PEGs
Apuntes de PEG.js
- Introducción a los PEGs
- PEGJS
- La Calculadora Simple
- Eliminación de la Recursividad por la Izquierda en PEGs
- Dangling else: Asociando un else con su if mas cercano
PEG.js Example
altjs.coffee is the code for the "AltJS language in 5 minutes" section
presented in the second half of the talk.
Extra Special Bonus!
idgrep.coffee (and idgrep.js) is another example of using Esprima
to do static analysis on JavaScript code.
Repositorios
- Ejemplos en PEGjs
[~/local/src/javascript/PLgrado/pegjs/examples(master)]$ pwd -P /Users/casiano/local/src/javascript/PLgrado/pegjs/examples
Bactracking en los PEGs
- PEG parsers don't backtrack like other recursive-descent parsers or Prolog do.
- Rather, when confronted with a choice, a PEG parser will try every option until one succeeds.
- Once one succeeds, it will commit to it no matter how the rule was invoked.
El siguiente ejemplo ilustra el backtracking en los PEGs:
var PEG = require ("pegjs");
var grammar1 = `
start = "test" / "test ;"
`;
var parser = PEG.buildParser(grammar1);
var input = 'test';
console.log("OK: "+parser.parse(input));
try {
// This input will not be accepted
var input = 'test ;';
console.log(parser.parse(input));
}
catch (e) {
console.log(e);
}
var grammar2 = `
start = "test" !" ;" / "test ;"
`;
var parser = PEG.buildParser(grammar2);
var input = 'test ;';
console.log("OK: "+parser.parse(input));
Cuando se ejecuta produce la salida:
[~/srcPLgrado/pegjs/examples(master)]$ node backtracking.js
OK: test
{ [SyntaxError: Expected end of input but " " found.]
message: 'Expected end of input but " " found.',
expected: [ { type: 'end', description: 'end of input' } ],
found: ' ',
offset: 4,
line: 1,
column: 5,
name: 'SyntaxError' }
OK: test ;
Ejemplos
> parser = PEG.buildParser(`d = 'c'\ns = ('a' / 'b')+\n`, {allowedStartRules: ['d', 's']})
{ SyntaxError: [Function: peg$SyntaxError],
parse: [Function: peg$parse] }
> r = parser.parse('c')
'c'
> r = parser.parse('aba', {startRule: "s"})
[ 'a', 'b', 'a' ]
> parser = PEG.buildParser(`d = 'c'\ns = ('a' / 'b')+\n`, {allowedStartRules: ['d', 's'], output: "source"}); typeof parser
'string'
[~/pegjs/examples(master)]$ cat initializer.js
"use strict";
const PEG = require("pegjs");
const grammar = `
{
const util = require("util");
var g = "visible variable";
console.log("Inside Initializer! options = "+util.inspect(options));
}
start = 'a' { console.log('g = '+g); return 1; }
/ & { console.log('inside predicate: g = '+g); return true; } 'b' { return 2; }
`;
console.log("GRAMMAR:\n"+grammar);
const parser = PEG.buildParser(grammar);
console.log("PARSING 'a'");
let r = parser.parse("a", { x: 'hello' });
console.log(r);
console.log("PARSING 'b'");
r = parser.parse("b");
console.log(r);
Borrador
- borrador (work in progress. Don't look here)