A pseudo console for Dart, using Dartpad (includes our Option type)

dart_pseudo_console.dart

main() {
  // Press `run` button to evaluate the file.
  // You must explicitly print values, e.g. print("$myValue")
  
}


// CODE BELOW FOR OUR OPTION TYPE:

A identity<A>(A value) => value;

abstract class Option<A> {
  const Option();

  factory Option.of(A value) => value != null ? Some<A>(value) : empty();

  static Option<A> empty<A>() => None<A>();

  B fold<B>(B ifNone(), B ifSome(A value));

  Option<B> map<B>(B f(A value)) => fold(empty, (value) => Some(f(value)));

  Option<B> flatMap<B>(Option<B> f(A value)) =>
      fold(empty, (value) => f(value));

  Option<B> andThen<B>(Option<B> value) => fold(empty, (_) => value);

  Option<A> filter(bool f(A value)) =>
      fold(() => this, (value) => f(value) ? this : empty());

  Option<A> orElse(Option<A> fallback()) =>
      fold(fallback, (value) => Some(value));

  A getOrElse(A fallback()) => fold(fallback, identity);

  bool isDefined() => fold(() => false, (_) => true);

  bool isEmpty() => !isDefined();

//   REMOVED:
//   Either<B, A> toEither<B>(B fallback()) =>
//       fold(() => Left(fallback()), (value) => Right(value));

  List<A> toList() => fold(() => const [], (value) => [value]);

  A orNull() => fold(() => null, identity);

  /// Returns `true` if this is a `Some` and its value is equal to `reference`
  /// (as determined by `==`), returns `false` otherwise.
  bool contains(A reference()) =>
      fold(() => false, (value) => value == reference());

  /// Returns `true` if `None` or returns the result of the application of
  /// the given predicate to the `Some` value.
  bool forall(bool reference(A a)) =>
      fold(() => true, (value) => reference(value));

  /// Returns `false` if `None` or returns the result of the application of
  /// the given predicate to the `Some` value.
  bool exists(bool reference(A a)) =>
      fold(() => false, (value) => reference(value));
}

class Some<A> extends Option<A> {
  final A value;

  const Some(this.value) : super();

  @override
  B fold<B>(B ifNone(), B ifSome(A value)) => ifSome(value);

  @override
  bool operator ==(Object other) => other is Some && other.value == value;

  @override
  int get hashCode => value.hashCode;

  @override
  String toString() => 'Some<$A>($value)';
}

class None<A> extends Option<A> {
  const None();

  @override
  B fold<B>(B ifNone(), B ifSome(A value)) => ifNone();

  @override
  bool operator ==(Object other) => other is None;

  @override
  int get hashCode => 0;

  @override
  String toString() => 'None<$A>';
}