class ParseResult<A> {
 final bool isSuccess;
 final bool isCommitted;
 /// [:null:] if [:!isSuccess:]
 final A value;
 final String text;
 final Position position;
 final Expectations expectations;

 String get errorMessage {
   final pos = expectations.position;
   final maxSeenChar = (pos.offset < text.length)
       ? "'${text[pos.offset]}'"
       : 'eof';
   final prelude =
       'line ${pos.line}, character ${pos.character}:';
   final expected = expectations.expected;
   if (expected.isEmpty) {
     return '$prelude unexpected $maxSeenChar.';
   } else {
     final or = _humanOr(new List.from(expected));
     return "$prelude expected $or, got $maxSeenChar.";
   }
 }

 ParseResult(this.text, this.expectations, this.position, this.isSuccess,
             this.isCommitted, this.value);

 ParseResult copy({String text, Expectations expectations, int position,
                   bool isSuccess, bool isCommitted,
                   Object value: const Undefined()}) {
   return new ParseResult(
       (text != null)               ? text         : this.text,
       (expectations != null)       ? expectations : this.expectations,
       (position != null)           ? position     : this.position,
       (isSuccess != null)          ? isSuccess    : this.isSuccess,
       (isCommitted != null)        ? isCommitted  : this.isCommitted,
       (value != const Undefined()) ? value        : this.value);
 }

 String get _rest => text.substring(position.offset);
 get _shortRest => _rest.length < 10 ? _rest : '${_rest.substring(0, 10)}...';

 toString() {
   final c = isCommitted ? '*' : '';
   return isSuccess
       ? 'success$c: {value: $value, rest: "$_shortRest"}'
       : 'failure$c: {message: $errorMessage, rest: "$_shortRest"}';
 }
}