It would be great to have union types just for ext...

# language-proposals

[Ayfri]

It would be great to have union types just for external declarations in K/JS, so it's a lot easier to create typings for a TypeScript library. I think it would be a pretty easy task to implement it just in that specific context so this is why I post a message specifically for this

[dmitriy.novozhilov]

It won't be easy, because it means full support of union types in type system, which is main blocker from design and implementation sides

[Ayfri]

Ah, but doesn't they exist in a way when using multiple return types in a when ?

[dmitriy.novozhilov]

No. when expressions are nothing more than select function from inference point of view

when {
  ... -> x
  ... -> y
  ... -> z
}
// same as
fun <K> select(vararg value: K): K

select(x, y, z)

[Ayfri]

Then what happens there ? 🤔

[dmitriy.novozhilov]

Boolean & String is an intersection type, not union

[dmitriy.novozhilov]

interface A
interface B
interface C : A, B
interface D : A, B

fun test(c: C, d: D) {
    val x = select(c, d)
    // type of x is CommonSuperType(C, D) = A & B
}

[Ayfri]

Ah okay I see, thanks for the information :)