guys, is there any discussion or explanation about...

# announcements

[evkaky]

guys, is there any discussion or explanation about, why no union/intersection types (like in typescript) in kotlin?
I don’t think, that it’s a must have feature, but I think this topic is quite interesting and was already discussed

[janvladimirmostert]

It would be useful

[karelpeeters]

There is some discussion here: https://discuss.kotlinlang.org/t/union-types/77/2

[darkmoon_uk]

Isn't a sealed class what you are after? These are very powerful and flexible, and can behave semantically like a `union`ed type in C (even if the underlying memory layout/usage doesn't follow suit).

[karelpeeters]

Well for example this isn't really possible: (made-up syntax) typedef Value = String | Int, you can emulate it with container sealed classes but that's a lot of boilerplate.

[darkmoon_uk]

Ah, ok, yes it'd be:

sealed class Value {
    data class String( value: String ) : Value()
    data class Int( value: Int ) : Value()
}

[darkmoon_uk]

Or some variant using generics

[darkmoon_uk]

but either way, as you say, a tonne more boilerplate than your pseudocode.

[darkmoon_uk]

Your proposal looks like it'd be a headache for the type system though; not sure how that'd work, but maybe I just need to be enlightened 🙂

[karelpeeters]

This is not my proposal, I can't say I've ever needed them. There are a ton of languages that do this, from Haskell to C++.