petitparser/lib/src/debug/progress.dart

part of debug;

/**
 * Returns a transformed [parser] that when being used to read input
 * visually prints its progress while progressing.
 *
 * For example, the snippet
 *
 *     var parser = letter() & word().star();
 *     progress(parser).parse('f123');
 *
 * produces the following output:
 *
 *     * Instance of 'SequenceParser'
 *     * Instance of 'CharacterParser'[letter expected]
 *     ** Instance of 'PossessiveRepeatingParser'[0..*]
 *     ** Instance of 'CharacterParser'[letter or digit expected]
 *     *** Instance of 'CharacterParser'[letter or digit expected]
 *     **** Instance of 'CharacterParser'[letter or digit expected]
 *     ***** Instance of 'CharacterParser'[letter or digit expected]
 *
 * Jumps backwards mean that the parser is back-tracking. Often choices can
 * be reordered to such expensive parses.
 */
Parser progress(Parser parser, [OutputHandler output = print]) {
  return transformParser(parser, (each) {
    return new ContinuationParser(each, (continuation, context) {
      output('${_repeat(1 + context.position, '*')} $each');
      return continuation(context);
    });
  });
}