packages/petitparser/lib/src/debug/trace.dart

part of petitparser.debug;

/// Returns a transformed [parser] that when being used to read input prints a
/// trace of all activated parsers and their respective parse results.
///
/// For example, the snippet
///
///     var parser = letter() & word().star();
///     trace(parser).parse('f1');
///
/// produces the following output:
///
///     Instance of 'SequenceParser'
///       Instance of 'CharacterParser'[letter expected]
///       Success[1:2]: f
///       Instance of 'PossessiveRepeatingParser'[0..*]
///         Instance of 'CharacterParser'[letter or digit expected]
///         Success[1:3]: 1
///         Instance of 'CharacterParser'[letter or digit expected]
///         Failure[1:3]: letter or digit expected
///       Success[1:3]: [1]
///     Success[1:3]: [f, [1]]
///
/// Indentation signifies the activation of a parser object. Reverse indentation
/// signifies the returning of a parse result either with a success or failure
/// context.
Parser trace(Parser parser, [OutputHandler output = print]) {
  var level = 0;
  return transformParser(parser, (each) {
    return new ContinuationParser(each, (continuation, context) {
      output('${_repeat(level, '  ')}$each');
      level++;
      var result = continuation(context);
      level--;
      output('${_repeat(level, '  ')}$result');
      return result;
    });
  });
}