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PEG library for the Go language

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Copyright (c) Richard Lyman. All rights reserved.
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Table of Contents

  • Name and Pronunciation
  • Introduction
  • Grammar Definitions

Name and Pronunciation

There is a PEG/Packrat library Neotoma (a scientific classification for pack rats) -- Amotoen is Neotoma written in reverse, not that there is any particular quality or relationship shared between the two; I was just taking the shortest path to a unique name that might have been tangentially related to PEGs. You might even successfully define 'shortest path' as some stream-of-consciousness process.

I tend to pronounce Amotoen with a style that follows: Am-o-toe-n. I tend to place emphasis on the 'toe'.


Amotoen is a Clojure library that supports PEG style definitions of grammars that can produce parsers. While there are academic papers available that rigorously define PEG, I've found that PEGs, or Parsing Expression Grammar(s), are best explained by the related Wikipedia page.

The clj-peg library was a predecessor to Amotoen and as such, Amotoen syntax might be reminiscent of the syntax in clj-peg. There are, however, significant differences between using clj-peg and using Amotoen. The most significant of those differences can be found in the lack of macros, gen-class, or gen-interface. The clj-peg library used the dirty three in the core, and Amotoen avoids all of them entirely.

Amotoen uses runtime processing of a given grammar, where clj-peg used macro time expansion of a grammar. Amotoen uses protocols instead of interfaces. Amotoen internally uses nils instead of gen-classed Errors. All of these changes result in far greater ease of use as well as increased maintainence.

In other words: Amotoen is better than clj-peg. Amotoen is not AOT'd.

Grammar Definitions

The grammar for Amotoen grammars is:

    :Whitespace     '(| \space \newline \tab \,)
    :_*             '(* :Whitespace)
    :_              [:Whitespace '(* :Whitespace)]
    :Grammar        [\{ :_* :Rule '(* [:_ :Rule]) :_* \}]
    :Rule           [:Keyword :_ :Body]
    :Keyword        [\: :ValidKeywordChar '(* :ValidKeywordChar)]
    :Body           '(| :Keyword :Char :Grouping)
    :Grouping       '(| :Sequence :Either :ZeroOrMore :ZeroOrOne :AnyNot)
    :Sequence       [\[     :_* :Body '(* [:_* :Body])  :_* \]]
    :Either         [\( \|  :_  :Body '(* [:_* :Body])  :_* \)]
    :ZeroOrMore     [\( \*  :_  :Body                   :_* \)]
    :ZeroOrOne      [\( \?  :_  :Body                   :_* \)]
    :AnyNot         [\( \%  :_  '(| :Keyword :Char)     :_* \)]
    :Char           [\\ '(| :TabChar :SpaceChar :NewlineChar (% \space))]
    :TabChar        (pegs "tab")
    :SpaceChar      (pegs "space")
    :NewlineChar    (pegs "newline")
    :ValidKeywordChar (lpegs '| "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789:/*+!_?-")

I'll be adding more documentation on writing grammars soon...

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