trace function

Parser trace (Parser parser, [ OutputHandler output = print ])

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

final 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.

Implementation

Parser trace(Parser parser, [OutputHandler output = print]) {
  var level = 0;
  return transformParser(parser, (each) {
    return ContinuationParser(each, (continuation, context) {
      output('${'  ' * level}$each');
      level++;
      final result = continuation(context);
      level--;
      output('${'  ' * level}$result');
      return result;
    });
  });
}