Example of a Result type in Dart 3.

result.dart

sealed class Result<T, E> {
  const Result();

  const factory Result.value(T value) = ValueResult._;
  const factory Result.error(E error) = ErrorResult._;
}

final class ValueResult<T, E> extends Result<T, E> {
  final T value;

  const ValueResult._(this.value);
}

final class ErrorResult<T, E> extends Result<T, E> {
  final E error;

  const ErrorResult._(this.error);
}

void main() {
  final willSucceed = parseInt('5');

  // This doesn't work. Two frictions:
  // 1. I need to re-add the types (<int, String>) which is not super valuable.
  // 2. The else case is not inferred to be ErrorResult, even though it must be.
  if (willSucceed is ValueResult<int, String>) {
    print('value: ${willSucceed.value}');
  } else {
    print('error: ${willSucceed.error}');
  }

  // Let's try if pattern matching (https://dart.dev/language/branches#if-case).
  // 1. Works, though still manages to be more characters than if-is.
  // 2. The else case is still not inferred to be ErrorResult.
  if (willSucceed case ValueResult(value: final value)) {
    print('value: $value');
  } else {
    print('error: ${willSucceed.error}');
  }

  // Let's try switch pattern matching (https://dart.dev/language/branches#exhaustiveness-checking).
  // 1. Works!
  // 2. It's definitely the most characters of all the options, but uh, it works.
  switch (willSucceed) {
    case final ValueResult<int, String> result:
      print('value: ${result.value}');
    case final ErrorResult<int, String> result:
      print('error: ${result.error}');
  }
  
  // Let's try the other switch statement (https://dart.dev/language/patterns#switch-statements-and-expressions).
  // 1. Works!
  // 2. It's still cumbersome to type out both result types.
  switch (willSucceed) {
    case ValueResult(value: final value):
      print('value: $value');
    case ErrorResult(error: final error):
      print('error: $error');
  }
  
  // The shorthand is even nicer.
  switch (willSucceed) {
    case ValueResult(: final value):
      print('value: $value');
    case ErrorResult(: final error):
      print('error: $error');
  }
}

Result<int, String> parseInt(String s) {
  final parsed = int.tryParse(s);
  return parsed != null ? Result.value(parsed) : Result.error('bad: $s');
}