README.md

GLL.js

Generalised Parser Combinators in JavaScript

This library is an JavaScript implementation of Vegard Øye's brilliant tutorial on implementing a GLL framework in Racket:

https://github.com/epsil/gll

About

Generalised parsers are special because they can handle any ambiguous context-free grammar. They work by performing a breadth-first search on all possible transitions.

This library provides combinators for creating and composing generalised parsers. Any combination of parsers can be composed together to form a valid parser.

This implementation uses memoisation and a streaming API for performance improvements. Best case performance is O(n) according to input size, if the grammar is LL(1). The worse case performance is O(n^3), if the grammar is highly ambiguous.

Examples

Highly-ambiguous grammar

S := b | S S | S S S

val input = 'bbbbbbb';
var S = gll.makeParser(function() {
    return gll.alt(
        gll.string('b'),
        gll.seq(S, S),
        gll.seq(S, S, S)
    );
});
var result = S(input);

if(!result.isEmpty && result.val.isSuccess) {
    console.log('Got a sequence of b characters');
}

Balanced parentheses

S := ( S ) | ε

var input = '(())';
var balancedParens = gll.makeParser(function() {
    return gll.alt(
        gll.seq(gll.string('('), balancedParens, gll.string(')')),
        gll.string('')
    );
});
var result = balancedParens(input);

if(!result.isEmpty && result.val.isSuccess) {
    console.log('Input had balanced parenthesis');
}

Combinators

succeed(val)

Always succeeds with the given value, without consuming the input string.

string(match)

Succeeds if the input string equals the match string.

regexp(pattern)

Succeeds if the input string matches the RegExp pattern.

seq(...)

Succeeds if the input string succeeds for all supplied parsers, in order.

alt(...)

Succeeds if the input string succeeds for any supplied parsers, in order.

red(p, fn)

Runs the supplied function over a successful result, if the supplied parser succeeds.

Resources

• General Parser Combinators in Racket, Vegard Øye, 2012
• Generalized Parser Combinators, Daniel Spiewak, 2010