[SOLVED] Dart list of exactly two types

Dart list of exactly two types

Is it possible in Dart to have a list of exactly two types like you can in TypeScript? For example, something like this would compile:

List<int|String> foo = [
    1,2,3,4,'baz'
];

while this wouldn't:

List<int|String> foo = [
    1,2,false,4,'baz',[],3.0
];

Solution

No.

The Dart type system does not contain general union types. It cannot express "must be int or String" in the type system

The two available exceptions are:

Something or Null (nullable types, like int?).
Something or a Future of the same thing (FutureOr).

Those two union types are known by the type system For all other combinations, all you can do is to pick a type which is a super-type of both the types. In this situation, the nearest common supertype is Object. The type system won't prevent you from putting booleans or lists into that.