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The Parsatron

Born from Haskell's Parsec library, The Parsatron is a functional parser library. The Parsatron provides a lot of very small functions that can be combined into larger ones to very quickly write parsers for languages.

Like all parser combinator libraries, The Parsatron produces recursive-descent parsers that are best suited for LL(1) grammars. However, The Parsatron offers infinite lookahead which means you can try and parse any insane thing you'd like and if it doesn't work out, fall back to where you started. It's a feature that's worked out well for others. I'm sure you'll find something useful to do with it.


You can use The Parsatron by including

[the/parsatron "0.0.7"]

in your project.clj dependencies. It's available for download from Clojars.

ClojureScript Support

The Parsatron has been ported to ClojureScript and is included in the library distribution. There are a few small differences between ClojureScript and Clojure that follow The Parsatron into the browser, namely:

  • The Parsatron makes liberal use of macros: >>, let->> and defparser must be included via :require-macros
  • ClojureScript has no notion of a character type like Clojure does. The Parsatron considers Strings of length 1 to be characters

The Parsatron's ClojureScript tests can be run by first building them:

lein cljsbuild once

and then opening the html file test/resources/parsatron_test.html


A basic syntax checker for a certain profane esoteric programming language could be defined as follows:

(defparser instruction []
  (choice (char \>)
          (char \<)
          (char \+)
          (char \-)
          (char \.)
          (char \,)
          (between (char \[) (char \]) (many (instruction)))))

(defparser bf []
  (many (instruction))

The defparser forms create new parsers that you can combine into other, more complex parsers. As you can see in this example, those parsers can be recursive.

The choice, char, between and many functions you see are themselves combinators, provided gratis by the library. Some, like choice, many, and between, take parsers as arguments and return you a new one, wholly different, but exhibiting eerily familiar behavior. Some, like char, take less exotic input (in this case, a humble character) and return more basic parsers, that perform what is asked of them without hestitation or spite.

You execute a parser over some input via the run form.

(run (bf) ",>++++++[<-------->-],[<+>-]<.")

Currently, The Parsatron only provides character-oriented parsers, but the ideas it's built on are powerful enough that with the right series of commits, it can be made to run over sequence of arbitrary "tokens". Clojure's handling of sequences and sequence-like things is a feature deeply ingrained in the language's ethos. Look for expansion in this area.

Beyond just verifying that a string is a valid member of some language, The Parsatron offers you facilities for interacting with and operating on the things you parse via sequencing of multiple parsers and binding their results. The macros >> and let->> embody this facility.

As an example, bencoded strings are prefixed by their length and a colon:

(defparser ben-string []
  (let->> [length (integer)]
    (>> (char \:)
        (times length (any-char)))))

let->> allows you to capture and name the result of a parser so it's value may be used later. >> is very similar to Clojure's do in that it executes it's forms in order, but "throws away" all but the value of the last form.

(run (ben-string) "4:spam") ;; => [\s \p \a \m]


Copyright (C) 2011 Nate Young

Distributed under the Eclipse Public License, the same as Clojure.

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