Union types: They can be introduces in a limited,...

# language-proposals

[louiscad]

Union types:

They can be introduces in a limited, yet useful way: In the only place where we also have intersection types, which is the where clause that specifies type arguments constraints for a function.

I think that would satisfy many use cases where we currently write multiple functions (often extensions).

The when expression would also need to be tweaked to make use of this I think, and probably some compiler magic, and maybe @JvmOverloads, unless these functions are required to have the union type constrained generic argument to be reified, which requires the function to be inline, and always inlining the code.

[Iaroslav Postovalov]

inline fun <reified T> T.printIntOrString() where T : Int or String = when (this) {
  is Int -> print("int! $this")
  is String -> print ("string. $this")
}

[Iaroslav Postovalov]

🤔

[alexsullivan114]

➕ to Union Types, though I'm interested in seeing the proposed syntax for declaring a union type. In Typescript declaring a union type is trivial, and it's really nice. In Kotlin you can effectively mimic them with sealed classes, but it takes a lot more boilerplate.

[jw]

The syntax is the least interesting part of union types though. The underlying implementation on the JVM, JS, and native, and how those are exposed to Java, JS, and C is far more interesting and challenging to get right.

[louiscad]

If it only works for reified type parameters (as suggested in this proposal), interop is out of the question since it's a Kotlin only feature that only applies to inlined code.

[Astronaut4449]

I miss algebraic data types so much 😢

[louiscad]

@Astronaut4449 Kotlin has a version of them in the form of `sealed class`es.

[Astronaut4449]

I know, but that's sometimes just not enough.