are unions/sum types a thing yet?

# announcements

[Bernhard]

are unions/sum types a thing yet?

[Casey Brooks]

I don’t think Kotlin should support union types, they really go against the idea of type safety (with them, a single object could be one of several disparate types; ambiguity ensues). You might check out Arrow, which has many such asbtractions, such as Either https://arrow-kt.io/docs/arrow/core/either/ which do not introduce ambiguity to the type system

[Casey Brooks]

For javascript interop, you have the dynamic type already for cases where JS objects have that union type, and you can cast/convert that to the Kotlin type as appropriate

[streetsofboston]

With sealed classes and smart-casting, you can create your own sum-types, like the Either from Arrow.

[Bernhard]

yeah, sealed classes don't work with existing types do they?

[streetsofboston]

No, that is correct. You’d have to do Either<Int, String> or something.

[Bernhard]

I see, never thought of using the type this way

[Bernhard]

map & co won't make sense ofc but that doesn't matter in this case

[streetsofboston]

sealed class Either<out L, out R>
class Left<out L>(val left: L) : Either<L, Nothing>()  { ... }
class Right<out R>(val right: R): Either<Nothing, R> { ... }

And add some extension functions to ‘map’, ‘flatMap’, etc.

[GarouDan]

You could create an issue for it, and if receive enough votes could be interesting. I’d vote for it

[Paul Woitaschek]

I really would love to see union types

[Paul Woitaschek]

I have an implementation similar to adapter delegates by hannes Dorfmann and it would greatly benefit from union types

[karelpeeters]

This is the discussions post about it: https://discuss.kotlinlang.org/t/union-types/77, and this is the Shelved issue: https://youtrack.jetbrains.com/issue/KT-13108

[yen]

Nullable types in Kotlin are a form of union types, aren't they?

[karelpeeters]

Sure, a very limited form.

[raulraja]

https://github.com/arrow-kt/arrow-meta/pull/64