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

Issue

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?

Solution

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 '!';
  }
}

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");
}

Answered By - Michael Horn