backtracking.js

const P = require('parsimmon');
const Chalk = require('chalk');

function wants(x) {
  process.stdout.write(Chalk.bold.red('( wants ' + Chalk.cyan(x) + ' ) '));
  return x;
}

function got(x) {
  console.log(Chalk.bold.green('( GOT ' + Chalk.cyan(x) + ' )'));
  return x;
}

function log(x) {
  console.log(Chalk.bold(x));
  return x;
}

const parser =
  P.seq(
    P.of('\nPARSE START').map(log),
    P.of('===========').map(log),
    P.of('[').map(wants).then(P.string('[').map(got)),
    P.alt(
      P.of('a').map(wants).then(P.string('a').map(got)),
      P.of('b').map(wants).then(P.string('b').map(got)),
      P.of('c').map(wants).then(P.string('c').map(got))
    ),
    P.of(']').map(wants).then(P.string(']').map(got))
  );

parser.parse('[a]');
parser.parse('[b]');
parser.parse('[c]');
parser.parse('[]');
console.log();