Is there a way to specify both `in` and `out` boun...

# getting-started

[Blake Anderson]

Is there a way to specify both in and out bounds for a generic type? For example, the union of List<String> and List<Object> can be described by both in String and out Object, but it doesn't seem like there's any way to express this.

[Youssef Shoaib [MOD]]

List is already out by default. Regardless, anything that's out Object (or out Any? in Kotlin) is just in whatever because anything that's returned out will be considered the most generic possible type Any?

[Blake Anderson]

Using List as an example, probably should have used MutableList to be more accurate. More accurate example:

class Parent;
class Child;
class ReadWrite<T: Parent> {
    fun read(): T
    fun write(value: T)
}

I want to allow value to be an instance of Child - effectively, T: out Parent & in Child. This doesn't seem possible, and I can't hack my way around this by defining fun write(value: Child) because that causes an ambiguous overload for T = Child.

[Sam]

It sounds like maybe you just want two different type parameters. As a general pattern:

interface Read<out T>
interface Write<in T>
interface ReadWrite<out R, in W: R>: Read<R>, Write<W>

Then, as you say, the union of ReadWrite<A, A> and ReadWrite<B, B> is ReadWrite<A, B> 😍. Very occasionally I've found myself wishing List followed this pattern as well.

[Sam]

But the downside of having to write MutableList<T, T> everywhere instead of MutableList<T> might be too high a price to pay 😄

[Blake Anderson]

Good call. Certainly not ideal but gets the job done and I couldn't come up with a workaround so this is great. A typealias would help with the few places the definition is used, but apparently you can't have nested typealiases which is unfortunate. Either way, I don't need that often and the messy generics in the class is far better than having to cast things around at the use site. Appreciate it!

https://github.com/Rhovas/Interpreter/commit/eb60d1711d596a065e3386f2d20b2f87372cc548#diff-15687bb6ec713efe65b2fd04[…]b9db29c56f323a5f3b119ee3535eL3