BSD-2-Clause licensed by Megaparsec contributors, Paolo Martini, Daan Leijen
Maintained by Mark Karpov
This version can be pinned in stack with:megaparsec-9.6.1@sha256:8d8f8ee5aca5d5c16aa4219afd13687ceab8be640f40ba179359f2b42a628241,3323


License FreeBSD Hackage Stackage Nightly Stackage LTS CI

This is an industrial-strength monadic parser combinator library. Megaparsec is a feature-rich package that tries to find a nice balance between speed, flexibility, and quality of parse errors.


The project provides flexible solutions to satisfy common parsing needs. The section describes them shortly. If you’re looking for comprehensive documentation, see the section about documentation.

Core features

The package is built around MonadParsec, an MTL-style monad transformer. Most features work with all instances of MonadParsec. One can achieve various effects combining monad transformers, i.e. building a monadic stack. Since the common monad transformers like WriterT, StateT, ReaderT and others are instances of the MonadParsec type class, one can also wrap ParsecT in these monads, achieving, for example, backtracking state.

On the other hand ParsecT is an instance of many type classes as well. The most useful ones are Monad, Applicative, Alternative, and MonadParsec.

Megaparsec includes all functionality that is typically available in Parsec-like libraries and also features some special combinators:

  • parseError allows us to end parsing and report an arbitrary parse error.
  • withRecovery can be used to recover from parse errors “on-the-fly” and continue parsing. Once parsing is finished, several parse errors may be reported or ignored altogether.
  • observing makes it possible to “observe” parse errors without ending parsing.

In addition to that, Megaparsec features high-performance combinators similar to those found in Attoparsec:

  • tokens makes it easy to parse several tokens in a row (string and string' are built on top of this primitive). This is about 100 times faster than matching a string token by token. tokens returns “chunk” of original input, meaning that if you parse Text, it’ll return Text without repacking.
  • takeWhile and takeWhile1 are about 150 times faster than approaches involving many, manyTill and other similar combinators.
  • takeP allows us to grab n tokens from the stream and returns them as a “chunk” of the stream.

Megaparsec is about as fast as Attoparsec if you write your parser carefully (see also the section about performance).

The library can currently work with the following types of input stream out-of-the-box:

  • String = [Char]
  • ByteString (strict and lazy)
  • Text (strict and lazy)

It’s also possible to make it work with custom token streams by making them an instance of the Stream type class.

Error messages

  • Megaparsec has typed error messages and the ability to signal custom parse errors that better suit the user’s domain of interest.

  • Since version 8, the location of parse errors can independent of current offset in the input stream. It is useful when you want a parse error to point to a particular position after performing some checks.

  • Instead of a single parse error Megaparsec produces so-called ParseErrorBundle data type that helps to manage multi-error messages and pretty-print them. Since version 8, reporting multiple parse errors at once has become easier.

External lexers

Megaparsec works well with streams of tokens produced by tools like Alex. The design of the Stream type class has been changed significantly in the recent versions, but user can still work with custom streams of tokens.

Character and binary parsing

Megaparsec has decent support for Unicode-aware character parsing. Functions for character parsing live in the Text.Megaparsec.Char module. Similarly, there is Text.Megaparsec.Byte module for parsing streams of bytes.


Text.Megaparsec.Char.Lexer is a module that should help you write your lexer. If you have used Parsec in the past, this module “fixes” its particularly inflexible Text.Parsec.Token.

Text.Megaparsec.Char.Lexer is intended to be imported using a qualified import, it’s not included in Text.Megaparsec. The module doesn’t impose how you should write your parser, but certain approaches may be more elegant than others. An especially important theme is parsing of white space, comments, and indentation.

The design of the module allows one quickly solve simple tasks and doesn’t get in the way when the need to implement something less standard arises.

Text.Megaparsec.Byte.Lexer is also available for users who wish to parse binary data.


Megaparsec is well-documented. See the current version of Megaparsec documentation on Hackage.


You can find the most complete Megaparsec tutorial here. It should provide sufficient guidance to help you start with your parsing tasks.


Despite being flexible, Megaparsec is also fast. Here is how Megaparsec compares to Attoparsec (the fastest widely used parsing library in the Haskell ecosystem):

Test case Execution time Allocated Max residency
CSV (Attoparsec) 76.50 μs 397,784 10,544
CSV (Megaparsec) 64.69 μs 352,408 9,104
Log (Attoparsec) 302.8 μs 1,150,032 10,912
Log (Megaparsec) 337.8 μs 1,246,496 10,912
JSON (Attoparsec) 18.20 μs 128,368 9,032
JSON (Megaparsec) 25.45 μs 203,824 9,176

You can run the benchmarks yourself by executing:

$ nix-build -A benches.parsers-bench
$ cd result/bench
$ ./bench-memory
$ ./bench-speed

More information about benchmarking and development can be found here.

Comparison with other solutions

There are quite a few libraries that can be used for parsing in Haskell, let’s compare Megaparsec with some of them.

Megaparsec vs Attoparsec

Attoparsec is another prominent Haskell library for parsing. Although both libraries deal with parsing, it’s usually easy to decide which you will need in particular project:

  • Attoparsec is sometimes faster but not that feature-rich. It should be used when you want to process large amounts of data where performance matters more than quality of error messages.

  • Megaparsec is good for parsing of source code or other human-readable texts. It has better error messages and it’s implemented as a monad transformer.

So, if you work with something human-readable where the size of input data is moderate, it makes sense to go with Megaparsec, otherwise Attoparsec may be a better choice.

Megaparsec vs Parsec

Since Megaparsec is a fork of Parsec, we are bound to list the main differences between the two libraries:

  • Better error messages. Megaparsec has typed error messages and custom error messages, it can also report multiple parse errors at once.

  • Megaparsec can show the line on which parse error happened as part of parse error. This makes it a lot easier to figure out where the error happened.

  • Some quirks and bugs of Parsec are fixed.

  • Better support for Unicode parsing in Text.Megaparsec.Char.

  • Megaparsec has more powerful combinators and can parse languages where indentation matters.

  • Better documentation.

  • Megaparsec can recover from parse errors “on the fly” and continue parsing.

  • Megaparsec allows us to conditionally process parse errors inside a running parser. In particular, it’s possible to define regions in which parse errors, should they happen, will get a “context tag”, e.g. we could build a context stack like “in function definition foo”, “in expression x”, etc.

  • Megaparsec is faster and supports efficient operations tokens, takeWhileP, takeWhile1P, takeP, like Attoparsec.

If you want to see a detailed change log, may be helpful. Also see this original announcement for another comparison.

Megaparsec vs Trifecta

Trifecta is another Haskell library featuring good error messages. These are the common reasons why Trifecta may be problematic to use:

  • Complicated, doesn’t have any tutorials available, and documentation doesn’t help much.

  • Trifecta can parse String and ByteString natively, but not Text.

  • Depends on lens, which is a very heavy dependency. If you’re not into lens, you may not like the API.

Idris has switched from Trifecta to Megaparsec which allowed it to have better error messages and fewer dependencies.

Megaparsec vs Earley

Earley is a newer library that allows us to safely parse context-free grammars (CFG). Megaparsec is a lower-level library compared to Earley, but there are still enough reasons to choose it:

  • Megaparsec is faster.

  • Your grammar may be not context-free or you may want introduce some sort of state to the parsing process. Almost all non-trivial parsers require state. Even if your grammar is context-free, state may allow for additional niceties. Earley does not support that.

  • Megaparsec’s error messages are more flexible allowing to include arbitrary data in them, return multiple error messages, mark regions that affect any error that happens in those regions, etc.

In other words, Megaparsec is less safe but also more powerful.

Related packages

The following packages are designed to be used with Megaparsec (open a PR if you want to add something to the list):

Prominent projects that use Megaparsec

Some prominent projects that use Megaparsec:

  • Idris—a general-purpose functional programming language with dependent types
  • Dhall—an advanced configuration language
  • hnix—re-implementation of the Nix language in Haskell
  • Hledger—an accounting tool
  • MMark—strict markdown processor for writers

Links to announcements and blog posts

Here are some blog posts mainly announcing new features of the project and describing what sort of things are now possible:


Issues (bugs, feature requests or otherwise feedback) may be reported in the GitHub issue tracker for this project.

Pull requests are also welcome. If you would like to contribute to the project, you may find this document helpful.


Copyright © 2015–present Megaparsec contributors
Copyright © 2007 Paolo Martini
Copyright © 1999–2000 Daan Leijen

Distributed under FreeBSD license.


Megaparsec follows SemVer.

Megaparsec 9.6.1

  • Exposed Text.Megaparsec.State, so that the new functions (initialState and initialPosState) can be actually imported from it. PR 549.

Megaparsec 9.6.0

  • Added the functions initialState and initialPosState to Text.Megaparsec.State. Issue 449.

Megaparsec 9.5.0

  • Dropped a number of redundant constraints here and there. PR 523.

  • Added a MonadWriter instance for ParsecT. PR 534.

Megaparsec 9.4.1

  • Removed Monad m constraints in several places where they were introduced in 9.4.0. Issue 532.

Megaparsec 9.4.0

  • dbg now prints hints among other debug information. PR 530.

  • Hints are no longer lost in certain methods of MTL instances for ParsecT. Issue 528.

  • Added a new method to the MonadParsec type class—mkParsec. This can be used to construct “new primitives” with arbitrary behavior at the expense of having to dive into Megaparsec’s internals. PR 514.

Megaparsec 9.3.1

  • Fixed a bug related to processing of tabs when error messages are rendered. Issue 524.

Megaparsec 9.3.0

  • Now label can override more than one group of hints in the parser it wraps. Issue 482.

  • takeP n now returns the empty chunk of the input stream when n is negative, similar to when n == 0. Issue 497.

  • Added the MonadParsecDbg type class in Text.Megaparsec.Debug. The type class allows us to use dbg in MTL monad transformers. Issue 488.

  • Introduced the ShareInput and NoShareInput newtype wrappers in Text.Megaparsec.Stream in order to allow the user to choose how the input should be sliced and shared during the parsing. Issue 492.

Megaparsec 9.2.2

  • Fixed a space leak in the implementations of the reachOffset and reachOffsetNoLine methods of TraversableStream. Issue 486.

Megaparsec 9.2.1

  • Builds with mtl-2.3 and transformers-0.6.

Megaparsec 9.2.0

  • Added parsers for binary representations (little/big endian) of numbers in Text.Megaparsec.Byte.Binary.

Megaparsec 9.1.0

  • Added dbg' in Text.Megaparsec.Debug for debugging parsers that have unshowable return values.

  • Documentation improvements.

Megaparsec 9.0.1

Megaparsec 9.0.0

  • Split the Stream type class. The methods showTokens and tokensLength have been put into a separate type class VisualStream, while reachOffset and reachOffsetNoLine are now in TraversableStream. This should make defining Stream instances for custom streams easier.

  • Defined Stream instances for lists and Seqs.

  • Added the functions hspace and hspace1 to the Text.Megaparsec.Char and Text.Megaparsec.Byte modules.

Megaparsec 8.0.0

  • The methods failure and fancyFailure of MonadParsec are now ordinary functions and live in Text.Megaparsec. They are defined in terms of the new parseError method of MonadParsec. This method allows us to signal parse errors at a given offset without manipulating parser state manually.

  • Megaparsec now supports registration of “delayed” parse errors. On lower level we added a new field called stateParseErrors to the State record. The type also had to change from State s to State s e. This field contains the list of registered ParseErrors that do not end parsing immediately but still will cause failure in the end if the list is not empty. Users are expected to register parse errors using the three functions: registerParseError, registerFailure, and registerFancyFailure. These functions are analogous to those without the register prefix, except that they have “delayed” effect.

  • Added the tokensLength method to the Stream type class to improve support for custom input streams.

  • Added the setErrorOffset function to set offset of ParseErrors.

  • Changed type signatures of reachOffset and reachOffsetNoLine methods of the Stream type class. Instead of three-tuple reachOffset now returns two-tuple because SourcePos is already contained in the returned PosState record.

  • Generalized decimal, binary, octal, and hexadecimal parsers in lexer modules so that they Num instead of just Integral.

  • Dropped support for GHC 8.2.x and older.

Megaparsec 7.0.5

  • Dropped support for GHC 7.10.

  • Adapted the code to MonadFail changes in base-4.13.

  • Separated the test suite into its own package. The reason is that we can avoid circular dependency on hspec-megaparsec and thus avoid keeping copies of its source files in our test suite, as we had to do before. Another benefit is that we can export some auxiliary functions in megaparsec-tests which can be used by other test suites, for example in the parser-combinators-tests package.

    Version of megaparsec-tests will be kept in sync with versions of megaparsec from now on.

Megaparsec 7.0.4

  • Numerous documentation corrections.

Megaparsec 7.0.3

  • Fixed the build with mtl older than 2.2.2.

Megaparsec 7.0.2

  • Fixed the property test for char' which was failing in the case when there is a character with different upper and title cases.

  • More descriptive error messages when elabel or ulabel from Text.Megaparsec.Error.Builder are used with empty strings.

  • Typo fixes in the docs.

Megaparsec 7.0.1

  • Fixed a bug in errorBundlePretty. Previously the question sign ? was erroneously inserted before offending line in 2nd and later parse errors.

Megaparsec 7.0.0


  • Dropped the Text.Megaparsec.Perm module. Use Control.Applicative.Permutations from parser-combinators instead.

  • Dropped the Text.Megaparsec.Expr module. Use Control.Monad.Combinators.Expr from parser-combinators instead.

  • The debugging function dbg has been moved from Text.Megaparsec to its own module Text.Megaparsec.Debug.

  • Dropped support for GHC 7.8.


  • Moved some general combinators from Text.Megaparsec.Char and Text.Megaparsec.Byte to Text.Megaparsec, renaming some of them for clarity.

    Practical consequences:

    • Now there is the single combinator that is a generalization of char for arbitrary streams. Text.Megaparsec.Char and Text.Megaparsec.Byte still contain char as type-constrained versions of single.

    • Similarly, now there is the chunk combinator that is a generalization of string for arbitrary streams. The string combinator is still re-exported from Text.Megaparsec.Char and Text.Megaparsec.Byte for compatibility.

    • satisfy does not depend on type of token, and so it now lives in Text.Megaparsec.

    • anyChar was renamed to anySingle and moved to Text.Megaparsec.

    • notChar was renamed to anySingleBut and moved to Text.Megaparsec.

    • oneOf and noneOf were moved to Text.Megaparsec.

  • Simplified the type of the token primitive. It now takes just a matching function Token s -> Maybe a as the first argument and the collection of expected items Set (ErrorItem (Token s)) as the second argument. This makes sense because the collection of expected items cannot depend on what we see in the input stream.

  • The label primitive now doesn’t prepend the phrase “the rest of” to the label when its inner parser produces hints after consuming input. In that case label has no effect.

  • Fixed the Text.Megaparsec.Char.Lexer.charLiteral so it can accept longer escape sequences (max length is now 10).

  • Added the binDigitChar functions in Text.Megaparsec.Byte and Text.Megaparsec.Char.

  • Added the binary functions in Text.Megaparsec.Byte.Lexer and Text.Megaparsec.Char.Lexer.

  • Improved case-insensitive character matching in the cases when e.g. isLower and isUpper both return False. Functions affected: Text.Megaparsec.Char.char'.

  • Renamed getPosition to getSourcePos.

  • Renamed getTokensProcessed to getOffset, setTokensProcessed to setOffset.

  • Dropped getTabWidth and setTabWidth because tab width is irrelevant to parsing process now, it’s only relevant for pretty-printing of parse errors, which is handled separately.

  • Added and withParsecT in Text.Megaparsec.Internal to allow changing the type of the custom data component in parse errors.

Parser state and input stream

  • Dropped stacks of source positions. Accordingly, the functions pushPosition and popPosition from Text.Megaparsec and sourcePosStackPretty from Text.Megaparsec.Error were removed. The reason for this simplification is that I could not find any code that uses the feature and it makes manipulation of source positions hairy.

  • Introduced PosState for calculating SourcePos from offsets and getting offending line for displaying on pretty-printing of parse errors. It’s now contained in both State and ParseErrorBundle.

  • Dropped positionAt1, positionAtN, advance1, and advanceN methods from Stream. They are no longer necessary because reachOffset (and its specialized version reachOffsetNoLine) takes care of SourcePos calculation.

Parse errors

  • ParseError now contains raw offset in input stream instead of SourcePos. errorPos was dropped from Text.Megaparsec.Error.

  • ParseError is now parametrized over stream type s instead of token type t.

  • Introduced ParseErrorBundle which contains one or more ParseError equipped with all information that is necessary to pretty-print them together with offending lines from the input stream. Functions like runParser now return ParseErrorBundle instead of plain ParseError.

    By default there will be only one ParseError in such a bundle, but it’s possible to add more parse errors to a bundle manually. During pretty-printing, the input stream will be traversed only once.

  • The primary function for pretty-printing of parse errors—errorBundlePretty always prints offending lines now. parseErrorPretty is still there, but it probably won’t see a lot of use from now on. parseErrorPretty' and parseErrorPretty_ were removed. parseTest' was removed because parseTest always prints offending lines now.

  • Added attachSourcePos function in Text.Megaparsec.Error.

  • The ShowToken type class has been removed and its method showTokens now lives in the Stream type class.

  • The LineToken type class is no longer necessary because the new method reachOffset of the type class Stream does its job.

  • In Text.Megaparsec.Error the following functions were added: mapParseError, errorOffset.

  • Implemented continuous highlighting in parse errors. For this we added the errorComponentLen method to the ShowErrorComponent type class.

Parse error builder

  • The functions err and errFancy now accept offsets at which the parse errors are expected to have happened, i.e. Ints. Thus posI and posN are no longer necessary and were removed.

  • ET is now parametrized over the type of stream s instead of token type t.

  • Combinators like utoks and etoks now accept chunks of input stream directly, i.e. Tokens s instead of [Token s] which should be more natural and convenient.

Megaparsec 6.5.0

  • Added Text.Megaparsec.Internal, which exposes some internal data structures and data constructor of ParsecT.

Megaparsec 6.4.1

  • scientific now correctly backtracks after attempting to parse fractional and exponent parts of a number. float correctly backtracks after attempting to parse optional exponent part (when it comes after fractional part, otherwise it’s obligatory).

Megaparsec 6.4.0

  • Text.Megaparsec now re-exports Control.Monad.Combinators instead of Control.Applicative.Combinators from parser-combinators because the monadic counterparts of the familiar combinators are more efficient and not as leaky.

    This may cause minor breakage in certain cases:

    • You import Control.Applicative and in that case there will be a name conflict between Control.Applicative.many and Control.Monad.Combinator.many now (the same for some).

    • You define a polymorphic helper in terms of combinator(s) from Control.Applicative.Combinators and use Applicative or Alternative constraint. In this case you’ll have to adjust the constraint to be Monad or MonadPlus respectively.

    Also note that the new Control.Monad.Combinators module we re-export now re-exports empty from Control.Applicative.

  • Fix the atEnd parser. It now does not produce hints, so when you use it, it won’t contribute to the “expecting end of input” component of parse error.

Megaparsec 6.3.0

  • Added an IsString instance for ParsecT. Now it is possible to write "abc" rather than string "abc".

  • Added the customFailure combinator, which is a special case of fancyFailure.

  • Made implementation of sconcat and mconcat of ParsecT more efficient.

Megaparsec 6.2.0

  • float in Text.Megaparsec.Char.Lexer and Text.Megaparsec.Byte.Lexer now does not accept plain integers. This is the behavior we had in version 5 of the library.

Megaparsec 6.1.1

  • Fixed the bug when tokens used cok continuation even when matching an empty chunk. Now it correctly uses eok in this case.

Megaparsec 6.1.0

  • Improved rendering of offending line in parseErrorPretty' in the presence of tab characters.

  • Added parseErrorPretty_, which is just like parseErrorPretty' but allows to specify tab width to use.

  • Adjusted hint generation so when we backtrack a consuming parser with try, we do not create hints from its parse error (because it’s further in input stream!). This was a quite subtle bug that stayed unnoticed for several years apparently.

Megaparsec 6.0.2

  • Allow parser-combinators-0.2.0.

Megaparsec 6.0.1

  • Fixed a typo in

  • Added some text that clarifies how to parametrize the ParseError type.

Megaparsec 6.0.0


  • Re-organized the module hierarchy. Some modules such as Text.Megaparsec.Prim do not exist anymore. Stream definitions were moved to Text.Megaparsec.Stream. Generic combinators are now re-exported from the Control.Applicative.Combinators from the package parser-combinators. Just import Text.Megaparsec and you should be OK. Add Text.Megaparsec.Char if you are working with a stream of Chars or Text.Megaparsec.Byte if you intend to parse binary data, then add qualified modules you need (permutation parsing, lexing, expression parsing, etc.). Text.Megaparsec.Lexer was renamed to Text.Megaparsec.Char.Lexer because many functions in it has the Token s ~ Char constraint. There is also Text.Megaparsec.Byte.Lexer now, although it has fewer functions.

  • Dropped per-stream modules, the Parser type synonym is to be defined manually by user.

  • Added a MonadFix instance for ParsecT.

  • More lightweight dependency tree, dropped exceptions and QuickCheck dependencies.

  • Added dependency on case-insensitive.

Source positions

  • Now Pos contains an Int inside, not Word.

  • Dropped unsafePos and changed type of mkPos so it throws from pure code if its argument is not a positive Int.

  • Added pos1 constant that represents the Pos with value 1 inside.

  • Made InvalidPosException contain the invalid Int value that was passed to mkPos.

Parse errors

  • Changed the definition of ParseError to have separate data constructors for “trivial” errors (unexpected/expected tokens) and “fancy” errors (everything else).

  • Removed the ErrorComponent type class, added ErrorFancy instead. ErrorFancy is a sum type which can represent fail messages, incorrect indentation, and custom data (we use Void for that by default to “disable” it). This is better than the typeclass-based approach because every instance of ErrorComponent needed to have constructors for fail and indentation massages anyway, leading to duplication of code (for example for parse error component rendering).

  • Added Functor instances for ErrorItem and ErrorFancy.

  • Added the function errorPos to get error positions from ParseError (previously it was a record selector in ParseError).

  • Control characters in parse error are displayed in a readable form even when they are part of strings, for example: {<newline> ({ followed by the newline character). Previously control characters were rendered in readable form only as standalone tokens.

  • Added Text.Megaparsec.Error.Builder module to help construct ParseErrors easily. It is useful for testing and debugging. Previously we had something like that in the hspec-megaparsec package, but it does not hurt to ship it with the library.

  • Added parseErrorPretty' allowing to display offending line in parse errors.

  • Added LineToken type class for tokens that support operations necessary for selecting and displaying relevant line of input (used in parseErrorPretty').

  • Added parseTest' function that is just like parseTest, but also prints offending line in parse errors. This is powered by the new parseErrorPretty'.


  • Introduced the new Text.Megaparsec.Stream module that is the home of Stream type class. In version 6, the type class has been extended significantly to improve performance and make some combinators more general.


  • Changed signatures of failure and token, they only can signal trivial errors now.

  • Added a new method of MonadParsec type class called fancyFailure for signalling non-trivial failures. Signatures of some functions (failure, token) have been changed accordingly.

  • Added takeWhileP, takeWhile1P and takeP to MonadParsec.

  • Added takeRest non-primitive combinator to consume the rest of input.

  • Added atEnd which returns True when end of input has been reached.

  • Dropped oneOf' and noneOf' from Text.Megaparsec.Char. These were seldom (if ever) used and are easily re-implemented.

  • Added notChar in Text.Megaparsec.Char.

  • Added space1 in Text.Megaparsec.Char. This parser is like space but requires at least one space character to be present to succeed.

  • Added new module Text.Megaparsec.Byte, which is similar to Text.Megaparsec.Char, but for token streams of the type Word8 instead of Char.

  • integer was dropped from Text.Megaparsec.Char.Lexer. Use decimal instead.

  • number was dropped from Text.Megaparsec.Char.Lexer. Use scientific instead.

  • decimal, octal, and hexadecimal are now polymorphic in their return type and can be used to parse any instance of Integral.

  • float is now polymorphic in its return type and can be used to parse any instance of RealFloat.

  • Added new module Text.Megaparsec.Byte.Lexer, which provides some functions (white space and numeric helpers) from Text.Megaparsec.Char.Lexer for streams with token type Word8.

Megaparsec 5.3.1

  • Various updates to the docs.

  • Allowed QuickCheck-2.10.

Megaparsec 5.3.0

  • Added the match combinator that allows to get collection of consumed tokens along with result of parsing.

  • Added the region combinator which allows to process parse errors happening when its argument parser is run.

  • Added the getNextTokenPosition, which returns position where the next token in the stream begins.

  • Defined Semigroup and Monoid instances of ParsecT.

  • Dropped support for GHC 7.6.

  • Added an ErrorComponent instance for ().

Megaparsec 5.2.0

  • Added MonadParsec instance for RWST.

  • Allowed many to run parsers that do not consume input. Previously this signalled an error which was ugly. Of course, in most cases giving many a parser that do not consume input will lead to non-termination bugs, but there are legal cases when this should be allowed. The test suite now contains an example of this. Non-termination issues is something inherited from the power Megaparsec gives (with more power comes more responsibility), so that error case in many really does not solve the problem, it was just a little ah-hoc guard we got from Parsec’s past.

  • The criterion benchmark was completely re-written and a new weigh benchmark to analyze memory consumption was added.

  • Performance improvements: count (marginal improvement, simpler implementation), count' (considerable improvement), and many (marginal improvement, simpler implementation).

  • Added stateTokensProcessed field to parser state and helper functions getTokensProcessed and setTokensProcessed. The field contains number of processed tokens so far. This allows, for example, create wrappers that return just parsed fragment of input stream alongside with result of parsing. (It was possible before, but very inefficient because it required traversing entire input stream twice.)

  • IndentNone option of indentBlock now picks whitespace after it like its sisters IndentMany and IndentSome do, see #161.

  • Fixed a couple of quite subtle bugs in indentBlock introduced by changing behaviour of skipLineComment in version 5.1.0. See #178 for more information.

Megaparsec 5.1.2

  • Stopped using property tests with dbg helper to avoid flood of debugging info when test suite is run.

  • Fixed the build with QuickCheck versions older than 2.9.0.

Megaparsec 5.1.1

  • Exported the observing primitive from Text.Megaparsec.

Megaparsec 5.1.0

  • Defined displayException for ParseError, so exceptions are displayed in human-friendly form now. This works with GHC 7.10 and later.

  • Line comments parsed by skipLineComment now may end at the end of input and do not necessarily require a newline to be parsed correctly. See #119.

  • Exposed parseErrorTextPretty function in Text.Megaparsec.Error to allow to render ParseErrors without stack of source positions.

  • Eliminated the old-tests test suite — Parsec legacy. The cases that are not already obviously covered in the main test suite were included into it.

  • Added Arbitrary instances for the following data types: Pos, SourcePos, ErrorItem, Dec, ParseError and State. This should make testing easier without the need to add orphan instances every time. The drawback is that we start to depend on QuickCheck, but that’s a fair price.

  • The test suite now uses the combination of Hspec and the hpesc-megaparsec package, which also improved the latter (that package is the recommended way to test Megaparsec parsers).

  • The try combinator now truly backtracks parser state when its argument parser fails (either consuming input or not). Most users will never notice the difference though. See #142.

  • Added the dbg function that should be helpful for debugging.

  • Added observing primitive combinator that allows to “observe” parse errors without ending parsing (they are returned in Left, while normal results are wrapped in Right).

  • Further documentation improvements.

Megaparsec 5.0.1

  • Derived NFData instances for Pos, InvalidPosException, SourcePos, ErrorItem, Dec, ParseError, and State.

  • Derived Data instance for ParseError, Data and Typeable instances for SourcePos and State.

  • Minor documentation improvements.

Megaparsec 5.0.0

General changes

  • Removed parseFromFile and StorableStream type-class that was necessary for it. The reason for removal is that reading from file and then parsing its contents is trivial for every instance of Stream and this function provides no way to use newer methods for running a parser, such as runParser'. So, simply put, it adds little value and was included in 4.x versions for compatibility reasons.

  • Moved position-advancing function from arguments of token and tokens functions to Stream type class (named updatePos). The new function allows to handle custom streams of tokens where every token contains information about its position in stream better (for example when stream of tokens is produced with happy/alex).

  • Support for include files (stack of positions instead of flat position) added. The new functions pushPosition and popPosition can be used to move “vertically” in the stack of positions. getPosition and setPosition still work on top (“current file”) level, but user can get full stack via getParserState if necessary. Note that ParseError and pretty-printing for it also support the new feature.

  • Added type function Token associated with Stream type class. The function returns type of token corresponding to specific token stream.

  • Type ParsecT (and also type synonym Parsec) are now parametrized over type of custom component in parse errors.

  • Parameters of MonadParsec type class are: e — type of custom component in parse errors, s — type of input stream, and m — type of underlying monad.

  • Type of failure primitive combinator was changed, now it accepts three arguments: set of unexpected items, set of expected items, and set of custom data.

  • Type of token primitive combinator was changed, now in case of failure a triple-tuple is returned with elements corresponding to arguments of failure primitive. The token primitive can also be optionally given an argument of token type to use in error messages (as expected item) in case of end of input.

  • unexpected combinator now accepts argument of type ErrorItem instead of plain String.

  • General performance improvements and improvements in speed of some combinators, manyTill in particular.

Error messages

  • The module Text.Megaparsec.Pos was completely rewritten. The new module uses Pos data type with smart constructors to ensure that things like line and column number can be only positive. SourcePos on the other hand does not require smart constructors anymore and its constructors are exported. Show and Read instances of SourcePos are derived and pretty-printing is done with help of sourcePosPretty function.

  • The module Text.Megaparsec.Error was completely rewritten. A number of new types and type-classes are introduced: ErrorItem, Dec, ErrorComponent, and ShowErrorComponent. ParseError does not need smart constructors anymore and its constructor and field selectors are exported. It uses sets (from the containers package) instead of sorted lists to enumerate unexpected and expected items. The new definition is also parametrized over token type and custom data type which can be passed around as part of parse error. Default “custom data” component is Dec, which see. All in all, we have completely well-typed and extensible error messages now. Show and Read instances of ParseError are derived and pretty-printing is done with help of parseErrorPretty.

  • The module Text.Megaparsec.ShowToken was eliminated and type class ShowToken was moved to Text.Megaparsec.Error. The only method of that class in now named showTokens and it works on streams of tokens, where single tokes are represented by NonEmpty list with single element.

Built-in combinators

  • Combinators oneOf, oneOf', noneOf, and noneOf' now accept any instance of Foldable, not only String.


  • Error messages about incorrect indentation levels were greatly improved. Now every such message contains information about desired ordering between “reference” indentation level and actual indentation level as well as values of these levels. The information is stored in ParseError in well-typed form and can be pretty-printed when necessary. As part of this improvement, type of indentGuard was changed.

  • incorrectIndent combinator is introduced in Text.Megaparsec.Lexer module. It allows to fail with detailed information regarding incorrect indentation.

  • Introduced scientific parser that can parse arbitrary big numbers without error or memory overflow. float still returns Double, but it’s defined in terms of scientific now. Since Scientific type can reliably represent integer values as well as floating point values, number now returns Scientific instead of Either Integer Double (Integer or Double can be extracted from Scientific value anyway). This in turn makes signed parser more natural and general, because we do not need ad-hoc Signed type class anymore.

  • Added skipBlockCommentNested function that should help parse possibly nested block comments.

  • Added lineFold function that helps parse line folds.

Megaparsec 4.4.0

  • Now state returned on failure is the exact state of parser at the moment when it failed, which makes incremental parsing feature much better and opens possibilities for features like “on-the-fly” recovering from parse errors.

  • The count combinator now works with Applicative instances (previously it worked only with instances of Alternative). It’s now also faster.

  • tokens and parsers built upon it (such as string and string') backtrack automatically on failure now, that is, when they fail, they never consume any input. This is done to make their consumption model match how error messages are reported (which becomes an important thing as user gets more control with primitives like withRecovery). This means, in particular, that it’s no longer necessary to use try with tokens-based parsers. This new feature does not affect performance in any way.

  • New primitive parser withRecovery added. The parser allows to recover from parse errors “on-the-fly” and continue parsing. Once parsing is finished, several parse errors may be reported or ignored altogether.

  • eitherP combinator added.

  • Removed Enum instance of Message type. This was Parsec’s legacy that we should eliminate now. Message does not constitute enumeration, toEnum was never properly defined for it. The idea to use fromEnum to determine type of Message is also ugly, for this purpose new functions isUnexpected, isExpected, and isMessage are defined in Text.Megaparsec.Error.

  • Minor tweak in signature of MonadParsec type class. Collection of constraints changed from Alternative m, Monad m, Stream s t to Alternative m, MonadPlus m, Stream s t. This is done to make it easier to write more abstract code with older GHC where such primitives as guard are defined for instances of MonadPlus, not Alternative.

Megaparsec 4.3.0

  • Canonicalized Applicative/Monad instances. Thanks to Herbert Valerio Riedel.

  • Custom messages in ParseError are printed each on its own line.

  • Now accumulated hints are not used with ParseError records that have only custom messages in them (created with Message constructor, as opposed to Unexpected or Expected). This strips “expected” line from custom error messages where it’s unlikely to be relevant anyway.

  • Added higher-level combinators for indentation-sensitive grammars: indentLevel, nonIndented, and indentBlock.

Megaparsec 4.2.0

  • Made newPos constructor and other functions in Text.Megaparsec.Pos smarter. Now it’s impossible to create SourcePos with non-positive line number or column number. Unfortunately we cannot use Numeric.Natural because we need to support older versions of base.

  • ParseError is now a monoid. mergeError is used as mappend.

  • Added functions addErrorMessages and newErrorMessages to add several messages to existing error and to construct error with several attached messages respectively.

  • parseFromFile now lives in Text.Megaparsec.Prim. Previously we had 5 nearly identical definitions of the function, varying only in type-specific readFile function. Now the problem is solved by introduction of StorableStream type class. All supported stream types are instances of the class out of box and thus we have polymorphic version of parseFromFile.

  • ParseError is now instance of Exception (and Typeable).

  • Introduced runParser' and runParserT' functions that take and return parser state. This makes it possible to partially parse input, resume parsing, specify non-standard initial textual position, etc.

  • Introduced failure function that allows to fail with arbitrary collection of messages. unexpected is now defined in terms of failure. One consequence of this design decision is that failure is now method of MonadParsec, while unexpected is not.

  • Removed deprecated combinators from Text.Megaparsec.Combinator:

    • chainl
    • chainl1
    • chainr
    • chainr1
  • number parser in Text.Megaparsec.Lexer now can be used with signed combinator to parse either signed Integer or signed Double.

Megaparsec 4.1.1

  • Fixed bug in implementation of sepEndBy and sepEndBy1 and removed deprecation notes for these functions.

  • Added tests for sepEndBy and sepEndBy1.

Megaparsec 4.1.0

  • Relaxed dependency on base, so that minimal required version of base is now This allows Megaparsec to compile with GHC 7.6.x.

  • Text.Megaparsec and Text.Megaparsec.Prim do not export data types Consumed and Reply anymore because they are rather low-level implementation details that should not be visible to end-user.

  • Representation of file name and textual position in error messages was made conventional.

  • Fixed some typos is documentation and other materials.

Megaparsec 4.0.0

General changes

  • Renamed many1some as well as other parsers that had many1 part in their names.

  • The following functions are now re-exported from Control.Applicative: (<|>), many, some, optional. See #9.

  • Introduced type class MonadParsec in the style of MTL monad transformers. Eliminated built-in user state since it was not flexible enough and can be emulated via stack of monads. Now all tools in Megaparsec work with any instance of MonadParsec, not only with ParsecT.

  • Added new function parseMaybe for lightweight parsing where error messages (and thus file name) are not important and entire input should be parsed. For example it can be used when parsing of single number according to specification of its format is desired.

  • Fixed bug with notFollowedBy always succeeded with parsers that don’t consume input, see #6.

  • Flipped order of arguments in the primitive combinator label, see #21.

  • Renamed tokenPrimtoken, removed old token, because tokenPrim is more general and original token is little used.

  • Made token parser more powerful, now its second argument can return Either [Message] a instead of Maybe a, so it can influence error message when parsing of token fails. See #29.

  • Added new primitive combinator hidden p which hides “expected” tokens in error message when parser p fails.

  • Tab width is not hard-coded anymore. It can be manipulated via getTabWidth and setTabWidth. Default tab-width is defaultTabWidth, which is 8.

Error messages

  • Introduced type class ShowToken and improved representation of characters and strings in error messages, see #12.

  • Greatly improved quality of error messages. Fixed entire Text.Megaparsec.Error module, see #14 for more information. Made possible normal analysis of error messages without “render and re-parse” approach that previous maintainers had to practice to write even simplest tests, see module Utils.hs in old-tests for example.

  • Reduced number of Message constructors (now there are only Unexpected, Expected, and Message). Empty “magic” message strings are ignored now, all the library now uses explicit error messages.

  • Introduced hint system that greatly improves quality of error messages and made code of Text.Megaparsec.Prim a lot clearer.

Built-in combinators

  • All built-in combinators in Text.Megaparsec.Combinator now work with any instance of Alternative (some of them even with Applicative).

  • Added more powerful count' parser. This parser can be told to parse from m to n occurrences of some thing. count is defined in terms of count'.

  • Removed optionMaybe parser, because optional from Control.Applicative does the same thing.

  • Added combinator someTill.

  • These combinators are considered deprecated and will be removed in future:

    • chainl
    • chainl1
    • chainr
    • chainr1
    • sepEndBy
    • sepEndBy1

Character parsing

  • Renamed some parsers:

    • alphaNumalphaNumChar
    • digitdigitChar
    • endOfLineeol
    • hexDigithexDigitChar
    • letterletterChar
    • lowerlowerChar
    • octDigitoctDigitChar
    • spacespaceChar
    • spacesspace
    • upperupperChar
  • Added new character parsers in Text.Megaparsec.Char:

    • asciiChar
    • charCategory
    • controlChar
    • latin1Char
    • markChar
    • numberChar
    • printChar
    • punctuationChar
    • separatorChar
    • symbolChar
  • Descriptions of old parsers have been updated to accent some Unicode-specific moments. For example, old description of letter stated that it parses letters from “a” to “z” and from “A” to “Z”. This is wrong, since it used Data.Char.isAlpha predicate internally and thus parsed many more characters (letters of non-Latin languages, for example).

  • Added combinators char', oneOf', noneOf', and string' which are case-insensitive variants of char, oneOf, noneOf, and string respectively.


  • Rewritten parsing of numbers, fixed #2 and #3 (in old Parsec project these are number 35 and 39 respectively), added per bug tests.

    • Since Haskell report doesn’t say anything about sign, integer and float now parse numbers without sign.

    • Removed natural parser, it’s equal to new integer now.

    • Renamed naturalOrFloatnumber — this doesn’t parse sign too.

    • Added new combinator signed to parse all sorts of signed numbers.

  • Transformed Text.Parsec.Token into Text.Megaparsec.Lexer. Little of Parsec’s code remains in the new lexer module. New module doesn’t impose any assumptions on user and should be vastly more useful and general. Hairy stuff from original Parsec didn’t get here, for example built-in Haskell functions are used to parse escape sequences and the like instead of trying to re-implement the whole thing.


  • Renamed the following functions:

    • permutemakePermParser
    • buildExpressionParsermakeExprParser
  • Added comprehensive QuickCheck test suite.

  • Added benchmarks.

Parsec 3.1.9

  • Many and various updates to documentation and package description (including the homepage links).

  • Add an Eq instance for ParseError.

  • Fixed a regression from 3.1.6: runP is again exported from module Text.Parsec.

Parsec 3.1.8

  • Fix a regression from 3.1.6 related to exports from the main module.

Parsec 3.1.7

  • Fix a regression from 3.1.6 related to the reported position of error messages. See bug #9 for details.

  • Reset the current error position on success of lookAhead.

Parsec 3.1.6

  • Export Text instances from Text.Parsec.

  • Make Text.Parsec exports more visible.

  • Re-arrange Text.Parsec exports.

  • Add functions crlf and endOfLine to Text.Parsec.Char for handling input streams that do not have normalized line terminators.

  • Fix off-by-one error in Token.charControl.

Parsec 3.1.4 & 3.1.5

  • Bump dependency on text.

Parsec 3.1.3

  • Fix a regression introduced in 3.1.2 related to positions reported by error messages.