Megaparsec

This is an industrial-strength monadic parser combinator library. Megaparsec
is a fork of Parsec library originally
written by Daan Leijen.
Features
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 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:
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.
Lexer
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.
Documentation
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.
Tutorials
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.
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.
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
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:
Authors
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.
Contribution
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.
License
Copyright © 2015–2016 Megaparsec contributors
Copyright © 2007 Paolo Martini
Copyright © 1999–2000 Daan Leijen
Distributed under FreeBSD license.