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A monadic LL(infinity) parser combinator library

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Authors: @jneen and @laughinghan

Maintainer: @wavebeem

Parsimmon is a small library for writing big parsers made up of lots of little parsers. The API is inspired by parsec and Promises/A+.

Parsimmon supports IE7 and newer browsers, along with Node.js. It can be used as a standard Node module through npm (named parsimmon), or directly in the browser through a script tag, where it exports a global variable called Parsimmon. To download the latest browser build, use the unpkg version. For more information on how to use unpkg, see the unpkg homepage.

API Documentation

Full API documentation in


See the examples directory for annotated examples of parsing JSON, Lisp, and math.


A Parsimmon parser is an object that represents an action on a stream of text, and the promise of either an object yielded by that action on success or a message in case of failure. For example, Parsimmon.string('foo') yields the string 'foo' if the beginning of the stream is 'foo', and otherwise fails.

The method .map is used to transform the yielded value. For example,

  .map(function(x) { return x + 'bar'; })

will yield 'foobar' if the stream starts with 'foo'. The parser

  .map(function(x) { return Number(x) * 2; })

will yield the number 24 when it encounters the string '12'.

Calling .parse(string) on a parser parses the string and returns an object with a boolean status flag, indicating whether the parse succeeded. If it succeeded, the value attribute will contain the yielded value. Otherwise, the index and expected attributes will contain the index of the parse error (with offset, line and column properties), and a sorted, unique array of messages indicating what was expected.

The error object can be passed along with the original source to Parsimmon.formatError(source, error) to obtain a human-readable error string.


Feel free to ask a question by filing a GitHub Issue. I'm happy to help point you in the right direction with the library, and hopefully improve the documentation so less people get confused in the future.


Contributions are not just pull requests. Issues clearly describing bugs or confusing parts of Parsimmon are welcome! Also, documentation enhancements and examples are very desirable. Feeling overwhelmed about contributing? Open an issue about what you want to contribute and I'm happy to help you through to completion!


Thanks to @bd82 we have a good benchmark comparing Parsimmon CPU performance to several other parser libraries with a simple JSON parser example.

Fantasy Land


Parsimmon is also compatible with fantasyland. It is a Semigroup, an Applicative Functor, and a Monad.