Container types and the `map()` function in Dart

main.dart

void main() {
  print(f(true).map((value) => value + 1));
  print(f(false).map((value) => value + 1));
  
  print([1, 2, 3].map((value) => "x ${value + 1}"));
  print([].map((value) => value + 1));
}

// Methods like
// map, flatMap / expand, filter / where, firstWhere, reduce, fold
// are typically defined on "containers" like collections (array, set, etc.),
// `Optional<T>`, `Result<T, E>`, `Either<T, U>`.

class Result<Value, Error> {
  Result.success(Value value): this.value = value, this.error = null;
  Result.failure(this.error): this.value = null;
  
  Result<T, Error> map<T>(T Function(Value) f) {
    if (value != null) {
      return Result.success(f(value!));
    } else {
      return Result.failure(error);
    }
  }
  
  String toString() => "Result(value: $value, error: $error)";
  
  final Value? value;
  final Error? error;
}

Result<int, FError> f(bool allGood) {
  if (allGood) {
    return Result.success(42);
  } else {
    return Result.failure(FError.unknown);
  }
}

enum FError { unknown }