Manipulating Haskell source: abstract syntax, lexer, parser, and pretty-printer https://github.com/haskell-suite/haskell-src-exts

Version on this page:1.21.0
LTS Haskell 13.25:1.20.3
Stackage Nightly 2019-06-12:1.21.0
Latest on Hackage:1.21.0

See all snapshots haskell-src-exts appears in

BSD-3-Clause licensed by Niklas Broberg
Maintained by Matthew Pickering

Module documentation for 1.21.0

There are no documented modules for this package.

Haskell Source Extensions

haskell-src-exts is a package for handling and manipulating Haskell source code. It is a descendant of the haskell-src package that is part of the standard libraries, but extends this to support a number of syntactic extensions, e.g. MPTCs, fundeps, GADTs, TH etc. The aim is to support all extensions recognized by the community, as determined by what is implemented in compilers and tools.

Apart from the more standard extensions supported by e.g. GHC, haskell-src-exts provides support for HaRP (Haskell Regular Patterns) and HSX (Haskell Source with XML) syntax.

Package structure

The modules that comprise haskell-src-exts all reside in the hierarchic namespace Language.Haskell.Exts. Notable exposed modules include:

  • Language.Haskell.Exts - Imports and re-exports all the below, and also defines some functions that combine functionality from several modules.
  • Language.Haskell.Exts.Syntax - The abstract syntax tree that the other modules work on.
  • Language.Haskell.Exts.Build - Combinators for building abstract syntax.
  • Language.Haskell.Exts.Parser - Functions for parsing Haskell source code into an abstract syntax representation.


The haskell-src-exts Package is distributed under a derived BSD-style license. It derives from several sources, all of which are distributable under BSD-style or compatible licenses. See the file LICENSE for the complete license text.


1.20.3 --> 1.21.0
* DerivingVia (#420)
* TypeInType (#414)
* Parse TH-quoted list constructor (#415)

1.20.2 --> 1.20.3
* GHC 8.6.1 compatability (MonadFail)
* Remove cpphs dependency

1.20.1 --> 1.20.2
* Add fixity of (&)
* GHC 8.4.1 compatability

1.20.0 --> 1.20.1
* Fix two test failures

1.19.1 --> 1.20.0

* GHC 8.2 updates
* Add some missing Generics instances (#380)
* Support Template Haskell pattern splices (#352)
* Fix overloaded labels starting with _ (#362)
* Relax MultiParamTypeClass check (#333)
* DeriveAnyClass (#337)
* Deriving Strategies (#387)
* Unboxed Sums
* Expression holes in infix expressions
* Support multiple pattern synonyms per pattern synonym signature
* COMPLETE pragmas
* Fix #309, remove unnecessary semi-colon from pretty printer
* Refine ExplicitNamespaces to properly export type operators

1.19.0 --> 1.19.1

* Revert making fields of `SrcLoc` strict. (#340)

1.18.2 --> 1.19.0

* Add JavaScriptFFI to known extensions (#335)
* Remove space when pretty printing `SrcLoc`. (#307)
* Don't reverse arguments in a context. (#328)
* Add support for EmptyCase extension (#255)

1.18.1 --> 1.18.2

* Fix overzealous lexing of type applications

1.18.0 --> 1.18.1

* Fix overzealous lexing of overloaded labels.

1.17.1 --> 1.18.0

* Remove simplified AST
* GHC 8.0 update: Overloaded labels, type applications, pattern synonym
extensions, StrictData, injective type families

AST changes:

* Add TyQuasiQuote (#279).

Other changes:

* Only treat 'role' as a keyword in certain contexts (#286).
* Prettyprint '!' in patterns correctly when BangPatterns are enabled (#287).
* Parse magic hash with multiple trailing hashes (#300).
* Apply fixities inside pattern synonym builders
* Fix lexing of idents with multiple trailing #

** 1.17.x

1.17.0 --> 1.17.1

Bug Fix:

* Fix bug in parsing view patterns (#284) --> 1.17.0

AST changes:

* Replaced VarA with AppA in Asst (#168).
* Promoted list/tuple members changed from Promoted to Type (#162).
* Update PatBind, Match, and Alt from containing Binds to Maybe Binds to
distinguish between empty where clauses and where clauses with no
binds (#244).
* Add RoleAnnotDecl and Role to support Role annotations (#215).
* Move NameSpace field from EVar/IVar to EAbs/IAbs.

Other changes:

* Add standalone parsers for ImportDecl.
* Fix pretty-printer bugs for HaRP (#160).
* Insert parentheses when pretty-printing non-atomic bang types (#169).
* Un-reverse confusion of left and right arrow (#175).
* Prettyprint option pragmas like 1.15 (#172).
* Conditionally insert lines when pretty-printing declarations (#171).
* Distinguish deriving (Show) from deriving Show (#189).
* Allow parsing of unicode subscript and superscript functions (#173).
* Pretty print unboxed tuples with spaces (#193).
* Improve performance when parsing long extension lists (#200).
* Properly pretty print constructor and class operators (#204).
* Read Haskell source files as UTF-8 (#223).
* Fix ExplicitNamespaces parsing (#216).
* PolyKinds implies KindSignatures (#220).
* Preserve location information for infix binds (#205).
* Preserve positional information in checkPattern (#231).
* Maintain correct line numbers when parsing multiline GHC_OPTIONS (#218).
* Correctly parse "*" with TypeOperators (#81).
* Export the "pretty" method (#222).
* Add javascript calling convention for foreign imports (#236).
* Add non-greedy parsers for module heads (#191).
* Add a flag to disable arity checking when parsing (#260).
* Parse "-" in type signatures (#206).
* Add support for type wildcards and expression holes (#252).
* Add support for Pattern Synonyms (#197).
* Bump the happy lower bound (#250).
* Make test suite pass on GHC 7.12 (#224).
* Support linking Haddock comments to AST nodes (#213).
* Parse multiline LANGUAGE pragmas (#217).
* Parse trailing where (#25).
* Parse modules starting with pragmas and indented
"module" keyword (#122).
* Use pretty-show to get human readable test outputs.
* Respect fixity declarations inside where/let/class in
`applyFixities` (#212).
* Correctly parse the combination of view patterns and bang patterns. (#276)

** 1.16.x

1.16.0 -->

Include generated InternalParser.hs in the release tarball
(See http://ro-che.info/articles/2014-03-08-happy-alex-ghc-7.8 for details) --> 1.16.0

New language features:

* DoRec
* Closed type families
* GADT records
* ExplicitNamespaces
* Type equality constraints
* PolyKinds
* DataKinds
* Default associated types
* Instance signatures
* OVERLAP pragma
* Parallel arrays
* MINIMAL pragma
* Default signatures
* Safe Haskell
* Binary literals
* Qualified record puns
* Type splices

AST changes:

* PatBind no longer contains the optional Maybe Type field. The type
signature is now represented as part of the pattern (PatTypeSig)
* GuardedAlt and GuardedAlts types are replaced with the isomorphic
GuardedRhs and Rhs types
* Move parentheses from the Context to Asst type
* Change DeclHead and InstHead to make it possible to represent
various ways to put parentheses around parts of declarations. Introduce a new
type InstRule for the same purpose.
* Change representation of negated patterns
* Make strictness annotations on data type fields part of those fields' Type

Notable bug fixes:

* Allow statements, and not just expression, in MultiWayIf conditions
* Allow equality constraints when GADTs are enabled
* Fix parsing and printing of pragma modifiers, such as CONLIKE
* Don't treat 'capi' or 'interruptible' as a keyword except after 'foreign'
* Let ImpredicativeTypes imply ExplicitForAll
* Let RecursiveDo enable 'rec' keyword
* Fix pretty-printing of the unary minus sign
* Do not require FlexibleContexts for things like Ord (i Int) => ...
* Fix pretty-printing of deriving clauses
* Parse "Constraint" as a kind with ConstraintKinds enabled
* Support qualified quasi-quoters
* Enable ScopedTypeVariables when parsing rules
* Only permit record wildcards in last position
* Allow n-parameter infix data declaration heads

Other changes:

* Expose showToken from Language.Haskell.Exts.Lexer
* Show instance for SrcLoc and SrcSpan are changed (now they do not contain
* field names)
* Add DeriveGeneric to known extensions

** 1.15.x

1.15.0 -->

* Eliminate inexhaustive patterns (and consequent bugs)
* The test suite for 1.15.0 wouldn't build due to a missing file in the source
distribution. Fixed. --> 1.15.0

* Add support for extensions:
- MultiWayIf
- LambdaCase
- DataKinds
* Remove support for old (deprecated) Generics
* Derive GHC's Generic instances for datatypes
* Derive some missing Data and Typeable instances
* Multiple bug fixes
* Add missing Functor and Applicative instances for monads
* Remove support for GHCs older than 7.4

** 1.14.x

1.14.0 -->

Nothing has actually changed here (apart from the CHANGELOG). This release is
done to include InternalParser.hs generated with happy 1.19 so that it compiles
with GHC 7.8.

1.13.6 --> 1.14.0

* Modernize the Extension datatype in L.H.E.Extension, following the lead
of Cabal, to allow negative and positive extension modifiers (turning
features on and off). You need to worry about backwards-incompatible
changes if any of the following pertains to you:
1) If you use the Extension datatype programmatically - it has changed
significantly (see documentation).
2) The ParseMode record now has one more field
(baseLanguage :: Language), which might give you a type error.
3) The behavior of the (extensions :: [Extension]) field has changed,
which could bite you if you pass custom extensions in the ParseMode.
Previously, the ParseMode defaulted to the list of extensions accepted
by Haskell2010, and if you set the list explicitly you would override
this. Now, the defaults are { baseLanguage = Haskell2010, extensions = [] },
and explicitly setting a list of extensions will be interpreted on top of
Haskell2010. See further the documentation for L.H.E.Extension.

* Add support for the 'capi' calling convention. It is enabled with the CApiFFI
extension. It's been included since GHC 7.4, and advertised since 7.6.

* Add support for the 'interruptible' FFI safety annotation, enabled with
the InterruptibleFFI extension.

* Give better error message when lexing newline fails. In particular, fix the bug
when the parser would crash if the file didn't end with a newline.

* Support unboxed tuple expressions and patterns.

* Fix bug in lexing of primitive integer literals in hex or octal notation.

* Disallow negative primitive word literals
(such as W# (-0x8000000000000000##)).

* Allow phase control for SPECIALIZE pragma.

* Derive Foldable and Traversable instances for all annotated AST types.

* Fix bug with pretty-printing WARNING and DEPRECATED pragmas.

** 1.13.x

1.13.5 --> 1.13.6

* Allow 0-parameter type classes when MultiParamTypeClasses is on.

* Add support for NondecreasingIndentation. Previously it was always on,
which is the default behavior in GHC. Now it is on only if enabled, or
when using the newly added ghcDefaults extension set (instead of haskell2010).

1.13.4 --> 1.13.5

* Expose Language.Haskell.Exts.Lexer, which implements
a standalone token stream lexer. The module is
re-exported both by Language.Haskell.Exts and by

1.13.3 --> 1.13.4

* Fix bug where operators starting with # written in
parentheses would not be parsed when UnboxedTuples is
turned on. Now works.

* Allow 'family' and 'forall' as (non-type) varid's. This
adds one more shift/reduce conflict to the parser, and
its resolution means that '{-# RULES "name" forall = ... #-}'
is not allowed.

* Complete the set of FFI calling conventions from the Haskell
2010 report (even if no compiler implements them). Also
include the 'js' calling convention, supported by UHC.

1.13.2 --> 1.13.3

* Fundep premises are now allowed to be empty.

* Fix the bug where the lexer would crash on a LINE pragma
that did not include a line number.

* Fix the bug where the lexer would require the # of a
MagicHash-style type constructor to be succeeded by at
least one character in the file.

* Fix long-standing bug where the parser would crash with
an ugly "Internal error" error message if encountering
an extra }.

* Report errors at the right place for function arity
mismatches. Earlier they were reported at end of file,
now they are reported where the function is declared.

* Lexer now properly fails on line-breaks in string literals.

* Lexer now handles character escapes up to 0x10FFFF (unicode).

1.13.1 --> 1.13.2

* Fix the bug with the precedence of unary prefix minus.
Previously it was resolved as binding more tightly
than any infix operator, now it is correctly treated
as having the same fixity as binary infix minus.

1.13.0 --> 1.13.1

* Allow an optional semi before the closing tag of
an element. This achieves a similar effect for
XmlSyntax in do blocks as DoAndIfThenElse does for
the if construct. No more need to indent the closing
tag one step further than the opening tag.

* Add a dummy 'noLoc :: SrcLoc' to L.H.E.SrcLoc, to
use when generating code. It could definitely be
done more elegantly, but not without inducing another
major version bump, so later.

* Fix a regression from 1.11.x where the parser would crash
upon encountering non-simple class/data declaration
heads, e.g. 'data A [a]'. Now fails with a parse error
as intended.

1.12.0 --> 1.13.0

* Add extensions DoAndIfThenElse and NPlusKPatterns to

* DoAndIfThenElse is now supported, at long last,
making HSE compatible with Haskell2010

* Introduce haskell98 and haskell2010 extension groups,
exported from Language.Haskell.Exts.Extensions.

* Backwards-incompatible change: default parse mode
is now to use haskell2010, which means the following
features are recognized by default: DoAndIfThenElse,
PatternGuards, ForeignFunctionInterface, EmptyDataDecls.
NPlusKPatterns is no longer recognized by default.

** 1.12.x

1.11.1 --> 1.12.0

* Move from old [$...| quasi-quote syntax to the new [...| one.
The old syntax is still recognized while parsing.

* Allow symbols as variables when TypeOperators is enabled.

* Rename ExplicitForall in ExplicitForAll, to be consistent
with GHC and the Haskell' process.

** 1.11.x

1.10.2 --> 1.11.1

* API change: the fixities field in ParseMode is now of type
Maybe [Fixity]. If the field is Nothing the parsing will
not try to do any fixity resolution whatsoever, otherwise
it behaves as before.

* API change, bug fix: The Fixity type contains a QName rather
than an Op to name the operator. The operator must match
the given QName exactly (i.e., unqualified names only match
unqualified names, and qualified names only match qualified
names) for applyFixities to perform fixups.

* Bug fix: End-of-file inside an OPTIONS pragma no longer loops.

** 1.10.x

1.10.1 --> 1.10.2

* Fix a missing case in the Functor declaration for Decl. Thanks
to Malcolm Wallace for the patch!

1.10.0 --> 1.10.1

* Enable the unicode version of DoubleColon (x2237). Thanks
to Andr�s Sicard-Ram�rez for the patch!

1.9.6 --> 1.10.0

* Ensure that implied extensions are always picked up, not only
when using the parseFile* family of functions as previously.

* Add the newly devised <%>...</%> syntax to the XmlSyntax support.
This causes changes to pretty much everything, including adding
a case to the AST which prompts the major version bump.

** 1.9.x

1.9.5 --> 1.9.6

* Fix a bug (#203) where the lexer loops on malformed quasi-quoters.

* Fix a bug with pretty-printing RULES pragmas.

1.9.4 --> 1.9.5

* Fix a bug where deriving clauses for GADT-style data declarations
were not properly indented.

* Pretty-printing patterns is now more accurate in inserting (and not
inserting) parentheses when needed.

1.9.3 --> 1.9.4

* Pretty-printer now inserts parentheses in clever places when
printing kinds.

* Pretty-printing expressions is now far more accurate in inserting
(and not inserting) parentheses when needed.

* Pretty-printing negative expressions no longer inserts a superfluous
space between the - and the expression.

1.9.2 --> 1.9.3

* Constructors for newtype declarations must now have exactly one
argument. This is only when using the classic syntax, not with
GADT-style syntax.

* Fix a bug where preceding commas in tuple sections were counted
one too few.

1.9.1 --> 1.9.2

* Fix a bug with pretty-printing lexer tokens.

* Fix a bug where non-colon TypeOperators could not be used in
prefix mode.

1.9.0 --> 1.9.1

* Export parseFileContentsWithExts from .Exts.

1.8.2 --> 1.9.0

* OptionPragma is renamed to the more descriptive ModulePragma,
and adds a constructor AnnModulePragma for handling ANN pragmas
preceding module header.

* Add instances for Eq/Ord/Data/Typeable for Fixity.

* Add 'parseFileWithComments' and 'parseFileContentsWithComments'
to L.H.Exts .

* More informative error messages when HSX tags are mismatched.

** 1.8.x

1.8.1 --> 1.8.2

* Don't insert redundant parentheses around record constructions
and updates.

1.8.0 --> 1.8.1

* Fix three bugs with the handling of ANN. I must have been really
tired when implementing that support.

1.7.2 --> 1.8.0

* Add an instance Show Fixity (derived).

* Support for the new ANN and INLINE_CONLIKE pragmas.

* Export knownExtensions from .Extension.

* Remove support for CFILES and INCLUDE pragmas. The support wasn't
correct anyway, as it assumed the pragmas appeared at the top of
files. As CFILES/INCLUDE pragmas can (and do) appear anywhere,
there's no hope to support them in the AST. Better to remove the
support altogether. Files with CFILES/INCLUDE pragmas can still
be parsed of course, but those pragmas will be handled as comments.

* Parsing with ignoreLinePragmas = False now correctly updates the
file name.

* Allow the whole SPECIALISE/INLINE family of pragmas in instance
declarations. The InsInline constructor is removed, and is now
represented by InsDecl (InlineSig ...).

* Fix a bug with line numbering and quasi quotes, and a similar one
with line numbering and CDATA.

* Fix a few minor bugs in the exactPrinter.

* Fix the strange handling of so called strings in LINE pragmas.

** 1.7.x

1.7.1 --> 1.7.2

* Fixes a bug in lexing LINE pragmas (used when ignoreLinePragmas
is set to False).

1.7.0 --> 1.7.1

* UnicodeSyntax now also enables the forall symbol (U+2200).

1.6.1 --> 1.7.0

* Operators defined on the form

(a `op` b) c = ...

could not be handled by the (annotated) AST, nor the parser. I had to
change the definition of the AST node for InfixMatch to allow a list
of right-hand subpatterns, i.e.

InfixMatch l (Pat l) (Name l) (Pat l) ...

has become

InfixMatch l (Pat l) (Name l) [Pat l] ...

I also had an epiphany and fixed the issue that would arise with
exact printing of prefix definitions including parentheses, so
that now works too!

** 1.6.x

1.6.0 --> 1.6.1

* UnicodeSyntax now works not only for identifiers, but also for
->, <- and =>, as well as Arrows arrows and kind stars.

1.5.3 --> 1.6.0

* (=~=) turns out to be too general at Functor (for intuitive and not
technical reasons), so is specialised to Annotated to closer mirror
the original intention.

* applyFixities is hoisted to a monad, and now fails on ambiguous infix

** 1.5.x

1.5.2 --> 1.5.3

* Several small bug fixes in the exact printer, and fail more gracefully
if the number of srcInfoPoints doesn't match the needs of the node.

1.5.1 --> 1.5.2

* Fix a bug in the exact printer that made it always print the first token
at position (0,0).

* In fixing the above, Annotated is now a superclass of ExactP. It was already
a superclass in spirit, and nothing can break from this since ExactP is only
exported abstractly.

1.5.0 --> 1.5.1

* The pretty printer now introduces parentheses for non-atomic arguments to
function application. Note that infix applications are left untouched, no
parentheses will be inserted there, as it is assumed that fixities are
already properly resolved.

* Fix a bug in the pretty printer where view patterns and n+k patterns were
not properly parenthesised.

1.4.0 --> 1.5.0

* Add support for acting on LINE pragmas while parsing, i.e. updating the source
position according to info given in LINE pragmas. This is done conditionally
based on a new flag ignoreLinePragmas in the ParseMode, hence the need to
increase the major version.

** 1.4.x

1.3.5 --> 1.4.0

* The AST node for Proc in the simple AST is changed to include a SrcLoc argument,
to make it consistent with similar nodes e.g. Lambda. This is specifically needed
for transformation of patterns in HSX.

** 1.3.x

1.3.4 --> 1.3.5

* Added an entry point in the parser for statements, and an instance Parseable Stmt
to go with it.

* Ensured that .Annotated exports all relevant parseXXX(WithYYY) functions.

1.3.3 --> 1.3.4

* Operator fixities are now resolved in patterns.

1.3.2 --> 1.3.3

* Fixes a bug where qualified keywords are rejected even if the extension that
enables the keyword in question is not turned on.

1.3.0 --> 1.3.2

(Let's forget 1.3.1 ever existed.)

* Fix a bug where declarations of infix operators were not properly merged as FunBinds.

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