Dart - How to create custom types on the fly like in Typescript?

I want to create a custom type in Dart like I would do in typescript. This type should be a subtype of String, accepting only some values.

For example, in Typescript I would do:

type myType = 'HELLO' | 'WORLD' | '!'

How can I do the same stuff in Dart?

Original Answer (see updates below): This isn't possible at the language level in Dart - There are a couple alternatives though.

You could simply define an enum along with a method to derive a string from your enum:

enum MyType {
  hello,
  world,
  exclamationPoint,
}

String myTypeToString(MyType value) {
  switch (value) {
    case MyType.hello: 
      return 'HELLO';
    case MyType.world: 
      return 'WORLD';
    case MyType.exclamationPoint: 
      return '!';
  }
}

Update 2: In Dart 3 we have yet another way, using sealed classes:

sealed class MyType {
  @override
  String toString() {
    return switch (this) {
        Hello() => "Hello",
        World() => "world",
        Exclamation() => "!"
    };
  }
}

class Hello extends MyType {}
class World extends MyType {}
class Exclamation extends MyType {}

https://dart.dev/language/class-modifiers#sealed

Old Update: Now that Dart 2.17 supports declaring methods on enums, it's possible to do this a bit more cleanly than in my original answer:

enum MyType {
  hello,
  world,
  exclamationPoint;

  @override
  String toString() {
    switch (this) {
      case MyType.hello: 
        return 'HELLO';
      case MyType.world: 
        return 'WORLD';
      case MyType.exclamationPoint: 
        return '!';
    }
  }
}

Or you could define a class with three named constructors, and override the toString method:

class MyType {
  final String _value;

  MyType.hello(): _value = 'HELLO';
  MyType.world(): _value = 'WORLD';
  MyType.exclamationPoint(): _value = '!';

  @override
  String toString() {
    return _value;
  }
}

// Usage:
void main() {
  final hello = MyType.hello();
  final world = MyType.world();
  final punctuation = MyType.exclamationPoint();

  // Prints "HELLO, WORLD!"
  print("$hello, $world$punctuation");
}