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.
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 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
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 typed error messages and the ability to signal custom parse
errors that better suit user's domain of interest.
Since version 8, 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 single parse error Megaparsec produces so-called
ParseErrorBundle data type that helps to manage multi-error messages and
pretty-print them easily and efficiently. Since version 8, reporting
multiple parse errors at once has become much easier.
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
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 the most complete Megaparsec tutorial here. It
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 Megaparsec.
Despite being flexible, Megaparsec is also fast. Here is how Megaparsec
compares to Attoparsec (the fastest widely used parsing
library in the Haskell ecosystem):
You can run the benchmarks yourself by executing:
$ nix-bulid -A benches.parsers-bench
$ cd result/bench
More information about benchmarking and development can be found
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 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
Some quirks and bugs of Parsec are fixed.
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.
Megaparsec is faster and supports efficient operations
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. These are the common reasons why Trifecta may be problematic to
Complicated, doesn't have any tutorials available, and documentation
doesn't help at all.
Trifecta can parse
ByteString natively, but not
lens, which is a very heavy dependency. If you're not into
lens and would like to keep your code “vanilla”, you may not like the
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.
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
- hnix—re-implementation of the Nix
language in Haskell
- Hledger—an accounting tool
- MMark—strict markdown processor for
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
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.