Union type for primitive types data in dart

Question

How can I define a variable with only two possible primitive types, such as (String or num) for example, in Dart? Is there a way to do this? like union types in TypeScript.

const x:string|number;

Answer 1

Related Info/Resources:

This question is a possible duplicate of this question here.

There WAS a union package that has been discontinued. It was replaced by the freezed package, which includes the functionality you are looking for but with a bunch of other stuff as well.

I think you may be interested in dart's sealed classes or the either package.

Ideas for implementing from scratch:

To achieve the functionality you are looking for, you could create your own class that takes a dynamic type in its constructor, and throws an error if the type does not evaluate to one of the types that you want to enforce.

Answer 2

I'll add another recommendation for Freezed. It's a good package that will solve your problem and can also be used for things like data classes w/ JSON support.

If you don't want to add dependencies / use code generation / whatever, you can write your own sealed class:

sealed class MyUnion {
  const MyUnion();

  static MyUnion fromResponse(Object response) {
    return switch (response) {
      int val => MyInt(val),
      String val => MyString(val),
      _ => throw UnimplementedError('Unsupported type ${response.runtimeType}'),
    };
  }
}

class MyInt extends MyUnion {
  const MyInt(this.val);
  final int val;
}

class MyString extends MyUnion {
  const MyString(this.val);
  final String val;
}

void main() {
  printUnion(const MyInt(1));
  printUnion(const MyString('Hello World'));
  printUnion(MyUnion.fromResponse(10));
  printUnion(MyUnion.fromResponse('ten'));
}

void printUnion(MyUnion arg) {
  switch (arg) {
    case MyInt(:final val):
      print('Found int: $val');
    case MyString(:final val):
      print('Found String: $val');
  }
}

As an aside: Dart does not have primitive types. It has built in types, but every type except Null is a subtype of Object.