where method

@useResult Parser<R> where(Predicate<R> predicate, { FailureFactory<R>? failureFactory, @Deprecated('Use `failureFactory` instead') Callback<R, String>? failureMessage, @Deprecated('Use `failureFactory` instead') Callback<R, int>? failurePosition, })

Returns a parser that evaluates the predicate with the successful parse result. If the predicate returns true the parser proceeds with the parse result, otherwise a parse failure is created using the optionally specified failureMessage and failurePosition callbacks.

The function failureMessage receives the parse result and is expected to return an error string of the failed predicate. If no function is provided a default error message is created.

Similarly, the failurePosition receives the parse result and is expected to return the position of the error of the failed predicate. If no function is provided the parser fails at the beginning of the delegate.

The following example parses two characters, but only succeeds if they are equal:

final inner = any() & any();
final parser = inner.where(
    (value) => value[0] == value[1],
    failureFactory: (context, success) =>
        context.failure('characters do not match'));
parser.parse('aa');   // ==> Success: ['a', 'a']
parser.parse('ab');   // ==> Failure: characters do not match

Implementation

@useResult
Parser<R> where(
  Predicate<R> predicate, {
  FailureFactory<R>? failureFactory,
  @Deprecated('Use `failureFactory` instead')
      Callback<R, String>? failureMessage,
  @Deprecated('Use `failureFactory` instead')
      Callback<R, int>? failurePosition,
}) =>
    WhereParser<R>(
        this,
        predicate,
        failureFactory ??
            ((failureMessage != null || failurePosition != null)
                ? (context, success) => context.failure(
                    failureMessage?.call(success.value) ??
                        'unexpected "${success.value}"',
                    failurePosition?.call(success.value))
                : (context, success) =>
                    context.failure('unexpected "${success.value}"')));