskell now supports universe-polymorphic postulates. The core changes are that the translation of function types has been changed from

T[[ Pi (x : A) B ]] =
  if A has a Haskell kind then
    forall x. () -> T[[ B ]]
  else if x in fv B then
    undef
  else
    T[[ A ]] -> T[[ B ]]

into

T[[ Pi (x : A) B ]] =
  if x in fv B then
    forall x. T[[ A ]] -> T[[ B ]]  -- Note: T[[A]] not Unit.
  else
    T[[ A ]] -> T[[ B ]],

and that the translation of constants (postulates, constructors and literals) has been changed from

T[[ k As ]] =
  if COMPILED_TYPE k T then
    T T[[ As ]]
  else
    undef

into

T[[ k As ]] =
  if COMPILED_TYPE k T then
    T T[[ As ]]
  else if COMPILED k E then
    ()
  else
    undef.

For instance, assuming a Haskell definition

  type AgdaIO a b = IO b,

we can set up universe-polymorphic IO in the following way:

postulate
  IO     : ∀ {ℓ} → Set ℓ → Set ℓ
  return : ∀ {a} {A : Set a} → A → IO A
  _>>=_  : ∀ {a b} {A : Set a} {B : Set b} →
           IO A → (A → IO B) → IO B

{-# COMPILED_TYPE IO AgdaIO              #-}
{-# COMPILED return  (\_ _ -> return)    #-}
{-# COMPILED _>>=_   (\_ _ _ _ -> (>>=)) #-}

This is accepted because (assuming that the universe level type is translated to the Haskell unit type ())

(\_ _ -> return)
  : forall a. () -> forall b. () -> b -> AgdaIO a b
  = T [[ ∀ {a} {A : Set a} → A → IO A ]]

and

(\_ _ _ _ -> (>>=))
  : forall a. () -> forall b. () ->
      forall c. () -> forall d. () ->
        AgdaIO a c -> (c -> AgdaIO b d) -> AgdaIO b d
  = T [[ ∀ {a b} {A : Set a} {B : Set b} →
           IO A → (A → IO B) → IO B ]].

Epic

New Epic backend pragma: STATIC.

In the Epic backend, functions marked with the STATIC pragma will be normalised before compilation. Example usage:
```
{-# STATIC power #-}

power : ℕ → ℕ → ℕ
power 0       x = 1
power 1       x = x
power (suc n) x = power n x * x
```
Occurrences of power 4 x will be replaced by ((x * x) * x) * x.
Some new optimisations have been implemented in the Epic backend:
- Removal of unused arguments.
A worker/wrapper transformation is performed so that unused arguments can be removed by Epic’s inliner. For instance, the map function is transformed in the following way:
```
map_wrap : (A B : Set) → (A → B) → List A → List B
map_wrap A B f xs = map_work f xs

map_work f []       = []
map_work f (x ∷ xs) = f x ∷ map_work f xs
```
If map_wrap is inlined (which it will be in any saturated call), then A and B disappear in the generated code.

Unused arguments are found using abstract interpretation. The bodies of all functions in a module are inspected to decide which variables are used. The behaviour of postulates is approximated based on their types. Consider return, for instance:
```
postulate return : {A : Set} → A → IO A
```
The first argument of return can be removed, because it is of type Set and thus cannot affect the outcome of a program at runtime.
- Injection detection.
At runtime many functions may turn out to be inefficient variants of the identity function. This is especially true after forcing. Injection detection replaces some of these functions with more efficient versions. Example:
```
inject : {n : ℕ} → Fin n → Fin (1 + n)
inject {suc n} zero    = zero
inject {suc n} (suc i) = suc (inject {n} i)
```
Forcing removes the Fin constructors’ ℕ arguments, so this function is an inefficient identity function that can be replaced by the following one:
```
inject {_} x = x
```
To actually find this function, we make the induction hypothesis that inject is an identity function in its second argument and look at the branches of the function to decide if this holds.

Injection detection also works over data type barriers. Example:
```
forget : {A : Set} {n : ℕ} → Vec A n → List A
forget []       = []
forget (x ∷ xs) = x ∷ forget xs
```
Given that the constructor tags (in the compiled Epic code) for Vec.[] and List.[] are the same, and that the tags for Vec._∷_ and List._∷_ are also the same, this is also an identity function. We can hence replace the definition with the following one:
```
forget {_} xs = xs
```
To get this to apply as often as possible, constructor tags are chosen after injection detection has been run, in a way to make as many functions as possible injections.

Constructor tags are chosen once per source file, so it may be advantageous to define conversion functions like forget in the same module as one of the data types. For instance, if Vec.agda imports List.agda, then the forget function should be put in Vec.agda to ensure that vectors and lists get the same tags (unless some other injection function, which puts different constraints on the tags, is prioritised).
- Smashing.
This optimisation finds types whose values are inferable at runtime:
- A data type with only one constructor where all fields are inferable is itself inferable.
- Set ℓ is inferable (as it has no runtime representation).
A function returning an inferable data type can be smashed, which means that it is replaced by a function which simply returns the inferred value.

An important example of an inferable type is the usual propositional equality type (_≡_). Any function returning a propositional equality can simply return the reflexivity constructor directly without computing anything.

This optimisation makes more arguments unused. It also makes the Epic code size smaller, which in turn speeds up compilation.

JavaScript

ECMAScript compiler backend.

A new compiler backend is being implemented, targetting ECMAScript (also known as JavaScript), with the goal of allowing Agda programs to be run in browsers or other ECMAScript environments.

The backend is still at an experimental stage: the core language is implemented, but many features are still missing.

The ECMAScript compiler can be invoked from the command line using the flag --js:
```
agda --js --compile-dir=<DIR> <FILE>.agda
```
Each source <FILE>.agda is compiled into an ECMAScript target <DIR>/jAgda.<TOP-LEVEL MODULE NAME>.js. The compiler can also be invoked using the Emacs mode (the variable agda2-backend controls which backend is used).

Note that ECMAScript is a strict rather than lazy language. Since Agda programs are total, this should not impact program semantics, but it may impact their space or time usage.

ECMAScript does not support algebraic datatypes or pattern-matching. These features are translated to a use of the visitor pattern. For instance, the standard library’s List data type and null function are translated into the following code:
```
exports["List"] = {};
exports["List"]["[]"] = function (x0) {
    return x0["[]"]();
  };
exports["List"]["_∷_"] = function (x0) {
    return function (x1) {
      return function (x2) {
        return x2["_∷_"](x0, x1);
      };
    };
  };

exports["null"] = function (x0) {
    return function (x1) {
      return function (x2) {
        return x2({
          "[]": function () {
            return jAgda_Data_Bool["Bool"]["true"];
          },
          "_∷_": function (x3, x4) {
            return jAgda_Data_Bool["Bool"]["false"];
          }
        });
      };
    };
  };
```
Agda records are translated to ECMAScript objects, preserving field names.

Top-level Agda modules are translated to ECMAScript modules, following the common.js module specification. A top-level Agda module Foo.Bar is translated to an ECMAScript module jAgda.Foo.Bar.

The ECMAScript compiler does not compile to Haskell, so the pragmas related to the Haskell FFI (IMPORT, COMPILED_DATA and COMPILED) are not used by the ECMAScript backend. Instead, there is a COMPILED_JS pragma which may be applied to any declaration. For postulates, primitives, functions and values, it gives the ECMAScript code to be emitted by the compiler. For data types, it gives a function which is applied to a value of that type, and a visitor object. For instance, a binding of natural numbers to ECMAScript integers (ignoring overflow errors) is:
```
data ℕ : Set where
  zero : ℕ
  suc  : ℕ → ℕ

{-# COMPILED_JS ℕ function (x,v) {
    if (x < 1) { return v.zero(); } else { return v.suc(x-1); }
  } #-}
{-# COMPILED_JS zero 0 #-}
{-# COMPILED_JS suc function (x) { return x+1; } #-}

_+_ : ℕ → ℕ → ℕ
zero  + n = n
suc m + n = suc (m + n)

{-# COMPILED_JS _+_ function (x) { return function (y) {
                      return x+y; };
  } #-}
```
To allow FFI code to be optimised, the ECMAScript in a COMPILED_JS declaration is parsed, using a simple parser that recognises a pure functional subset of ECMAScript, consisting of functions, function applications, return, if-statements, if-expressions, side-effect-free binary operators (no precedence, left associative), side-effect-free prefix operators, objects (where all member names are quoted), field accesses, and string and integer literals. Modules may be imported using the require (<module-id>) syntax: any impure code, or code outside the supported fragment, can be placed in a module and imported.

Tools

New flag --safe, which can be used to type-check untrusted code.

This flag disables postulates, primTrustMe, and “unsafe” OPTION pragmas, some of which are known to make Agda inconsistent.

Rejected pragmas:
```
--allow-unsolved-metas
--experimental-irrelevance
--guardedness-preserving-type-construtors
--injective-type-constructors
--no-coverage-check
--no-positivity-check
--no-termination-check
--sized-types
--type-in-type
```
Note that, at the moment, it is not possible to define the universe level or coinduction primitives when --safe is used (because they must be introduced as postulates). This can be worked around by type-checking trusted files in a first pass, without using --safe, and then using --safe in a second pass. Modules which have already been type-checked are not re-type-checked just because --safe is used.
Dependency graphs.

The new flag --dependency-graph=FILE can be used to generate a DOT file containing a module dependency graph. The generated file (FILE) can be rendered using a tool like dot.
The --no-unreachable-check flag has been removed.
Projection functions are highlighted as functions instead of as fields. Field names (in record definitions and record values) are still highlighted as fields.
Support for jumping to positions mentioned in the information buffer has been added.
The make install command no longer installs Agda globally (by default).

Release notes for Agda 2 version 2.2.10

Language

New flag: --without-K.

This flag makes pattern matching more restricted. If the flag is activated, then Agda only accepts certain case-splits. If the type of the variable to be split is D pars ixs, where D is a data (or record) type, pars stands for the parameters, and ixs the indices, then the following requirements must be satisfied:
- The indices ixs must be applications of constructors to distinct variables.
- These variables must not be free in pars.
The intended purpose of --without-K is to enable experiments with a propositional equality without the K rule. Let us define propositional equality as follows:
```
data _≡_ {A : Set} : A → A → Set where
  refl : ∀ x → x ≡ x
```
Then the obvious implementation of the J rule is accepted:
```
J : {A : Set} (P : {x y : A} → x ≡ y → Set) →
    (∀ x → P (refl x)) →
    ∀ {x y} (x≡y : x ≡ y) → P x≡y
J P p (refl x) = p x
```
The same applies to Christine Paulin-Mohring’s version of the J rule:
```
J′ : {A : Set} {x : A} (P : {y : A} → x ≡ y → Set) →
     P (refl x) →
     ∀ {y} (x≡y : x ≡ y) → P x≡y
J′ P p (refl x) = p
```
On the other hand, the obvious implementation of the K rule is not accepted:
```
K : {A : Set} (P : {x : A} → x ≡ x → Set) →
    (∀ x → P (refl x)) →
    ∀ {x} (x≡x : x ≡ x) → P x≡x
K P p (refl x) = p x
```
However, we have not proved that activation of --without-K ensures that the K rule cannot be proved in some other way.
Irrelevant declarations.

Postulates and functions can be marked as irrelevant by prefixing the name with a dot when the name is declared. Example:
```
postulate
  .irrelevant : {A : Set} → .A → A
```
Irrelevant names may only be used in irrelevant positions or in definitions of things which have been declared irrelevant.

The axiom irrelevant above can be used to define a projection from an irrelevant record field:
```
data Subset (A : Set) (P : A → Set) : Set where
  _#_ : (a : A) → .(P a) → Subset A P

elem : ∀ {A P} → Subset A P → A
elem (a # p) = a

.certificate : ∀ {A P} (x : Subset A P) → P (elem x)
certificate (a # p) = irrelevant p
```
The right-hand side of certificate is relevant, so we cannot define
```
certificate (a # p) = p
```
(because p is irrelevant). However, certificate is declared to be irrelevant, so it can use the axiom irrelevant. Furthermore the first argument of the axiom is irrelevant, which means that irrelevant p is well-formed.

As shown above the axiom irrelevant justifies irrelevant projections. Previously no projections were generated for irrelevant record fields, such as the field certificate in the following record type:
```
record Subset (A : Set) (P : A → Set) : Set where
  constructor _#_
  field
    elem         : A
    .certificate : P elem
```
Now projections are generated automatically for irrelevant fields (unless the flag --no-irrelevant-projections is used). Note that irrelevant projections are highly experimental.

Termination checker recognises projections.

Projections now preserve sizes, both in patterns and expressions. Example:

record Wrap (A : Set) : Set where
  constructor wrap
  field
    unwrap : A

open Wrap public

data WNat : Set where
  zero : WNat
  suc  : Wrap WNat → WNat

id : WNat → WNat
id zero    = zero
id (suc w) = suc (wrap (id (unwrap w)))

In the structural ordering unwrap w ≤ w. This means that

  unwrap w ≤ w < suc w,

and hence the recursive call to id is accepted.

Projections also preserve guardedness.

Tools

Hyperlinks for top-level module names now point to the start of the module rather than to the declaration of the module name. This applies both to the Emacs mode and to the output of agda --html.
Most occurrences of record field names are now highlighted as “fields”. Previously many occurrences were highlighted as “functions”.
Emacs mode: It is no longer possible to change the behaviour of the TAB key by customising agda2-indentation.
Epic compiler backend.

A new compiler backend is being implemented. This backend makes use of Edwin Brady’s language Epic (http://www.cs.st-andrews.ac.uk/~eb/epic.php) and its compiler. The backend should handle most Agda code, but is still at an experimental stage: more testing is needed, and some things written below may not be entirely true.

The Epic compiler can be invoked from the command line using the flag --epic:
```
agda --epic --epic-flag=<EPIC-FLAG> --compile-dir=<DIR> <FILE>.agda
```
The --epic-flag flag can be given multiple times; each flag is given verbatim to the Epic compiler (in the given order). The resulting executable is named after the main module and placed in the directory specified by the --compile-dir flag (default: the project root). Intermediate files are placed in a subdirectory called Epic.

The backend requires that there is a definition named main. This definition should be a value of type IO Unit, but at the moment this is not checked (so it is easy to produce a program which segfaults). Currently the backend represents actions of type IO A as functions from Unit to A, and main is applied to the unit value.

The Epic compiler compiles via C, not Haskell, so the pragmas related to the Haskell FFI (IMPORT, COMPILED_DATA and COMPILED) are not used by the Epic backend. Instead there is a new pragma COMPILED_EPIC. This pragma is used to give Epic code for postulated definitions (Epic code can in turn call C code). The form of the pragma is {-# COMPILED_EPIC def code #-}, where def is the name of an Agda postulate and code is some Epic code which should include the function arguments, return type and function body. As an example the IO monad can be defined as follows:
```
postulate
  IO     : Set → Set
  return : ∀ {A} → A → IO A
  _>>=_  : ∀ {A B} → IO A → (A → IO B) → IO B

{-# COMPILED_EPIC return (u : Unit, a : Any) -> Any =
                    ioreturn(a) #-}
{-# COMPILED_EPIC
      _>>=_ (u1 : Unit, u2 : Unit, x : Any, f : Any) -> Any =
        iobind(x,f) #-}
```
Here ioreturn and iobind are Epic functions which are defined in the file AgdaPrelude.e which is always included.

By default the backend will remove so-called forced constructor arguments (and case-splitting on forced variables will be rewritten). This optimisation can be disabled by using the flag --no-forcing.

All data types which look like unary natural numbers after forced constructor arguments have been removed (i.e. types with two constructors, one nullary and one with a single recursive argument) will be represented as “BigInts”. This applies to the standard Fin type, for instance.

The backend supports Agda’s primitive functions and the BUILTIN pragmas. If the BUILTIN pragmas for unary natural numbers are used, then some operations, like addition and multiplication, will use more efficient “BigInt” operations.

If you want to make use of the Epic backend you need to install some dependencies, see the README.
The Emacs mode can compile using either the MAlonzo or the Epic backend. The variable agda2-backend controls which backend is used.

Release notes for Agda 2 version 2.2.8

Language

Record pattern matching.

It is now possible to pattern match on named record constructors. Example:

record Σ (A : Set) (B : A → Set) : Set where
  constructor _,_
  field
    proj₁ : A
    proj₂ : B proj₁

map : {A B : Set} {P : A → Set} {Q : B → Set}
      (f : A → B) → (∀ {x} → P x → Q (f x)) →
      Σ A P → Σ B Q
map f g (x , y) = (f x , g y)

The clause above is internally translated into the following one:

map f g p = (f (Σ.proj₁ p) , g (Σ.proj₂ p))

Record patterns containing data type patterns are not translated. Example:

add : ℕ × ℕ → ℕ
add (zero  , n) = n
add (suc m , n) = suc (add (m , n))

Record patterns which do not contain data type patterns, but which do contain dot patterns, are currently rejected. Example:

Foo : {A : Set} (p₁ p₂ : A × A) → proj₁ p₁ ≡ proj₁ p₂ → Set₁
Foo (x , y) (.x , y′) refl = Set

Proof irrelevant function types.

Agda now supports irrelevant non-dependent function types:
```
f : .A → B
```
This type implies that f does not depend computationally on its argument. One intended use case is data structures with embedded proofs, like sorted lists:
```
postulate
  _≤_ : ℕ → ℕ → Set
  p₁  : 0 ≤ 1
  p₂  : 0 ≤ 1

data SList (bound : ℕ) : Set where
  []    : SList bound
  scons : (head : ℕ) →
          .(head ≤ bound) →
          (tail : SList head) →
          SList bound
```
The effect of the irrelevant type in the signature of scons is that scons’s second argument is never inspected after Agda has ensured that it has the right type. It is even thrown away, leading to smaller term sizes and hopefully some gain in efficiency. The type-checker ignores irrelevant arguments when checking equality, so two lists can be equal even if they contain different proofs:
```
l₁ : SList 1
l₁ = scons 0 p₁ []

l₂ : SList 1
l₂ = scons 0 p₂ []

l₁≡l₂ : l₁ ≡ l₂
l₁≡l₂ = refl
```
Irrelevant arguments can only be used in irrelevant contexts. Consider the following subset type:
```
data Subset (A : Set) (P : A → Set) : Set where
  _#_ : (elem : A) → .(P elem) → Subset A P
```
The following two uses are fine:
```
elimSubset : ∀ {A C : Set} {P} →
             Subset A P → ((a : A) → .(P a) → C) → C
elimSubset (a # p) k = k a p

elem : {A : Set} {P : A → Set} → Subset A P → A
elem (x # p) = x
```
However, if we try to project out the proof component, then Agda complains that variable p is declared irrelevant, so it cannot be used here:
```
prjProof : ∀ {A P} (x : Subset A P) → P (elem x)
prjProof (a # p) = p
```
Matching against irrelevant arguments is also forbidden, except in the case of irrefutable matches (record constructor patterns which have been translated away). For instance, the match against the pattern (p , q) here is accepted:
```
elim₂ : ∀ {A C : Set} {P Q : A → Set} →
        Subset A (λ x → Σ (P x) (λ _ → Q x)) →
        ((a : A) → .(P a) → .(Q a) → C) → C
elim₂ (a # (p , q)) k = k a p q
```
Absurd matches () are also allowed.

Note that record fields can also be irrelevant. Example:
```
record Subset (A : Set) (P : A → Set) : Set where
  constructor _#_
  field
    elem   : A
    .proof : P elem
```
Irrelevant fields are never in scope, neither inside nor outside the record. This means that no record field can depend on an irrelevant field, and furthermore projections are not defined for such fields. Irrelevant fields can only be accessed using pattern matching, as in elimSubset above.

Irrelevant function types were added very recently, and have not been subjected to much experimentation yet, so do not be surprised if something is changed before the next release. For instance, dependent irrelevant function spaces (.(x : A) → B) might be added in the future.

Mixfix binders.

It is now possible to declare user-defined syntax that binds identifiers. Example:

postulate
  State  : Set → Set → Set
  put    : ∀ {S} → S → State S ⊤
  get    : ∀ {S} → State S S
  return : ∀ {A S} → A → State S A
  bind   : ∀ {A B S} → State S B → (B → State S A) → State S A

syntax bind e₁ (λ x → e₂) = x ← e₁ , e₂

increment : State ℕ ⊤
increment = x ← get ,
            put (1 + x)

The syntax declaration for bind implies that x is in scope in e₂, but not in e₁.

You can give fixity declarations along with syntax declarations:

infixr 40 bind
syntax bind e₁ (λ x → e₂) = x ← e₁ , e₂

The fixity applies to the syntax, not the name; syntax declarations are also restricted to ordinary, non-operator names. The following declaration is disallowed:

syntax _==_ x y = x === y
```agda

Syntax declarations must also be linear; the following declaration
is disallowed:

```agda
syntax wrong x = x + x

Syntax declarations were added very recently, and have not been subjected to much experimentation yet, so do not be surprised if something is changed before the next release.

Prop has been removed from the language.

The experimental sort Prop has been disabled. Any program using Prop should typecheck if Prop is replaced by Set₀. Note that Prop is still a keyword.
Injective type constructors off by default.

Automatic injectivity of type constructors has been disabled (by default). To enable it, use the flag --injective-type-constructors, either on the command line or in an OPTIONS pragma. Note that this flag makes Agda anti-classical and possibly inconsistent:

Agda with excluded middle is inconsistent http://thread.gmane.org/gmane.comp.lang.agda/1367

See test/Succeed/InjectiveTypeConstructors.agda for an example.
Termination checker can count.

There is a new flag --termination-depth=N accepting values N >= 1 (with N = 1 being the default) which influences the behavior of the termination checker. So far, the termination checker has only distinguished three cases when comparing the argument of a recursive call with the formal parameter of the callee.

<: the argument is structurally smaller than the parameter

=: they are equal

?: the argument is bigger or unrelated to the parameter

This behavior, which is still the default (N = 1), will not recognise the following functions as terminating.
```
mutual

   f : ℕ → ℕ
   f zero          = zero
   f (suc zero)    = zero
   f (suc (suc n)) = aux n

   aux : ℕ → ℕ
   aux m = f (suc m)
```
The call graph
```
f --(<)--> aux --(?)--> f
```
yields a recursive call from f to f via aux where the relation of call argument to callee parameter is computed as “unrelated” (composition of < and ?).

Setting N >= 2 allows a finer analysis: n has two constructors less than suc (suc n), and suc m has one more than m, so we get the call graph:
```
f --(-2)--> aux --(+1)--> f
```
The indirect call f --> f is now labeled with (-1), and the termination checker can recognise that the call argument is decreasing on this path.

Setting the termination depth to N means that the termination checker counts decrease up to N and increase up to N-1. The default, N=1, means that no increase is counted, every increase turns to “unrelated”.

In practice, examples like the one above sometimes arise when with is used. As an example, the program
```
f : ℕ → ℕ
f zero          = zero
f (suc zero)    = zero
f (suc (suc n)) with zero
... | _ = f (suc n)
```
is internally represented as
```
mutual

  f : ℕ → ℕ
  f zero          = zero
  f (suc zero)    = zero
  f (suc (suc n)) = aux n zero

  aux : ℕ → ℕ → ℕ
  aux m k = f (suc m)
```
Thus, by default, the definition of f using with is not accepted by the termination checker, even though it looks structural (suc n is a subterm of suc suc n). Now, the termination checker is satisfied if the option --termination-depth=2 is used.

Caveats:
- This is an experimental feature, hopefully being replaced by something smarter in the near future.
- Increasing the termination depth will quickly lead to very long termination checking times. So, use with care. Setting termination depth to 100 by habit, just to be on the safe side, is not a good idea!
- Increasing termination depth only makes sense for linear data types such as ℕ and Size. For other types, increase cannot be recognised. For instance, consider a similar example with lists.
```
data List : Set where
  nil  : List
  cons : ℕ → List → List

mutual
  f : List → List
  f nil                  = nil
  f (cons x nil)         = nil
  f (cons x (cons y ys)) = aux y ys

  aux : ℕ → List → List
  aux z zs = f (cons z zs)
```
  Here the termination checker compares cons z zs to z and also to zs. In both cases, the result will be “unrelated”, no matter how high we set the termination depth. This is because when comparing cons z zs to zs, for instance, z is unrelated to zs, thus, cons z zs is also unrelated to zs. We cannot say it is just “one larger” since z could be a very large term. Note that this points to a weakness of untyped termination checking.
  
  To regain the benefit of increased termination depth, we need to index our lists by a linear type such as ℕ or Size. With termination depth 2, the above example is accepted for vectors instead of lists.
The codata keyword has been removed. To use coinduction, use the following new builtins: INFINITY, SHARP and FLAT. Example:
```
{-# OPTIONS --universe-polymorphism #-}

module Coinduction where

open import Level

infix 1000 ♯_

postulate
  ∞  : ∀ {a} (A : Set a) → Set a
  ♯_ : ∀ {a} {A : Set a} → A → ∞ A
  ♭  : ∀ {a} {A : Set a} → ∞ A → A

{-# BUILTIN INFINITY ∞  #-}
{-# BUILTIN SHARP    ♯_ #-}
{-# BUILTIN FLAT     ♭  #-}
```
Note that (non-dependent) pattern matching on SHARP is no longer allowed.

Note also that strange things might happen if you try to combine the pragmas above with COMPILED_TYPE, COMPILED_DATA or COMPILED pragmas, or if the pragmas do not occur right after the postulates.

The compiler compiles the INFINITY builtin to nothing (more or less), so that the use of coinduction does not get in the way of FFI declarations:
```
data Colist (A : Set) : Set where
  []  : Colist A
  _∷_ : (x : A) (xs : ∞ (Colist A)) → Colist A

{-# COMPILED_DATA Colist [] [] (:) #-}
```

Infinite types.

If the new flag --guardedness-preserving-type-constructors is used, then type constructors are treated as inductive constructors when we check productivity (but only in parameters, and only if they are used strictly positively or not at all). This makes examples such as the following possible:

data Rec (A : ∞ Set) : Set where
  fold : ♭ A → Rec A

-- Σ cannot be a record type below.

data Σ (A : Set) (B : A → Set) : Set where
  _,_ : (x : A) → B x → Σ A B

syntax Σ A (λ x → B) = Σ[ x ∶ A ] B

-- Corecursive definition of the W-type.

W : (A : Set) → (A → Set) → Set
W A B = Rec (♯ (Σ[ x ∶ A ] (B x → W A B)))

syntax W A (λ x → B) = W[ x ∶ A ] B

sup : {A : Set} {B : A → Set} (x : A) (f : B x → W A B) → W A B
sup x f = fold (x , f)

W-rec : {A : Set} {B : A → Set}
        (P : W A B → Set) →
        (∀ {x} {f : B x → W A B} → (∀ y → P (f y)) → P (sup x f)) →
        ∀ x → P x
W-rec P h (fold (x , f)) = h (λ y → W-rec P h (f y))

-- Induction-recursion encoded as corecursion-recursion.

data Label : Set where
  ′0 ′1 ′2 ′σ ′π ′w : Label

mutual

  U : Set
  U = Σ Label U′

  U′ : Label → Set
  U′ ′0 = ⊤
  U′ ′1 = ⊤
  U′ ′2 = ⊤
  U′ ′σ = Rec (♯ (Σ[ a ∶ U ] (El a → U)))
  U′ ′π = Rec (♯ (Σ[ a ∶ U ] (El a → U)))
  U′ ′w = Rec (♯ (Σ[ a ∶ U ] (El a → U)))

  El : U → Set
  El (′0 , _)            = ⊥
  El (′1 , _)            = ⊤
  El (′2 , _)            = Bool
  El (′σ , fold (a , b)) = Σ[ x ∶ El a ]  El (b x)
  El (′π , fold (a , b)) =   (x : El a) → El (b x)
  El (′w , fold (a , b)) = W[ x ∶ El a ]  El (b x)

U-rec : (P : ∀ u → El u → Set) →
        P (′1 , _) tt →
        P (′2 , _) true →
        P (′2 , _) false →
        (∀ {a b x y} →
         P a x → P (b x) y → P (′σ , fold (a , b)) (x , y)) →
        (∀ {a b f} →
         (∀ x → P (b x) (f x)) → P (′π , fold (a , b)) f) →
        (∀ {a b x f} →
         (∀ y → P (′w , fold (a , b)) (f y)) →
         P (′w , fold (a , b)) (sup x f)) →
        ∀ u (x : El u) → P u x
U-rec P P1 P2t P2f Pσ Pπ Pw = rec
  where
  rec : ∀ u (x : El u) → P u x
  rec (′0 , _)            ()
  rec (′1 , _)            _              = P1
  rec (′2 , _)            true           = P2t
  rec (′2 , _)            false          = P2f
  rec (′σ , fold (a , b)) (x , y)        = Pσ (rec _ x) (rec _ y)
  rec (′π , fold (a , b)) f              = Pπ (λ x → rec _ (f x))
  rec (′w , fold (a , b)) (fold (x , f)) = Pw (λ y → rec _ (f y))

The --guardedness-preserving-type-constructors extension is based on a rather operational understanding of ∞/♯_; it’s not yet clear if this extension is consistent.

Qualified constructors.

Constructors can now be referred to qualified by their data type. For instance, given
```
data Nat : Set where
  zero : Nat
  suc  : Nat → Nat

data Fin : Nat → Set where
  zero : ∀ {n} → Fin (suc n)
  suc  : ∀ {n} → Fin n → Fin (suc n)
```
you can refer to the constructors unambiguously as Nat.zero, Nat.suc, Fin.zero, and Fin.suc (Nat and Fin are modules containing the respective constructors). Example:
```
inj : (n m : Nat) → Nat.suc n ≡ suc m → n ≡ m
inj .m m refl = refl
```
Previously you had to write something like
```
inj : (n m : Nat) → _≡_ {Nat} (suc n) (suc m) → n ≡ m
```
to make the type checker able to figure out that you wanted the natural number suc in this case.

Reflection.

There are two new constructs for reflection:

quoteGoal x in e

In e the value of x will be a representation of the goal type (the type expected of the whole expression) as an element in a datatype of Agda terms (see below). For instance,
```
example : ℕ
example = quoteGoal x in {! at this point x = def (quote ℕ) [] !}
```
quote x : Name

If x is the name of a definition (function, datatype, record, or a constructor), quote x gives you the representation of x as a value in the primitive type Name (see below).

Quoted terms use the following BUILTINs and primitives (available from the standard library module Reflection):

-- The type of Agda names.

postulate Name : Set

{-# BUILTIN QNAME Name #-}

primitive primQNameEquality : Name → Name → Bool

-- Arguments.

Explicit? = Bool

data Arg A : Set where
  arg : Explicit? → A → Arg A

{-# BUILTIN ARG    Arg #-}
{-# BUILTIN ARGARG arg #-}

-- The type of Agda terms.

data Term : Set where
  var     : ℕ → List (Arg Term) → Term
  con     : Name → List (Arg Term) → Term
  def     : Name → List (Arg Term) → Term
  lam     : Explicit? → Term → Term
  pi      : Arg Term → Term → Term
  sort    : Term
  unknown : Term

{-# BUILTIN AGDATERM            Term    #-}
{-# BUILTIN AGDATERMVAR         var     #-}
{-# BUILTIN AGDATERMCON         con     #-}
{-# BUILTIN AGDATERMDEF         def     #-}
{-# BUILTIN AGDATERMLAM         lam     #-}
{-# BUILTIN AGDATERMPI          pi      #-}
{-# BUILTIN AGDATERMSORT        sort    #-}
{-# BUILTIN AGDATERMUNSUPPORTED unknown #-}

Reflection may be useful when working with internal decision procedures, such as the standard library’s ring solver.

Minor record definition improvement.

The definition of a record type is now available when type checking record module definitions. This means that you can define things like the following:
```
record Cat : Set₁ where
  field
    Obj  : Set
    _=>_ : Obj → Obj → Set
    -- ...

  -- not possible before:
  op : Cat
  op = record { Obj = Obj; _=>_ = λ A B → B => A }
```

Tools

The Goal type and context command now shows the goal type before the context, and the context is shown in reverse order. The Goal type, context and inferred type command has been modified in a similar way.
Show module contents command.

Given a module name M the Emacs mode can now display all the top-level modules and names inside M, along with types for the names. The command is activated using C-c C-o or the menus.
Auto command.

A command which searches for type inhabitants has been added. The command is invoked by pressing C-C C-a (or using the goal menu). There are several flags and parameters, e.g. -c which enables case-splitting in the search. For further information, see the Agda wiki:

http://wiki.portal.chalmers.se/agda/pmwiki.php?n=Main.Auto
HTML generation is now possible for a module with unsolved meta-variables, provided that the --allow-unsolved-metas flag is used.

Release notes for Agda 2 version 2.2.6

Language

Universe polymorphism (experimental extension).

To enable universe polymorphism give the flag --universe-polymorphism on the command line or (recommended) as an OPTIONS pragma.

When universe polymorphism is enabled Set takes an argument which is the universe level. For instance, the type of universe polymorphic identity is
```
id : {a : Level} {A : Set a} → A → A.
```
The type Level is isomorphic to the unary natural numbers and should be specified using the BUILTINs LEVEL, LEVELZERO, and LEVELSUC:
```
data Level : Set where
  zero : Level
  suc  : Level → Level

{-# BUILTIN LEVEL     Level #-}
{-# BUILTIN LEVELZERO zero  #-}
{-# BUILTIN LEVELSUC  suc   #-}
```
There is an additional BUILTIN LEVELMAX for taking the maximum of two levels:
```
max : Level → Level → Level
max  zero    m      = m
max (suc n)  zero   = suc n
max (suc n) (suc m) = suc (max n m)

{-# BUILTIN LEVELMAX max #-}
```
The non-polymorphic universe levels Set, Set₁ and so on are sugar for Set zero, Set (suc zero), etc.

At present there is no automatic lifting of types from one level to another. It can still be done (rather clumsily) by defining types like the following one:
```
data Lifted {a} (A : Set a) : Set (suc a) where
  lift : A → Lifted A
```
However, it is likely that automatic lifting is introduced at some point in the future.
Multiple constructors, record fields, postulates or primitives can be declared using a single type signature:
```
data Bool : Set where
  false true : Bool

postulate
  A B : Set
```

Record fields can be implicit:

record R : Set₁ where
  field
    {A}         : Set
    f           : A → A
    {B C} D {E} : Set
    g           : B → C → E

By default implicit fields are not printed.

Record constructors can be defined:
```
record Σ (A : Set) (B : A → Set) : Set where
  constructor _,_
  field
    proj₁ : A
    proj₂ : B proj₁
```
In this example _,_ gets the type
```
 (proj₁ : A) → B proj₁ → Σ A B.
```
For implicit fields the corresponding constructor arguments become implicit.

Note that the constructor is defined in the outer scope, so any fixity declaration has to be given outside the record definition. The constructor is not in scope inside the record module.

Note also that pattern matching for records has not been implemented yet.
BUILTIN hooks for equality.

The data type
```
data _≡_ {A : Set} (x : A) : A → Set where
  refl : x ≡ x
```
can be specified as the builtin equality type using the following pragmas:
```
{-# BUILTIN EQUALITY _≡_  #-}
{-# BUILTIN REFL     refl #-}
```
The builtin equality is used for the new rewrite construct and the primTrustMe primitive described below.
New rewrite construct.

If eqn : a ≡ b, where _≡_ is the builtin equality (see above) you can now write
```
f ps rewrite eqn = rhs
```
instead of
```
  f ps with a | eqn
  ... | ._ | refl = rhs
```
The rewrite construct has the effect of rewriting the goal and the context by the given equation (left to right).

You can rewrite using several equations (in sequence) by separating them with vertical bars (|):
```
f ps rewrite eqn₁ | eqn₂ | … = rhs
```
It is also possible to add with-clauses after rewriting:
```
f ps rewrite eqns with e
... | p = rhs
```
Note that pattern matching happens before rewriting—if you want to rewrite and then do pattern matching you can use a with after the rewrite.

See test/Succeed/Rewrite.agda for some examples.
A new primitive, primTrustMe, has been added:
```
  primTrustMe : {A : Set} {x y : A} → x ≡ y
```
Here _≡_ is the builtin equality (see BUILTIN hooks for equality, above).

If x and y are definitionally equal, then primTrustMe {x = x} {y = y} reduces to refl.

Note that the compiler replaces all uses of primTrustMe with the REFL builtin, without any check for definitional equality. Incorrect uses of primTrustMe can potentially lead to segfaults or similar problems.

For an example of the use of primTrustMe, see Data.String in version 0.3 of the standard library, where it is used to implement decidable equality on strings using the primitive boolean equality.
Changes to the syntax and semantics of IMPORT pragmas, which are used by the Haskell FFI. Such pragmas must now have the following form:
```
{-# IMPORT <module name> #-}
```
These pragmas are interpreted as qualified imports, so Haskell names need to be given qualified (unless they come from the Haskell prelude).
The horizontal tab character (U+0009) is no longer treated as white space.
Line pragmas are no longer supported.
The --include-path flag can no longer be used as a pragma.
The experimental and incomplete support for proof irrelevance has been disabled.

Tools

New intro command in the Emacs mode. When there is a canonical way of building something of the goal type (for instance, if the goal type is a pair), the goal can be refined in this way. The command works for the following goal types:
- A data type where only one of its constructors can be used to construct an element of the goal type. (For instance, if the goal is a non-empty vector, a cons will be introduced.)
- A record type. A record value will be introduced. Implicit fields will not be included unless showing of implicit arguments is switched on.
- A function type. A lambda binding as many variables as possible will be introduced. The variable names will be chosen from the goal type if its normal form is a dependent function type, otherwise they will be variations on x. Implicit lambdas will only be inserted if showing of implicit arguments is switched on.
This command can be invoked by using the refine command (C-c C-r) when the goal is empty. (The old behaviour of the refine command in this situation was to ask for an expression using the minibuffer.)
The Emacs mode displays Checked in the mode line if the current file type checked successfully without any warnings.
If a file F is loaded, and this file defines the module M, it is an error if F is not the file which defines M according to the include path.

Note that the command-line tool and the Emacs mode define the meaning of relative include paths differently: the command-line tool interprets them relative to the current working directory, whereas the Emacs mode interprets them relative to the root directory of the current project. (As an example, if the module A.B.C is loaded from the file <some-path>/A/B/C.agda, then the root directory is <some-path>.)
It is an error if there are several files on the include path which match a given module name.
Interface files are relocatable. You can move around source trees as long as the include path is updated in a corresponding way. Note that a module M may be re-typechecked if its time stamp is strictly newer than that of the corresponding interface file (M.agdai).
Type-checking is no longer done when an up-to-date interface exists. (Previously the initial module was always type-checked.)
Syntax highlighting files for Emacs (.agda.el) are no longer used. The --emacs flag has been removed. (Syntax highlighting information is cached in the interface files.)
The Agate and Alonzo compilers have been retired. The options --agate, --alonzo and --malonzo have been removed.
The default directory for MAlonzo output is the project’s root directory. The --malonzo-dir flag has been renamed to --compile-dir.
Emacs mode: C-c C-x C-d no longer resets the type checking state. C-c C-x C-r can be used for a more complete reset. C-c C-x C-s (which used to reload the syntax highlighting information) has been removed. C-c C-l can be used instead.
The Emacs mode used to define some “abbrevs”, unless the user explicitly turned this feature off. The new default is not to add any abbrevs. The old default can be obtained by customising agda2-mode-abbrevs-use-defaults (a customisation buffer can be obtained by typing M-x customize-group agda2 RET after an Agda file has been loaded).

Release notes for Agda 2 version 2.2.4

Important changes since 2.2.2:

Change to the semantics of open import and open module. The declaration
```
open import M <using/hiding/renaming>
```
now translates to
```
import A
open A <using/hiding/renaming>
```
instead of
```
import A <using/hiding/renaming>
open A
```
The same translation is used for open module M = E …. Declarations involving the keywords as or public are changed in a corresponding way (as always goes with import, and public always with open).

This change means that import directives do not affect the qualified names when open import/module is used. To get the old behaviour you can use the expanded version above.
Names opened publicly in parameterised modules no longer inherit the module parameters. Example:
```
module A where
  postulate X : Set

module B (Y : Set) where
  open A public
```
In Agda 2.2.2 B.X has type (Y : Set) → Set, whereas in Agda 2.2.4 B.X has type Set.
Previously it was not possible to export a given constructor name through two different open public statements in the same module. This is now possible.
Unicode subscript digits are now allowed for the hierarchy of universes (Set₀, Set₁, …): Set₁ is equivalent to Set1.

Release notes for Agda 2 version 2.2.2

Tools

The --malonzodir option has been renamed to --malonzo-dir.
The output of agda --html is by default placed in a directory called html.

Infrastructure

The Emacs mode is included in the Agda Cabal package, and installed by cabal install. The recommended way to enable the Emacs mode is to include the following code in .emacs:
```
(load-file (let ((coding-system-for-read 'utf-8))
                (shell-command-to-string "agda-mode locate")))
```

Release notes for Agda 2 version 2.2.0

Important changes since 2.1.2 (which was released 2007-08-16):

Language

Exhaustive pattern checking. Agda complains if there are missing clauses in a function definition.
Coinductive types are supported. This feature is under development/evaluation, and may change.

http://wiki.portal.chalmers.se/agda/agda.php?n=ReferenceManual.Codatatypes
Another experimental feature: Sized types, which can make it easier to explain why your code is terminating.
Improved constraint solving for functions with constructor headed right hand sides.

http://wiki.portal.chalmers.se/agda/agda.php?n=ReferenceManual.FindingTheValuesOfImplicitArguments
A simple, well-typed foreign function interface, which allows use of Haskell functions in Agda code.

http://wiki.portal.chalmers.se/agda/pmwiki.php?n=Docs.FFI
The tokens forall, -> and \ can be written as ∀, → and λ.
Absurd lambdas: λ () and λ {}.

http://thread.gmane.org/gmane.comp.lang.agda/440
Record fields whose values can be inferred can be omitted.
Agda complains if it spots an unreachable clause, or if a pattern variable “shadows” a hidden constructor of matching type.

http://thread.gmane.org/gmane.comp.lang.agda/720

Tools

Case-split: The user interface can replace a pattern variable with the corresponding constructor patterns. You get one new left-hand side for every possible constructor.

http://wiki.portal.chalmers.se/agda/pmwiki.php?n=Main.QuickGuideToEditingTypeCheckingAndCompilingAgdaCode
The MAlonzo compiler.

http://wiki.portal.chalmers.se/agda/pmwiki.php?n=Docs.MAlonzo
A new Emacs input method, which contains bindings for many Unicode symbols, is by default activated in the Emacs mode.

http://wiki.portal.chalmers.se/agda/pmwiki.php?n=Docs.UnicodeInput
Highlighted, hyperlinked HTML can be generated from Agda source code.

http://wiki.portal.chalmers.se/agda/pmwiki.php?n=Main.HowToGenerateWebPagesFromSourceCode
The command-line interactive mode (agda -I) is no longer supported, but should still work.

http://thread.gmane.org/gmane.comp.lang.agda/245
Reload times when working on large projects are now considerably better.

http://thread.gmane.org/gmane.comp.lang.agda/551

Libraries

A standard library is under development.

http://wiki.portal.chalmers.se/agda/pmwiki.php?n=Libraries.StandardLibrary

Documentation

The Agda wiki is better organised. It should be easier for a newcomer to find relevant information now.

http://wiki.portal.chalmers.se/agda/

Infrastructure

Easy-to-install packages for Windows and Debian/Ubuntu have been prepared.

http://wiki.portal.chalmers.se/agda/pmwiki.php?n=Main.Download
Agda 2.2.0 is available from Hackage.

http://hackage.haskell.org/

Depends on 36 packages:

Used by 6 packages:

Agda, Agda-executable, agda-server, agda-snippets, hakyll-agda, PandocAgda