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350 lines
14 KiB
Markdown
350 lines
14 KiB
Markdown
# Megaparsec
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[](http://opensource.org/licenses/BSD-2-Clause)
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[](https://hackage.haskell.org/package/megaparsec)
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[](http://stackage.org/nightly/package/megaparsec)
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[](http://stackage.org/lts/package/megaparsec)
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* [Features](#features)
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* [Core features](#core-features)
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* [Error messages](#error-messages)
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* [External lexers](#external-lexers)
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* [Character and binary parsing](#character-and-binary-parsing)
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* [Lexer](#lexer)
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* [Documentation](#documentation)
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* [Tutorials](#tutorials)
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* [Performance](#performance)
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* [Comparison with other solutions](#comparison-with-other-solutions)
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* [Megaparsec vs Attoparsec](#megaparsec-vs-attoparsec)
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* [Megaparsec vs Parsec](#megaparsec-vs-parsec)
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* [Megaparsec vs Trifecta](#megaparsec-vs-trifecta)
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* [Megaparsec vs Earley](#megaparsec-vs-earley)
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* [Related packages](#related-packages)
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* [Prominent projects that use Megaparsec](#prominent-projects-that-use-megaparsec)
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* [Links to announcements and blog posts](#links-to-announcements-and-blog-posts)
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* [Contribution](#contribution)
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* [License](#license)
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This is an industrial-strength monadic parser combinator library. Megaparsec
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is a feature-rich package that tries to find a nice balance between speed,
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flexibility, and quality of parse errors.
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## Features
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The project provides flexible solutions to satisfy common parsing needs. The
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section describes them shortly. If you're looking for comprehensive
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documentation, see the [section about documentation](#documentation).
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### Core features
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The package is built around `MonadParsec`, an MTL-style monad transformer.
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Most features work with all instances of `MonadParsec`. One can achieve
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various effects combining monad transformers, i.e. building a monadic stack.
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Since the common monad transformers like `WriterT`, `StateT`, `ReaderT` and
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others are instances of the `MonadParsec` type class, one can also wrap
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`ParsecT` *in* these monads, achieving, for example, backtracking state.
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On the other hand `ParsecT` is an instance of many type classes as well. The
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most useful ones are `Monad`, `Applicative`, `Alternative`, and
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`MonadParsec`.
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Megaparsec includes all functionality that is typically available in
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Parsec-like libraries and also features some special combinators:
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* `parseError` allows us to end parsing and report an arbitrary parse error.
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* `withRecovery` can be used to recover from parse errors “on-the-fly” and
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continue parsing. Once parsing is finished, several parse errors may be
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reported or ignored altogether.
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* `observing` makes it possible to “observe” parse errors without ending
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parsing.
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In addition to that, Megaparsec features high-performance combinators
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similar to those found in [Attoparsec][attoparsec]:
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* `tokens` makes it easy to parse several tokens in a row (`string` and
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`string'` are built on top of this primitive). This is about 100 times
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faster than matching a string token by token. `tokens` returns “chunk” of
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original input, meaning that if you parse `Text`, it'll return `Text`
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without repacking.
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* `takeWhile` and `takeWhile1` are about 150 times faster than approaches
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involving `many`, `manyTill` and other similar combinators.
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* `takeP` allows us to grab n tokens from the stream and returns them as a
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“chunk” of the stream.
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Megaparsec is about as fast as Attoparsec if you write your parser carefully
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(see also [the section about performance](#performance)).
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The library can currently work with the following types of input stream
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out-of-the-box:
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* `String = [Char]`
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* `ByteString` (strict and lazy)
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* `Text` (strict and lazy)
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It's also possible to make it work with custom token streams by making them
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an instance of the `Stream` type class.
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### Error messages
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* Megaparsec has typed error messages and the ability to signal custom parse
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errors that better suit the user's domain of interest.
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* Since version 8, the location of parse errors can independent of current
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offset in the input stream. It is useful when you want a parse error to
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point to a particular position after performing some checks.
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* Instead of a single parse error Megaparsec produces so-called
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`ParseErrorBundle` data type that helps to manage multi-error messages and
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pretty-print them. Since version 8, reporting multiple parse errors at
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once has become easier.
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### External lexers
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Megaparsec works well with streams of tokens produced by tools like Alex.
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The design of the `Stream` type class has been changed significantly in the
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recent versions, but user can still work with custom streams of tokens.
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### Character and binary parsing
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Megaparsec has decent support for Unicode-aware character parsing. Functions
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for character parsing live in the [`Text.Megaparsec.Char`][tm-char] module.
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Similarly, there is [`Text.Megaparsec.Byte`][tm-byte] module for parsing
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streams of bytes.
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### Lexer
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[`Text.Megaparsec.Char.Lexer`][tm-char-lexer] is a module that should help
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you write your lexer. If you have used `Parsec` in the past, this module
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“fixes” its particularly inflexible `Text.Parsec.Token`.
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[`Text.Megaparsec.Char.Lexer`][tm-char-lexer] is intended to be imported
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using a qualified import, it's not included in [`Text.Megaparsec`][tm]. The
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module doesn't impose how you should write your parser, but certain
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approaches may be more elegant than others. An especially important theme is
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parsing of white space, comments, and indentation.
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The design of the module allows one quickly solve simple tasks and doesn't
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get in the way when the need to implement something less standard arises.
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[`Text.Megaparsec.Byte.Lexer`][tm-byte-lexer] is also available for users
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who wish to parse binary data.
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## Documentation
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Megaparsec is well-documented. See the [current version of Megaparsec
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documentation on Hackage][hackage].
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## Tutorials
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You can find the most complete Megaparsec tutorial [here][the-tutorial]. It
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should provide sufficient guidance to help you start with your parsing
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tasks.
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## Performance
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Despite being flexible, Megaparsec is also fast. Here is how Megaparsec
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compares to [Attoparsec][attoparsec] (the fastest widely used parsing
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library in the Haskell ecosystem):
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Test case | Execution time | Allocated | Max residency
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------------------|---------------:|----------:|-------------:
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CSV (Attoparsec) | 76.50 μs | 397,784 | 10,544
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CSV (Megaparsec) | 64.69 μs | 352,408 | 9,104
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Log (Attoparsec) | 302.8 μs | 1,150,032 | 10,912
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Log (Megaparsec) | 337.8 μs | 1,246,496 | 10,912
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JSON (Attoparsec) | 18.20 μs | 128,368 | 9,032
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JSON (Megaparsec) | 25.45 μs | 203,824 | 9,176
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You can run the benchmarks yourself by executing:
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```
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$ nix-build -A benches.parsers-bench
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$ cd result/bench
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$ ./bench-memory
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$ ./bench-speed
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```
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More information about benchmarking and development can be found
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[here][hacking].
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## Comparison with other solutions
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There are quite a few libraries that can be used for parsing in Haskell,
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let's compare Megaparsec with some of them.
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### Megaparsec vs Attoparsec
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[Attoparsec][attoparsec] is another prominent Haskell library for parsing.
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Although both libraries deal with parsing, it's usually easy to decide which
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you will need in particular project:
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* *Attoparsec* is sometimes faster but not that feature-rich. It should be
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used when you want to process large amounts of data where performance
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matters more than quality of error messages.
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* *Megaparsec* is good for parsing of source code or other human-readable
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texts. It has better error messages and it's implemented as a monad
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transformer.
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So, if you work with something human-readable where the size of input data
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is moderate, it makes sense to go with Megaparsec, otherwise Attoparsec may
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be a better choice.
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### Megaparsec vs Parsec
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Since Megaparsec is a fork of [Parsec][parsec], we are bound to list the
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main differences between the two libraries:
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* Better error messages. Megaparsec has typed error messages and custom
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error messages, it can also report multiple parse errors at once.
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* Megaparsec can show the line on which parse error happened as part of
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parse error. This makes it a lot easier to figure out where the error
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happened.
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* Some quirks and bugs of Parsec are fixed.
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* Better support for Unicode parsing in [`Text.Megaparsec.Char`][tm-char].
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* Megaparsec has more powerful combinators and can parse languages where
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indentation matters.
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* Better documentation.
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* Megaparsec can recover from parse errors “on the fly” and continue
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parsing.
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* Megaparsec allows us to conditionally process parse errors inside a
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running parser. In particular, it's possible to define regions in which
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parse errors, should they happen, will get a “context tag”, e.g. we could
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build a context stack like “in function definition foo”, “in expression
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x”, etc.
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* Megaparsec is faster and supports efficient operations `tokens`,
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`takeWhileP`, `takeWhile1P`, `takeP`, like Attoparsec.
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If you want to see a detailed change log, `CHANGELOG.md` may be helpful.
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Also see [this original announcement][original-announcement] for another
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comparison.
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### Megaparsec vs Trifecta
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[Trifecta][trifecta] is another Haskell library featuring good error
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messages. These are the common reasons why Trifecta may be problematic to
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use:
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* Complicated, doesn't have any tutorials available, and documentation
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doesn't help much.
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* Trifecta can parse `String` and `ByteString` natively, but not `Text`.
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* Depends on `lens`, which is a very heavy dependency. If you're not into
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`lens`, you may not like the API.
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[Idris][idris] has switched from Trifecta to Megaparsec which allowed it to
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[have better error messages and fewer dependencies][idris-testimony].
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### Megaparsec vs Earley
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[Earley][earley] is a newer library that allows us to safely parse
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context-free grammars (CFG). Megaparsec is a lower-level library compared to
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Earley, but there are still enough reasons to choose it:
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* Megaparsec is faster.
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* Your grammar may be not context-free or you may want introduce some sort
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of state to the parsing process. Almost all non-trivial parsers require
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state. Even if your grammar is context-free, state may allow for
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additional niceties. Earley does not support that.
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* Megaparsec's error messages are more flexible allowing to include
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arbitrary data in them, return multiple error messages, mark regions that
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affect any error that happens in those regions, etc.
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In other words, Megaparsec is less safe but also more powerful.
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## Related packages
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The following packages are designed to be used with Megaparsec (open a PR if
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you want to add something to the list):
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* [`hspec-megaparsec`](https://hackage.haskell.org/package/hspec-megaparsec)—utilities
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for testing Megaparsec parsers with with
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[Hspec](https://hackage.haskell.org/package/hspec).
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* [`replace-megaparsec`](https://hackage.haskell.org/package/replace-megaparsec)—Stream
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editing and find-and-replace with Megaparsec.
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* [`cassava-megaparsec`](https://hackage.haskell.org/package/cassava-megaparsec)—Megaparsec
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parser of CSV files that plays nicely with
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[Cassava](https://hackage.haskell.org/package/cassava).
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* [`tagsoup-megaparsec`](https://hackage.haskell.org/package/tagsoup-megaparsec)—a
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library for easily using
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[TagSoup](https://hackage.haskell.org/package/tagsoup) as a token type in
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Megaparsec.
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## Prominent projects that use Megaparsec
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Some prominent projects that use Megaparsec:
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* [Idris](https://github.com/idris-lang/Idris-dev)—a general-purpose
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functional programming language with dependent types
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* [Dhall](https://github.com/dhall-lang/dhall-haskell)—an advanced
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configuration language
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* [hnix](https://github.com/haskell-nix/hnix)—re-implementation of the Nix
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language in Haskell
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* [Hledger](https://github.com/simonmichael/hledger)—an accounting tool
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* [MMark](https://github.com/mmark-md/mmark)—strict markdown processor for
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writers
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## Links to announcements and blog posts
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Here are some blog posts mainly announcing new features of the project and
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describing what sort of things are now possible:
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* [Megaparsec 8](https://markkarpov.com/post/megaparsec-8.html)
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* [Megaparsec 7](https://markkarpov.com/post/megaparsec-7.html)
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* [Evolution of error messages](https://markkarpov.com/post/evolution-of-error-messages.html)
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* [A major upgrade to Megaparsec: more speed, more power](https://markkarpov.com/post/megaparsec-more-speed-more-power.html)
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* [Latest additions to Megaparsec](https://markkarpov.com/post/latest-additions-to-megaparsec.html)
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* [Announcing Megaparsec 5](https://markkarpov.com/post/announcing-megaparsec-5.html)
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* [Megaparsec 4 and 5](https://markkarpov.com/post/megaparsec-4-and-5.html)
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* [The original Megaparsec 4.0.0 announcement][original-announcement]
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## Contribution
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Issues (bugs, feature requests or otherwise feedback) may be reported in
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[the GitHub issue tracker for this
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project](https://github.com/mrkkrp/megaparsec/issues).
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Pull requests are also welcome. If you would like to contribute to the
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project, you may find [this document][hacking] helpful.
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## License
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Copyright © 2015–present Megaparsec contributors\
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Copyright © 2007 Paolo Martini\
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Copyright © 1999–2000 Daan Leijen
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Distributed under FreeBSD license.
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[hackage]: https://hackage.haskell.org/package/megaparsec
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[the-tutorial]: https://markkarpov.com/tutorial/megaparsec.html
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[hacking]: ./HACKING.md
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[tm]: https://hackage.haskell.org/package/megaparsec/docs/Text-Megaparsec.html
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[tm-char]: https://hackage.haskell.org/package/megaparsec/docs/Text-Megaparsec-Char.html
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[tm-byte]: https://hackage.haskell.org/package/megaparsec/docs/Text-Megaparsec-Byte.html
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[tm-char-lexer]: https://hackage.haskell.org/package/megaparsec/docs/Text-Megaparsec-Char-Lexer.html
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[tm-byte-lexer]: https://hackage.haskell.org/package/megaparsec/docs/Text-Megaparsec-Byte-Lexer.html
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[attoparsec]: https://hackage.haskell.org/package/attoparsec
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[parsec]: https://hackage.haskell.org/package/parsec
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[trifecta]: https://hackage.haskell.org/package/trifecta
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[earley]: https://hackage.haskell.org/package/Earley
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[idris]: https://www.idris-lang.org/
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[idris-testimony]: https://twitter.com/edwinbrady/status/950084043282010117?s=09
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[parsers-bench]: https://github.com/mrkkrp/parsers-bench
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[fast-parser]: https://markkarpov.com/megaparsec/writing-a-fast-parser.html
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[original-announcement]: https://mail.haskell.org/pipermail/haskell-cafe/2015-September/121530.html
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