megaparsec: Monadic parser combinators

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This is industrial-strength monadic parser combinator library. Megaparsec is a fork of Parsec library originally written by Daan Leijen.

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Dependencies base (>=4.6 && <5), bytestring, containers (>=0.5 && <0.6), exceptions (>=0.6 && <0.9), fail (>=4.9 && <4.10), mtl (>=2 && <3), scientific (>=0.3.1 && <0.4), semigroups (>=0.18 && <0.19), tagged (>=0.8 && <0.9), text (>=0.2), transformers (>=0.4 && <0.6) [details]
License BSD-2-Clause
Author Megaparsec contributors, Paolo Martini <>, Daan Leijen <>
Maintainer Mark Karpov <>
Category Parsing
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Source repo head: git clone
Uploaded by mrkkrp at 2016-05-14T10:22:49Z
Distributions Arch:9.5.0, Debian:8.0.0, Fedora:9.3.1, LTSHaskell:9.5.0, NixOS:9.5.0, Stackage:9.6.1, openSUSE:9.5.0
Reverse Dependencies 276 direct, 426 indirect [details]
Downloads 100885 total (617 in the last 30 days)
Rating 2.75 (votes: 24) [estimated by Bayesian average]
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Status Docs available [build log]
Last success reported on 2016-05-14 [all 1 reports]

Readme for megaparsec-5.0.0

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This is an industrial-strength monadic parser combinator library. Megaparsec is a fork of Parsec library originally written by Daan Leijen.


This 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, a MTL-style monad transformer. All tools and features work with any instance of MonadParsec. You can achieve various effects combining monad transformers, i.e. building monad stack. Since most common monad transformers like WriterT, StateT, ReaderT and others are instances of MonadParsec, you can wrap ParsecT in these monads, achieving, for example, backtracking state.

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

The module Text.Megaparsec.Combinator (its functions are included in Text.Megaparsec) contains traditional, general combinators that work with any instance of Alternative and some even with instances of Applicative.

Role of Monad, Applicative, and Alternative should be obvious, so let's enumerate methods of MonadParsec type class. The class represents core, basic functions of Megaparsec parsing. The rest of library is built via combination of these primitives:

  • failure allows to fail with arbitrary collection of messages.

  • label allows to add a “label” to any parser, so when it fails the user will see the label in the error message where “expected” items are enumerated.

  • hidden hides any parser from error messages altogether, this is officially recommended way to hide things, prefer it to the label "" approach.

  • try enables backtracking in parsing.

  • lookAhead allows to parse something without consuming input.

  • notFollowedBy succeeds when its argument fails, it does not consume input.

  • withRecovery 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.

  • eof only succeeds at the end of input.

  • token is used to parse single token.

  • tokens makes it easy to parse several tokens in a row.

  • getParserState returns full parser state.

  • updateParserState applies given function on parser state.

This list of core functions is longer than in some other libraries. Our goal was efficient and readable implementation of functionality provided by every such primitive, not minimal number of them. You can read the comprehensive description of every primitive function in Megaparsec documentation.

Megaparsec can currently work with the following types of input stream:

  • String = [Char]

  • ByteString (strict and lazy)

  • Text (strict and lazy)

Error messages

Megaparsec 5 introduces well-typed error messages and ability to use custom data types to adjust the library to your domain of interest. No need to keep your info as shapeless bunch of strings anymore.

The default error component (Dec) has constructors corresponding to fail function and indentation-related error messages. It is a decent option that should work out-of-box for most parsing needs, while you are free to use your own custom error component when necessary with little effort.

This new design allowed Megaparsec 5 to have much more helpful error messages for indentation-sensitive parsing instead of plain “incorrect indentation” phrase.

Alex/Happy support

Megaparsec works well with streams of tokens produced by tools like Alex/Happy. Megaparsec 5 adds updatePos method to Stream type class that gives you full control over textual positions that are used to report token positions in error messages. You can update current position on per character basis or extract it from token — all cases are covered.

Character parsing

Megaparsec has decent support for Unicode-aware character parsing. Functions for character parsing live in Text.Megaparsec.Char (they all are included in Text.Megaparsec). The functions can be divided into several categories:

  • Simple parsers — parsers that parse certain character or several characters of the same kind. This includes newline, crlf, eol, tab, and space.

  • Parsers corresponding to categories of characters parse single character that belongs to certain category of characters, for example: controlChar, spaceChar, upperChar, lowerChar, printChar, digitChar, and others.

  • General parsers that allow you to parse a single character you specify or one of given characters, or any character except for given ones, or character satisfying given predicate. Case-insensitive versions of the parsers are available.

  • Parsers for sequences of characters parse strings. These are more efficient and provide better error messages than other approaches most programmers can come up with. Case-sensitive string parser is available as well as case-insensitive string'.

Permutation parsing

For those who are interested in parsing of permutation phrases, there is Text.Megaparsec.Perm. You have to import the module explicitly, it's not included in the Text.Megaparsec module.

Expression parsing

Megaparsec has a solution for parsing of expressions. Take a look at Text.Megaparsec.Expr. You have to import the module explicitly, it's not included in the Text.Megaparsec.

Given a table of operators that describes their fixity and precedence, you can construct a parser that will parse any expression involving the operators. See documentation for comprehensive description of how it works.


Text.Megaparsec.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.Lexer is intended to be imported qualified, 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 you quickly solve simple tasks and doesn't get in your way when you want to implement something less standard.

Since Megaparsec 5, all tools for indentation-sensitive parsing are available in Text.Megaparsec.Lexer module — no third party packages required.


Megaparsec is well-documented. All functions and data-types are thoroughly described. We pay attention to avoid outdated info or unclear phrases in our documentation. See the current version of Megaparsec documentation on Hackage for yourself.


You can visit site of the project which has several tutorials that should help you to start with your parsing tasks. The site also has instructions and tips for Parsec users who decide to switch.


Despite being quite flexible, Megaparsec is also faster than Parsec. The repository includes benchmarks that can be easily used to compare Megaparsec and Parsec. In most cases Megaparsec is faster, sometimes dramatically faster. If you happen to have some other benchmarks, I would appreciate if you add Megaparsec to them and let me know how it performs.

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 and Attoparsec

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

  • Attoparsec is much 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 monad transformer.

So, if you work with something human-readable where size of input data is usually not huge, just go with Megaparsec, otherwise Attoparsec may be a better choice.

Megaparsec and Parsec

Since Megaparsec is a fork of Parsec, it's necessary to list main differences between the two libraries:

  • Better error messages. We test our error messages using dense QuickCheck tests. Good error messages are just as important for us as correct return values of our parsers. Megaparsec will be especially useful if you write compiler or interpreter for some language.

  • Some quirks and “buggy features” (as well as plain bugs) of original Parsec are fixed. There is no undocumented surprising stuff in Megaparsec.

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

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

  • Comprehensive QuickCheck test suite covering nearly 100% of our code.

  • We have benchmarks to detect performance regressions.

  • Better documentation, with 100% of functions covered, without typos and obsolete information, with working examples. Megaparsec's documentation is well-structured and doesn't contain things useless to end users.

  • Megaparsec's code is clearer and doesn't contain “magic” found in original Parsec.

  • Megaparsec has well-typed error messages and custom error messages.

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

  • Megaparsec is faster.

If you want to see a detailed change log, may be helpful.

To be honest Parsec's development has seemingly stagnated. It has no test suite (only three per-bug tests), and all its releases beginning from version 3.1.2 (according or its change log) were about introducing and fixing regressions. Parsec is old and somewhat famous in Haskell community, so we understand there will be some kind of inertia, but we advise you use Megaparsec from now on because it solves many problems of original Parsec project. If you think you still have a reason to use original Parsec, open an issue.

Megaparsec and Parsers

There is Parsers package, which is great. You can use it with Megaparsec or Parsec, but consider the following:

  • It depends on both Attoparsec and Parsec, which means you always grab useless code installing it. This is ridiculous, by the way, because this package is supposed to be useful for parser builders, so they can write basic core functionality and get the rest “for free”. But with these useful functions you get two more parsers as dependencies.

  • It currently has a bug in definition of lookAhead for various monad transformers like StateT, etc. which is visible when you create backtracking state via monad stack, not via built-in features.

We intended to use Parsers library in Megaparsec at some point, but aside from already mentioned flaws the library has different conventions for naming of things, different set of “core” functions, etc., different approach to lexer. So it didn't happen, Megaparsec has minimal dependencies, it is feature-rich and self-contained.

The following packages are designed to be used with Megaparsec:


The project was started and is currently maintained by Mark Karpov. You can find complete list of contributors in file in official repository of the project. Thanks to all the people who propose features and ideas, although they are not in, without them Megaparsec would not be that good.


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

Pull requests are also welcome (and yes, they will get attention and will be merged quickly if they are good, we are progressive folks).

If you want to write a tutorial to be hosted on Megaparsec's site, open an issue or pull request here.


Copyright © 2015–2016 Megaparsec contributors
Copyright © 2007 Paolo Martini
Copyright © 1999–2000 Daan Leijen

Distributed under FreeBSD license.