This is an industrial-strength monadic parser combinator library. Megaparsec
is a feature-rich package that strikes 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.
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
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
Megaparsec includes all functionality that is typically available in
Parsec-like libraries and also features some combinators that are quite
unique to it:
failure allows us to report a parse error with unexpected and expected
fancyFailure provides a way to report custom parse errors.
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 (they are returned in
Left, while normal results are wrapped in
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' 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
takeWhile1 are about 150 times faster than approaches
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
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.
Megaparsec has well-typed error messages and the ability to signal custom
parse errors to better work in user's domain of interest.
Megaparsec 7 introduced the
ParseErrorBundle data type that helps to
manage multi-error messages and pretty-print them easily and efficiently.
That version of the library also made the practice of displaying offending
line the default, similar to how recent versions of GHC do it.
Megaparsec works well with streams of tokens produced by tools like Alex.
The design of the
Stream type class has been changed significantly in
versions 6 and 7, 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
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.Megaparsec.Char.Lexer is intended to be imported
using a qualified import, it's not included in
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 Megaparsec tutorials here. They should provide
sufficient guidance to help you start with your parsing tasks. The site also
has instructions and tips for Parsec users who decide to migrate to
Despite being flexible, Megaparsec is also fast. Here is how Megaparsec
7.0.0 compares to Attoparsec 0.13.2.2 (the fastest widely used
parsing library in the Haskell ecosystem):
The benchmarks were created to guide development of Megaparsec 6 and can be
If you think your Megaparsec parser is not efficient enough, take a look at
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 monad
So, if you work with something human-readable where size of input data is
moderate, just go with Megaparsec, otherwise Attoparsec may be a better
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 well-typed error messages and custom
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
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
indentation matters out-of-the-box.
Megaparsec can recover from parse errors “on the fly” and continue
Megaparsec allows us 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 like Attoparsec.
If you want to see a detailed change log,
CHANGELOG.md may be helpful.
Also see this original announcement for another
Megaparsec vs Trifecta
Trifecta is another Haskell library featuring good error
messages. Some reasons one may question choice of Trifecta is his/her
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.
Idris has recently switched from Trifecta to Megaparsec which
allowed it to have better error messages and fewer
Megaparsec vs Earley
Earley is a newer library that allows us 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:
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 us 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.
In other words, Megaparsec is less safe but also more powerful.
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
- Hledger—an accounting tool
- MMark—strict markdown processor for
- Stache—Mustache templates for
- Language Puppet—library
for manipulating Puppet manifests
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 so good.
Issues (bugs, feature requests or otherwise feedback) may be reported in
the GitHub issue tracker for this project.
Pull requests are also welcome.
Copyright © 2015–2018 Megaparsec contributors
Copyright © 2007 Paolo Martini
Copyright © 1999–2000 Daan Leijen
Distributed under FreeBSD license.