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, an MTL-style monad transformer.
All tools and features work with all instances of
MonadParsec. You can
achieve various effects combining monad transformers, i.e. building monad
stack. Since the standard common monad transformers like
ReaderT and others are instances of the
class, you can wrap
ParsecT in these monads, achieving, for example,
On the other hand
ParsecT is an instance of many type classes as well. The
most useful ones are
Megaparsec includes all functionality that is available in Parsec plus
features some combinators that are missing in other parsing libraries:
failure allows to fail reporting a parse error with unexpected and
fancyFailure allows to fail reporting custom error messages.
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.
observing allows to “observe” parse errors without ending parsing (they
are returned in
Left, while normal results are wrapped in
In addition to that, Megaparsec 6 features high-performance combinators
similar to those found in Attoparsec:
tokens makes it easy to parse several tokens in a row (
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
without any repacking.
takeWhile1 are about 150 times faster than approaches
manyTill and other similar combinators.
takeP allows to grab n tokens from the stream and returns them as a
“chunk” of the stream.
So now that we have matched the main “performance boosters” of Attoparsec,
Megaparsec 6 is not significantly slower than Attoparsec if you write your
Megaparsec can currently work with the following types of input stream
ByteString (strict and lazy)
Text (strict and lazy)
It's also simple to make it work with custom token streams, and Megaparsec
users have done so many times with great success.
Megaparsec 5 introduces well-typed error messages and the ability to use
custom data types to adjust the library to specific domain of interest. No
need to use a shapeless bunch of strings anymore.
The design of parse errors has been revised in version 6 significantly, but
custom errors are still easy (probably even easier now).
Megaparsec works well with streams of tokens produced by tools like Alex.
The design of the
Stream type class has been changed significantly in
version 6, but user can still work with custom streams of tokens without
Megaparsec has decent support for Unicode-aware character parsing. Functions
for character parsing live in the
The functions can be divided into several categories:
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 the given characters, or any character except for the given
ones, or character satisfying given predicate. Case-insensitive versions
of the parsers are available.
Parsers for sequences of characters parse strings. Case-sensitive
string parser is available as well as case-insensitive
Similarly, there is
for parsing streams of bytes.
For those who are interested in parsing of permutation phrases, there
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.Char.Lexer is intended to be imported via 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 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
Text.Megaparsec.Char.Lexer module—no third party packages
Text.Megaparsec.Byte.Lexer is also available for users who wish to parse
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 find Megaparsec
should provide sufficient guidance to help you to start with your parsing
tasks. The site also has instructions and tips for Parsec users who decide
to migrate to Megaparsec.
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.
Additional benchmarks created to guide development of Megaparsec 6 can be
found here. These compare 3 pairs
of parsers written using Attoparsec and Megaparsec.
If you think your Megaparsec parser is not efficient enough, take a look
at these instructions.
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 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
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
Since version 6, Megaparsec features the same fast primitives that
Attoparsec has, so in many cases the difference in speed is not that big.
Megaparsec now aims to be “one size fits all” ultimate solution to parsing,
so it can be used even to parse low-level binary formats.
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. 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 a
compiler or an interpreter for some language.
Megaparsec 6 can show 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 “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
Megaparsec has well-typed error messages and custom error messages.
Megaparsec can recover from parse errors “on the fly” and continue
Megaparsec allows to conditionally process parse errors inside your
parser before parsing is finished. 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. This is not possible with Parsec.
Megaparsec is faster and supports efficient operations on top of
takeP just like Attoparsec.
If you want to see a detailed change log,
CHANGELOG.md may be helpful.
Also see this original announcement for another
Parsec is old and somewhat famous in the 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 the original Parsec project. If
you think you still have a reason to use original Parsec, open an issue.
Megaparsec vs Trifecta
Trifecta is another Haskell
library featuring good error messages. It's probably good, but also
under-documented, and has
unfixed bugs and flaws that
Edward is too busy to fix (simply a fact, no offense intended). Other
reasons one may question choice of Trifecta is his/her parsing library:
Complicated, doesn't have any tutorials available, and documentation
doesn't help at all.
Trifecta can parse
ByteString natively, but not
Trifecta's error messages may be different with their own features, but
certainly not as flexible as Megaparsec's error messages in the latest
lens. This means you'll pull in half of Hackage as transitive
dependencies. Also if you're not into
lens and would like to keep your
code “vanilla”, you may not like the API.
Megaparsec vs Earley
Earley is a newer library that
allows to safely (it your code compiles, then it probably works) parse
context-free grammars (CFG). Megaparsec is a lower-level library compared to
Earley, but there are still enough reasons to choose it over Earley:
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
something of this sort. Even if your grammar is context-free, state may
allow to add some 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.
The approach Earley uses differs from the conventional monadic parsing. If
you work not alone, people you work with, especially beginners, will be
much more productive with libraries taking more traditional path to
parsing like Megaparsec.
IOW, Megaparsec is less safe but also more powerful.
Megaparsec vs 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. 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”.
It currently has a
bug feature in definition of
various monad transformers like
StateT, etc. which is visible when you
create backtracking state via monad stack, not via built-in features. The
feature makes it so
lookAhead will backtrack your parser state but not
your custom state added via
StateT. Kmett thinks this behavior is
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 lexing. 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:
Prominent projects that use Megaparsec
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:
The project was started and is currently maintained by Mark Karpov. You can
find the complete list of contributors in the
AUTHORS.md file in the
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).
Copyright © 2015–2017 Megaparsec contributors
Copyright © 2007 Paolo Martini
Copyright © 1999–2000 Daan Leijen
Distributed under FreeBSD license.