singletons 2.4

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This is the README file for the singletons library. This file contains all the documentation for the definitions and functions in the library.

The singletons library was written by Richard Eisenberg,, and with significant contributions by Jan Stolarek, There are two papers that describe the library. Original one, Dependently typed programming with singletons, is available here and will be referenced in this documentation as the “singletons paper”. A follow-up paper, Promoting Functions to Type Families in Haskell, is available here and will be referenced in this documentation as the “promotion paper”.

Ryan Scott,, is an active maintainer.

Purpose of the singletons library

The library contains a definition of singleton types, which allow programmers to use dependently typed techniques to enforce rich constraints among the types in their programs. See the singletons paper for a more thorough introduction.

The package also allows promotion of term-level functions to type-level equivalents. Accordingly, it exports a Prelude of promoted and singletonized functions, mirroring functions and datatypes found in Prelude, Data.Bool, Data.Maybe, Data.Either, Data.Tuple and Data.List. See the promotion paper for a more thorough introduction.


The singletons library requires GHC 8.4.1 or greater. Any code that uses the singleton generation primitives needs to enable a long list of GHC extensions. This list includes, but is not necessarily limited to, the following:

  • DefaultSignatures
  • EmptyCase
  • ExistentialQuantification
  • FlexibleContexts
  • FlexibleInstances
  • GADTs
  • InstanceSigs
  • KindSignatures
  • RankNTypes
  • ScopedTypeVariables
  • TemplateHaskell
  • TypeFamilies
  • TypeInType
  • TypeOperators
  • UndecidableInstances

You may also want

  • -Wno-redundant-constraints

as the code that singletons generates uses redundant constraints, and there seems to be no way, without a large library redesign, to avoid this.

Modules for singleton types

Data.Singletons exports all the basic singletons definitions. Import this module if you are not using Template Haskell and wish only to define your own singletons.

Data.Singletons.TH exports all the definitions needed to use the Template Haskell code to generate new singletons.

Data.Singletons.Prelude re-exports Data.Singletons along with singleton definitions for various Prelude types. This module provides a singletonized equivalent of the real Prelude. Note that not all functions from original Prelude could be turned into singletons.

Data.Singletons.Prelude.* modules provide singletonized equivalents of definitions found in the following base library modules: Data.Bool, Data.Maybe, Data.Either, Data.List, Data.Tuple, Data.Void and GHC.Base. We also provide singletonized Eq, Ord, Show, Enum, and Bounded typeclasses.

Data.Singletons.Decide exports type classes for propositional equality.

Data.Singletons.TypeLits exports definitions for working with GHC.TypeLits.

Modules for function promotion

Modules in Data.Promotion namespace provide functionality required for function promotion. They mostly re-export a subset of definitions from respective Data.Singletons modules.

Data.Promotion.TH exports all the definitions needed to use the Template Haskell code to generate promoted definitions.

Data.Promotion.Prelude and Data.Promotion.Prelude.* modules re-export all promoted definitions from respective Data.Singletons.Prelude modules. Data.Promotion.Prelude.List adds a significant amount of functions that couldn’t be singletonized but can be promoted. Some functions still don’t promote - these are documented in the source code of the module. There is also Data.Promotion.Prelude.Bounded module that provides promoted PBounded typeclass.

Functions to generate singletons

The top-level functions used to generate singletons are documented in the Data.Singletons.TH module. The most common case is just calling singletons, which I’ll describe here:

singletons :: Q [Dec] -> Q [Dec]

Generates singletons from the definitions given. Because singleton generation requires promotion, this also promotes all of the definitions given to the type level.

Usage example:

$(singletons [d|
  data Nat = Zero | Succ Nat
  pred :: Nat -> Nat
  pred Zero = Zero
  pred (Succ n) = n

Definitions used to support singletons

Please refer to the singletons paper for a more in-depth explanation of these definitions. Many of the definitions were developed in tandem with Iavor Diatchki.

data family Sing (a :: k)

The data family of singleton types. A new instance of this data family is generated for every new singleton type.

class SingI (a :: k) where
  sing :: Sing a

A class used to pass singleton values implicitly. The sing method produces an explicit singleton value.

data SomeSing k where
  SomeSing :: Sing (a :: k) -> SomeSing k

The SomeSing type wraps up an existentially-quantified singleton. Note that the type parameter a does not appear in the SomeSing type. Thus, this type can be used when you have a singleton, but you don’t know at compile time what it will be. SomeSing Thing is isomorphic to Thing.

class SingKind k where
  type Demote k :: *
  fromSing :: Sing (a :: k) -> Demote k
  toSing   :: Demote k -> SomeSing k

This class is used to convert a singleton value back to a value in the original, unrefined ADT. The fromSing method converts, say, a singleton Nat back to an ordinary Nat. The toSing method produces an existentially-quantified singleton, wrapped up in a SomeSing. The Demote associated kind-indexed type family maps the kind Nat back to the type Nat.

data SingInstance (a :: k) where
  SingInstance :: SingI a => SingInstance a
singInstance :: Sing a -> SingInstance a

Sometimes you have an explicit singleton (a Sing) where you need an implicit one (a dictionary for SingI). The SingInstance type simply wraps a SingI dictionary, and the singInstance function produces this dictionary from an explicit singleton. The singInstance function runs in constant time, using a little magic.

Equality classes

There are two different notions of equality applicable to singletons: Boolean equality and propositional equality.

  • Boolean equality is implemented in the type family (:==) (which is actually a synonym for the type family (==) from Data.Type.Equality) and the class SEq. See the Data.Singletons.Prelude.Eq module for more information.

  • Propositional equality is implemented through the constraint (~), the type (:~:), and the class SDecide. See modules Data.Type.Equality and Data.Singletons.Decide for more information.

Which one do you need? That depends on your application. Boolean equality has the advantage that your program can take action when two types do not equal, while propositional equality has the advantage that GHC can use the equality of types during type inference.

Instances of both SEq and SDecide are generated when singletons is called on a datatype that has deriving Eq. You can also generate these instances directly through functions exported from Data.Singletons.TH.

Show classes

Promoted and singled versions of the Show class (PShow and SShow, respectively) are provided in the Data.Singletons.Prelude.Show module. In addition, there is a ShowSing class provided in the Data.Singletons.ShowSing module, which facilitates the ability to write Show instances for Sing instances.

What is the difference between the two? Let’s use the False constructor as an example. If you used the PShow Bool instance, then the output of calling Show_ on False is "False", much like the value-level Show Bool instance (similarly for the SShow Bool instance). However, the ShowSing Bool instance is intended for printing the value of the singleton constructor SFalse, so calling showsSingPrec 0 SFalse yields "SFalse" (simiarly for the Show (Sing (SBool z)) instance).

Instance of PShow, SShow, ShowSing, and Show (for the singleton type) are generated when singletons is called on a datatype that has deriving Show. You can also generate these instances directly through functions exported from Data.Singletons.TH.

A promoted and singled Show instance is provided for Symbol, but it is only a crude approximation of the value-level Show instance for String. On the value level, showing Strings escapes special characters (such as double quotes), but implementing this requires pattern-matching on character literals, something which is currently impossible at the type level. As a consequence, the type-level Show instance for Symbols does not do any character escaping.

Pre-defined singletons

The singletons library defines a number of singleton types and functions by default:

  • Bool
  • Maybe
  • Either
  • Ordering
  • ()
  • tuples up to length 7
  • lists

These are all available through Data.Singletons.Prelude. Functions that operate on these singletons are available from modules such as Data.Singletons.Bool and Data.Singletons.Maybe.

Promoting functions

Function promotion allows to generate type-level equivalents of term-level definitions. Almost all Haskell source constructs are supported – see last section of this README for a full list.

Promoted definitions are usually generated by calling promote function:

$(promote [d|
  data Nat = Zero | Succ Nat
  pred :: Nat -> Nat
  pred Zero = Zero
  pred (Succ n) = n

Every promoted function and data constructor definition comes with a set of so-called “symbols”. These are required to represent partial application at the type level. Each function gets N+1 symbols, where N is the arity. Symbols represent application of between 0 to N arguments. When calling any of the promoted definitions it is important refer to it using their symbol name. Moreover, there is new function application at the type level represented by Apply type family. Symbol representing arity X can have X arguments passed in using normal function application. All other parameters must be passed by calling Apply.

Users also have access to Data.Promotion.Prelude and its submodules (Base, Bool, Either, List, Maybe and Tuple). These provide promoted versions of function found in GHC’s base library.

Note that GHC resolves variable names in Template Haskell quotes. You cannot then use an undefined identifier in a quote, making idioms like this not work:

type family Foo a where ...
$(promote [d| ... foo x ... |])

In this example, foo would be out of scope.

Refer to the promotion paper for more details on function promotion.

Classes and instances

This is best understood by example. Let’s look at a stripped down Ord:

class Eq a => Ord a where
  compare :: a -> a -> Ordering
  (<)     :: a -> a -> Bool
  x < y = case x `compare` y of
            LT -> True
	    EQ -> False
	    GT -> False

This class gets promoted to a “kind class” thus:

class PEq a => POrd a where
  type Compare (x :: a) (y :: a) :: Ordering
  type (:<)    (x :: a) (y :: a) :: Bool
  type x :< y = ... -- promoting `case` is yucky.

Note that default method definitions become default associated type family instances. This works out quite nicely.

We also get this singleton class:

class SEq a => SOrd a where
  sCompare :: forall (x :: a) (y :: a). Sing x -> Sing y -> Sing (Compare x y)
  (%:<)    :: forall (x :: a) (y :: a). Sing x -> Sing y -> Sing (x :< y)

  default (%:<) :: forall (x :: a) (y :: a).
                   ((x :< y) ~ {- RHS from (:<) above -})
		=> Sing x -> Sing y -> Sing (x :< y)
  x %:< y = ...  -- this is a bit yucky too

Note that a singletonized class needs to use default signatures, because type-checking the default body requires that the default associated type family instance was used in the promoted class. The extra equality constraint on the default signature asserts this fact to the type checker.

Instances work roughly similarly.

instance Ord Bool where
  compare False False = EQ
  compare False True  = LT
  compare True  False = GT
  compare True  True  = EQ

instance POrd Bool where
  type Compare 'False 'False = 'EQ
  type Compare 'False 'True  = 'LT
  type Compare 'True  'False = 'GT
  type Compare 'True  'True  = 'EQ

instance SOrd Bool where
  sCompare :: forall (x :: a) (y :: a). Sing x -> Sing y -> Sing (Compare x y)
  sCompare SFalse SFalse = SEQ
  sCompare SFalse STrue  = SLT
  sCompare STrue  SFalse = SGT
  sCompare STrue  STrue  = SEQ

The only interesting bit here is the instance signature. It’s not necessary in such a simple scenario, but more complicated functions need to refer to scoped type variables, which the instance signature can bring into scope. The defaults all just work.

On names

The singletons library has to produce new names for the new constructs it generates. Here are some examples showing how this is done:

  1. original datatype: Nat

    promoted kind: Nat

    singleton type: SNat (which is really a synonym for Sing)

  2. original datatype: /\

    promoted kind: /\

    singleton type: %/\

  3. original constructor: Succ

    promoted type: 'Succ (you can use Succ when unambiguous)

    singleton constructor: SSucc

    symbols: SuccSym0, SuccSym1

  4. original constructor: :+:

    promoted type: ':+:

    singleton constructor: :%+:

    symbols: :+:@#@$, :+:@#@$$, :+:@#@$$$

  5. original value: pred

    promoted type: Pred

    singleton value: sPred

    symbols: PredSym0, PredSym1

  6. original value: +

    promoted type: +

    singleton value: %+

    symbols: +@#@$, +@#@$$, +@#@$$$

  7. original class: Num

    promoted class: PNum

    singleton class: SNum

  8. original class: ~>

    promoted class: #~>

    singleton class: %~>

Special names

There are some special cases, listed below (with asterisks* denoting special treatment):

  1. original datatype: []

    promoted kind: []

    singleton type*: SList

  2. original constructor: []

    promoted type: '[]

    singleton constructor*: SNil

    symbols*: NilSym0

  3. original constructor: :

    promoted type: ':

    singleton constructor*: SCons

    symbols: :@#@$, :@#@$$, :@#@$$$

  4. original datatype: (,)

    promoted kind: (,)

    singleton type*: STuple2

  5. original constructor: (,)

    promoted type: '(,)

    singleton constructor*: STuple2

    symbols*: Tuple2Sym0, Tuple2Sym1, Tuple2Sym2

    All tuples (including the 0-tuple, unit) are treated similarly.

  6. original value: (.)

    promoted type*: (:.)

    singleton value: (%.)

    symbols: (.@#@$), (.@#@$$), (.@#@$$$)

    The promoted type is special because GHC can’t parse a type named (.).

  7. original value: (!)

    promoted type*: (:!)

    singleton value: (%!)

    symbols: (!@#@$), (!@#@$$), (!@#@$$$)

    The promoted type is special because GHC can’t parse a type named (!).

  8. original value: ___foo

    promoted type*: US___foo (”US” stands for “underscore”)

    singleton value*: ___sfoo

    symbols*: US___fooSym0

    All functions that begin with leading underscores are treated similarly.

Supported Haskell constructs

The following constructs are fully supported:

  • variables
  • tuples
  • constructors
  • if statements
  • infix expressions and types
  • _ patterns
  • aliased patterns
  • lists
  • sections
  • undefined
  • error
  • deriving Eq, Ord, Show, Bounded, and Enum
  • class constraints (though these sometimes fail with let, lambda, and case)
  • literals (for Nat and Symbol), including overloaded number literals
  • unboxed tuples (which are treated as normal tuples)
  • records
  • pattern guards
  • case
  • let
  • lambda expressions
  • ! and ~ patterns (silently but successfully ignored during promotion)
  • class and instance declarations
  • higher-kinded type variables (see below)
  • functional dependencies (with limitations – see below)

Higher-kinded type variables in class/data declarations must be annotated explicitly. This is due to GHC’s handling of complete user-specified kind signatures, or CUSKs. Briefly, singletons has a hard time conforming to the precise rules that GHC imposes around CUSKs and so needs a little help around kind inference here. See this pull request for more background.

singletons is slightly more conservative with respect to deriving than GHC is. The stock classes listed above (Eq, Ord, Show, Bounded, and Enum) are the only ones that singletons will derive without an explicit deriving strategy. To do anything more exotic, one must explicitly indicate one’s intentions by using the DerivingStrategies extension.

singletons fully supports the anyclass strategy as well as the stock strategy (at least, for the classes listed above). singletons does not support the newtype strategy, as there is not an equivalent of coerce at the type level.

The following constructs are supported for promotion but not singleton generation:

  • scoped type variables
  • overlapping patterns. Note that overlapping patterns are sometimes not obvious. For example, the filter function does not singletonize due to overlapping patterns:
filter :: (a -> Bool) -> [a] -> [a]
filter _pred []    = []
filter pred (x:xs)
  | pred x         = x : filter pred xs
  | otherwise      = filter pred xs

Overlap is caused by otherwise catch-all guard, which is always true and thus overlaps with pred x guard.

The following constructs are not supported:

  • list comprehensions
  • do
  • arithmetic sequences
  • datatypes that store arrows, Nat, or Symbol
  • literals (limited support)

Why are these out of reach? The first two depend on monads, which mention a higher-kinded type variable. GHC did not support higher-sorted kind variables, which are be necessary to promote/singletonize monads, and singletons has not be rewritten to accommodate this new ability. This bug report is a feature request looking for support for these constructs.

Arithmetic sequences are defined using Enum typeclass, which uses infinite lists.

As described in the promotion paper, promotion of datatypes that store arrows is currently impossible. So if you have a declaration such as

data Foo = Bar (Bool -> Maybe Bool)

you will quickly run into errors.

Literals are problematic because we rely on GHC’s built-in support, which currently is limited. Functions that operate on strings will not work because type level strings are no longer considered lists of characters. Function working on integer literals can be promoted by rewriting them to use Nat. Since Nat does not exist at the term level it will only be possible to use the promoted definition, but not the original, term-level one.

This is the same line of reasoning that forbids the use of Nat or Symbol in datatype definitions. But, see this bug report for a workaround.

Support for *

The built-in Haskell promotion mechanism does not yet have a full story around the kind * (the kind of types that have values). Ideally, promoting some form of TypeRep would yield *, but the implementation of TypeRep would have to be updated for this to really work out. In the meantime, users who wish to experiment with this feature have two options:

  1. The module Data.Singletons.TypeRepStar has all the definitions possible for making * the promoted version of TypeRep, as TypeRep is currently implemented. The singleton associated with TypeRep has one constructor:

    newtype instance Sing :: Type -> Type where
      STypeRep :: TypeRep a -> Sing a

    Thus, a TypeRep is stored in the singleton constructor. However, any datatypes that store TypeReps will not generally work as expected; the built-in promotion mechanism will not promote TypeRep to *.

  2. The module Data.Singletons.CustomStar allows the programmer to define a subset of types with which to work. See the Haddock documentation for the function singletonStar for more info.

Known bugs

  • Record updates don’t singletonize
  • Inference dependent on functional dependencies is unpredictably bad. The problem is that a use of an associated type family tied to a class with fundeps doesn’t provoke the fundep to kick in. This is GHC’s problem, in the end.


Changelog for singletons project


  • Restore the TyCon1, TyCon2, etc. types. It turns out that the new TyCon doesn’t work with kind-polymorphic tycons.


  • Require GHC 8.4.

  • Demote Nat is now Natural (from Numeric.Natural) instead of Integer. In accordance with this change, Data.Singletons.TypeLits now exposes GHC.TypeNats.natVal (which returns a Natural) instead of GHC.TypeLits.natVal (which returns an Integer).

  • The naming conventions for infix identifiers (e.g., (&*)) have been overhauled.

    • Infix functions (that are not constructors) are no longer prepended with a colon when promoted to type families. For instance, the promoted version of (&*) is now called (&*) as well, instead of (:&*) as before.

      There is one exception to this rule: the (.) function, which is promoted as (:.). The reason is that one cannot write (.) at the type level.

    • Singletons for infix functions are now always prepended with % instead of %:.

    • Singletons for infix classes are now always prepended with % instead of :%.

    • Singletons for infix datatypes are now always prepended with a %.

      (Before, there was an unspoken requirement that singling an infix datatype required that name to begin with a colon, and the singleton type would begin with :%. But now that infix datatype names can be things like (+), this requirement became obsolete.)

    The upshot is that most infix names can now be promoted using the same name, and singled by simply prepending the name with %.

  • The suffix for defunctionalized names of symbolic functions (e.g., (+)) has changed. Before, the promoted type name would be suffixed with some number of dollar signs (e.g., (+$) and (+$$)) to indicate defunctionalization symbols. Now, the promoted type name is first suffixed with @#@ and then followed by dollar signs (e.g., (+@#@$) and (+@#@$$)). Adopting this conventional eliminates naming conflicts that could arise for functions that consisted of solely $ symbols.

  • The treatment of undefined is less magical. Before, all uses of undefined would be promoted to GHC.Exts.Any and singled to undefined. Now, there is a proper Undefined type family and sUndefined singleton function.

  • As a consequence of not promoting undefined to Any, there is no need to have a special any_ function to distinguish the function on lists. The corresponding promoted type, singleton function, and defunctionalization symbols are now named Any, sAny, and AnySym{0,1,2}.

  • Rework the treatment of empty data types:

    • Generated SingKind instances for empty data types now use EmptyCase instead of simply erroring.
    • Derived PEq instances for empty data types now return True instead of False. Derived SEq instances now return True instead of erroring.
    • Derived SDecide instances for empty data types now return Proved bottom, where bottom is a divergent computation, instead of erroring.
  • Add Data.Singletons.Prelude.IsString and Data.Promotion.Prelude.IsString modules. IsString.fromString is now used when promoting or singling string literals when the -XOverloadedStrings extension is enabled (similarly to how Num.fromInteger is currently used when promoting or singling numeric literals).

  • Add Data.Singletons.Prelude.Void.

  • Add promoted and singled versions of div, mod, divMod, quot, rem, and quotRem to Data.Singletons.TypeLits that utilize the efficient Div and Mod type families from GHC.TypeNats. Also add sLog2 and defunctionalization symbols for Log2 from GHC.TypeNats.

  • Add (<>) and (%<>), the promoted and singled versions of AppendSymbol from GHC.TypeLits.

  • Add (%^), the singleton version of GHC.TypeLits.^.

  • Add unlines and unwords to Data.Singletons.Prelude.List.

  • Add promoted and singled versions of Show, including deriving support.

  • Add a ShowSing class, which facilitates the ability to write Show instances for Sing instances.

  • Permit derived Ord instances for empty datatypes.

  • Permit standalone deriving declarations.

  • Permit DeriveAnyClass (through the anyclass keyword of DerivingStrategies)

  • Add a value-level (@@), which is a synonym for applySing.

  • Add Eq, Ord, Num, Enum, and Bounded instances for SomeSing, which leverage the SEq, SOrd, SNum, SEnum, and SBounded instances, respectively, for the underlying Sing.

  • Rework the Sing (a :: *) instance in Data.Singletons.TypeRepStar such that it now uses type-indexed Typeable. The new Sing instance is now:

    newtype instance Sing :: Type -> Type where
      STypeRep :: TypeRep a -> Sing a

    Accordingly, the SingKind instance has also been changed:

    instance SingKind Type where
      type Demote Type = SomeTypeRepStar
    data SomeTypeRepStar where
      SomeTypeRepStar :: forall (a :: *). !(TypeRep a) -> SomeTypeRepStar

    Aside from cleaning up some implementation details, this change assures that toSing can only be called on TypeReps whose kind is of kind *. The previous implementation did not enforce this, which could lead to segfaults if used carelessly.

  • Instead of erroring, the toSing implementation in the SingKind (k1 ~> k2) instance now works as one would expect (provided the user adheres to some common-sense SingKind laws, which are now documented).

  • Add a demote function, which is a convenient shorthand for fromSing sing.

  • Add a Data.Singletons.Sigma module with a Sigma (dependent pair) data type.

  • Export defunctionalization symbols for Demote, SameKind, KindOf, (~>), Apply, and (@@)fromData.Singletons`.

  • Add an explicitly bidirectional pattern synonym Sing. Pattern matching on Sing brings a SingI ty constraint into scope from a singleton Sing ty.

  • Add an explicitly bidirectional pattern synonym FromSing. Pattern matching on any demoted (base) type gives us the corresponding singleton.

  • Add explicitly bidirectional pattern synonyms SLambda{2..8}. Pattern matching on any defunctionalized singleton yields a term-level Haskell function on singletons.

  • Remove the family of TyCon1, TyCon2, …, in favor of just TyCon. GHC 8.4’s type system is powerful enough to allow this nice simplification.


  • Documentation clarifiation in Data.Singletons.TypeLits, thanks to @ivan-m.

  • Demote was no longer a convenient way of calling DemoteRep and has been removed. DemoteRep has been renamed Demote.

  • DemoteRep is now injective.

  • Demoting a Symbol now gives Text. This is motivated by making DemoteRep injective. (If Symbol demoted to String, then there would be a conflict between demoting [Char] and Symbol.)

  • Generating singletons also now generates fixity declarations for the singletonized definitions, thanks to @int-index.

  • Though more an implementation detail: singletons no longer uses kind-level proxies anywhere, thanks again to @int-index.

  • Support for promoting higher-kinded type variables, thanks for @int-index.

  • Data.Singletons.TypeLits now exports defunctionalization symbols for KnownNat and KnownSymbol.

  • Better type inference support around constraints, as tracked in Issue #176.

  • Type synonym definitions are now ignored, as they should be.

  • Show instances for SNat and SSymbol, thanks to @cumber.

  • The singFun and unSingFun functions no longer use proxies, preferring TypeApplications.


  • With TypeInType, we no longer kind KProxy. @int-index has very helpfully removed the use of KProxy from singletons.

  • Drop support for GHC 7.x.

  • Remove bugInGHC. That function was intended to work around GHC’s difficulty in detecting exhaustiveness of GADT pattern matches. GHC 8 comes with a much better exhaustiveness checker, and so this function is no longer necessary.


  • Require th-desugar >= 1.6

  • Work with GHC 8. GHC 8 gives the opportunity to simplify some pieces of singletons, but these opportunities are not yet fully realized. For example, injective type families means that we no longer need Sing to be a data family; it could be a type family. This might drastically simplify the way functions are singletonized. But not yet!

  • singletons now outputs a few more type/kind annotations to help GHC do type inference. There may be a few more programs accepted than before. (This is the fix for #136.)


  • Lots more functions in Data.Singletons.Prelude.List: filter, find, elemIndex, elemIndices, findIndex, findIndices, intersect, intersectBy, takeWhile, dropWhile, dropWhileEnd, span, break, take, drop, splitAt, group, maximum, minimum, insert, sort, groupBy, lookup, partition, sum, product, length, replicate, transpose, (!!), nub, nubBy, unionBy, union, genericLength

  • Fix fixity of *.

  • Make haddock work.


  • Instance promotion now works properly – it was quite buggy in 1.0.

  • Classes and instances can now be singletonized.

  • Limited support for functional dependencies.

  • We now have promoted and singletonized versions of Enum, as well as Bounded.

  • Deriving Enum is also now supported.

  • Ditto for Num, which includes an instance for Nat, naturally.

  • Promoting a literal number now uses overloaded literals at the type level, using a type-level FromInteger in the type-level Num class.

  • Better support for dealing with constraints. Some previously-unsingletonizable functions that have constrained parameters now work.

  • No more orphan Quasi instances!

  • Support for functions of arity 8 (instead of the old limit, 7).

  • Full support for fixity declarations.

  • A raft of bugfixes.

  • Drop support for GHC 7.8. You must have GHC 7.10.2.

Fix bug #116, thus allowing locally-declared symbols to be used in GHC 7.10.


  • No more GHC 7.8.2 support – you must have GHC 7.8.3.


Update testsuite to work with th-desugar-1.5.2. No functional changes.


This is a maintenance release to support building (but not testing, due to GHC bug #10058) with 7.10. This release also targets th-desugar-1.5. Some types changed (using th-desugar’s new DsMonad instead of Quasi), but clients generally won’t need to make any changes, unless they, too, generalize over Quasi.


This is a complete rewrite of the package.

  • A much wider array of surface syntax is now accepted for promotion and singletonization, including let, case, partially-applied functions, and anonymous functions, where, sections, among others.

  • Classes and instances can be promoted (but not singletonized).

  • Derivation of promoted instances for Ord and Bounded.

This release can be seen as a “technology preview”. More features are coming soon.

This version drops GHC 7.6 support.


Template Haskell names are now more hygienic. In other words, singletons won’t try to gobble up something happened to be named Sing in your project. (Note that the Template Haskell names are not completely hygienic; names generated during singleton generation can still cause conflicts.)

If a function to be promoted or singletonized is missing a type signature, that is now an error, not a warning.

Added a new external module Data.Singletons.TypeLits, which contain the singletons for GHC.TypeLits. Some convenience functions are also provided.

The extension EmptyCase is no longer needed. This caused pain when trying to support both GHC 7.6.3 and 7.8.


Fix export list of Data.Singletons.TH, again again.

Add SEq instances for Nat and Symbol.


Fix export list of Data.Singletons.TH, again.


Fix export list of Data.Singletons.TH.


Make compatible with GHC HEAD, but HEAD reports core lint errors sometimes.

Change module structure significantly. If you want to derive your own singletons, you should import Data.Singletons.TH. The module Data.Singletons now exports functions only for the use of singletons.

New modules Data.Singletons.Bool, ...Maybe, ...Either, and ...List are just like their equivalents from Data., except for List, which is quite lacking in features.

For singleton equality, use Data.Singletons.Eq.

For propositional singleton equality, use Data.Singletons.Decide.

New module Data.Singletons.Prelude is meant to mirror the Haskell Prelude, but with singleton definitions.

Streamline representation of singletons, resulting in exponential speedup at execution. (This has not been rigorously measured, but the data structures are now exponentially smaller.)

Add internal support for TypeLits, because the TypeLits module no longer exports singleton definitions.

Add support for existential singletons, through the toSing method of SingKind.

Remove the SingE class, bundling its functionality into SingKind. Thus, the SingRep synonym has also been removed.

Name change: KindIs becomes KProxy.

Add support for singletonizing calls to error.

Add support for singletonizing empty data definitions.


Make compatible with GHC HEAD, but HEAD reports core lint errors sometimes.


Bug fix to make singletons compatible with GHC 7.6.1.

Added git info to cabal file.


Update to work with latest version of GHC (7.7.20130114).

Now use branched type family instances to allow for promotion of functions with overlapping patterns.

Permit promotion of functions with constraints by omitting constraints.


Update to work with latest version of GHC (7.7.20121031).

Removed use of Any to simulate kind classes; now using KindOf and OfKind from GHC.TypeLits.

Made compatible with GHC.TypeLits.


Added this changelog

Update to work with latest version of GHC (7.6.1). (There was a change to Template Haskell).

Moved library into Data.Singletons.


Update to work with latest version of GHC. (There was a change to Template Haskell).

Updated dependencies in cabal to include the newer version of TH.


Initial public release