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    Parsey is a parser for context-free grammars. It utilizes earley parsing, a top-down chart parsing algorithm that can parse a large set of grammars.

    Parsey takes a list of rules (which can be defined using the Rule constructor) and some input sentence, then constructs a parse tree for the given language. The many applications of this include abstract syntax tree creation, NLP, and arithmetic, to name a few.


    Install from npm:

    npm install parsey --save


    const Rule    = require('parsey').Rule;
    const Sym     = require('parsey').Sym;
    const parse   = require('parsey').parse;
    const sum     = new Sym('sum');
    const prod    = new Sym('prod');
    const factor  = new Sym('factor');
    // Rule( [lhs : Sym], [rhs : Array], [valuator : Function] )
    let grammar = [
      // sum
      new Rule(sum    , [sum, '+', prod]    , (x, _, y) => x + y),
      new Rule(sum    , [prod]              , (x) => x),
      // product
      new Rule(prod   , [prod, '*', factor] , (x, _, y) => x * y),
      new Rule(prod   , [factor]            , (x) => x),
      // factor
      new Rule(factor , ['(', sum, ')']     , (_, x) => x),
      new Rule(factor , [/\d+/]             , (n) => parseFloat(n))
    let expr = '2 + 3 * (1 + 4)';
    let parseTree = parse(exp, grammar);
    // => 17

    Rules and Sym(bol)s

    A Sym is nothing more than an object that references a specific symbol for use in grammar rules, with a name attribute that describes the symbol (names need not be unique).

    const Sym = require('parsey').Sym;
    let sum = new Sym('sum');
    let prod = new Sym('prod');

    Rules describe the structure of a language. They contain a left-hand side consisting of a sole symbol, a right-hand side consisting of a list of symbols, and an optional evaluation function.

    const Rule = require('parsey').Rule;
    //                     lhs        rhs               valuator
    let sumRule = new Rule(sum, [sum, '+', prod], (x, _, y) => x + y);

    This rule states that a sum can be any sum, followed by a +, followed by any prod. The rhs can be populated with Sym objects, strings, or RegExp objects.

    This rule's valuator is a function that takes 3 arguments (one for each symbol on the rhs), and adds the first and last (we don't care about the '+').

    The Rule class is extended from Array, so the rhs symbols correspond to a Rule's indices and all of the methods on Array.prototype will apply to the rhs.

    The CFG

    The CFG is a glorified array, a container for your rules. It has some methods like rule() and getSymbols(), which can be handy for creating rules from strings.

    const CFG = require('parsey').CFG;
    let grammar = new CFG();
    grammar.rule('S -> NP VP');
    grammar.rule('NP -> Det N');
    grammar.rule('VP -> V NP');
    grammar.rule('Det -> "the");
    grammar.rule('Det -> "a");
    grammar.rule('V -> /runs?/');
    grammar.rule('V -> "ran"');

    rule() will try to find existing symbols in the grammar that have the same names as the ones that appear in a production string, and will create new Sym objects if a symbol could not be found.

    It will also treat string-looking symbols like "the" as terminal symbols and leave them as strings, and likewise will turn things like /regex/ into RegExp objects, which are also terminals.


    Given a list of Rules, parsing becomes as simple as

    const parse = require('parsey').parse;
    parse('1 + 2 * 3', grammar);

    The parse() function returns a parse tree, which is a node of the following structure:

      item: [Rule],
      children: [Array]

    Elements in children can be either nodes of the above structure, or strings.


    // 1 * 2
      item: <Rule 'prod' [prod, '*', factor]>,
      children: [
          item: <Rule 'factor' [/\d+/]>,
          children: [ '1' ]
          item: <Rule 'factor' [/\d+/]>,
          children: [ '2' ]


    Check out examples to see parsey in action for various use cases!


    Testing parsey requres jasmine (2.0+). All specs can be found in spec/. To run all tests, simply run npm test in the project's root.




    npm i parsey

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