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.
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
ReaderT and others are instances of
you can wrap
ParsecT in these monads, achieving, for example,
On the other hand
ParsecT is instance of many type classes as well. The
most useful ones are
(its functions are included in
Text.Megaparsec) contains traditional,
general combinators that work with any instance of
Alternative and some
even with instances of
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
try enables backtracking in parsing.
lookAhead allows to parse something without consuming input.
notFollowedBy succeeds when its argument fails, it does not consume
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 function is longer than in some other libraries. Our goal
was easy 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:
Megaparsec has decent support for Unicode-aware character parsing. Functions
for character parsing live in
Text.Megaparsec.Char (they all are
Text.Megaparsec). The functions can be divided into several
Simple parsers — parsers that parse certain character or several
characters of the same kind. This includes
Parsers corresponding to categories of characters parse single character
that belongs to certain category of characters, for example:
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
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
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
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.
is a module that should help you write your lexer. If you have used
in the past, this module “fixes” its particularly inflexible
Text.Megaparsec.Lexer is intended to be imported qualified, it's not
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,
The design of the module allows you quickly solve simple tasks and doesn't
get in your way when you want to implemented something less standard.
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
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 matter
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
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
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
Megaparsec has more powerful combinators and can parse languages where
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
If you want to see a detailed change log,
CHANGELOG.md 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
Megaparsec and Parsers
There is Parsers package,
which is great. You can use it with Megaparsec or Parsec, but consider the
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
StateT, etc. which is visible when you create
backtracking state via monad stack, not via built-in features. See #27.
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 project was started and is currently maintained by Mark Karpov. You can
find complete list of contributors in
AUTHORS.md file in official
repository of the project. Thanks to all the people who propose features and
ideas, although they are not in
AUTHORS.md, 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 Megaparsec contributors
Copyright © 2007 Paolo Martini
Copyright © 1999–2000 Daan Leijen
Distributed under FreeBSD license.