Answer by Michael Horn for Dart - How to create custom types on the fly like in Typescript?

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");}