I was expecting to get an unchecked cast warning here where I do `k as C`, especially since I got an...

# compiler

[eygraber]

I was expecting to get an unchecked cast warning here where I do k as C, especially since I got an error when I tried lookup[C::class] = factory despite C being guaranteed to be a K. Anyone know why I didn't get a warning?

public class FooFactory<K : FooKey> {
  @PublishedApi internal val lookup: MutableMap<KClass<out K>, (K) -> Foo<out K>> = mutableMapOf()

  public inline fun <reified C : K> addFactory(
    noinline factory: (C) -> Foo<out C>
  ) {
    lookup[C::class] = { k ->
      factory(k as C)
    }
  }
}

[dmitriy.novozhilov]

C is reified parameter, so it will be inlined to specific type argument on each call site, which makes this cast not unchecked

[eygraber]

Even though the k parameter to the lambda is typed as K? So if C was FooKey.Bar wouldn't the cast be k: K as FooKey.Bar?

[dmitriy.novozhilov]

Yes, that's right
And because of type erasure, at runtime it will be k: FooKey as FooKey.Bar

[eygraber]

But then I could do lookup[FooKey.Bar::class]?.invoke(FooKey.Baz) which I would think would result in a ClassCastException

[dmitriy.novozhilov]

Not sure I understand what you mean
Could you provide the full snippet?

[eygraber]

That's all the code I currently have, but theoretically I could have

sealed interface FooKey {
  data object Bar : FooKey
  data object Baz : FooKey
}

and because I wrap the factory with a cast that assumes K is C, I could later retrieve a factory for C, but pass it something that isn't C as the K parameter which would cause a ClassCastException

[dmitriy.novozhilov]

Well, actually any as cast may cause ClassCastException

[dmitriy.novozhilov]

And in your original code there are type constraints, which are checked at compile time

C <: K <: FooKey

So you can not pass such C which won't be subtype of K

[eygraber]

So then why is it an error to say lookup[C::class] = factory?

[dmitriy.novozhilov]

lookup (value): (K) -> Foo<out K> = Function1<K, Foo<out K>>
       factory: (C) -> Foo<out K> = Function1<C, Foo<out K>>

Function1 declared as Function1<in T, out R>
factory may be subtype of lookup if in argument of factory is supertype of the corresponding argument of lookup, which is not correct (C <: K, not vice versa)

[eygraber]

Ah didn't realize that the parameter is in but I guess that makes sense 😅

[dmitriy.novozhilov]

Actually, Kotlin type system just can't express the relation between key and value, which you put in the lookup map
You has a relation that type of value depend on the type of specific key, which is possible only in system with dependent types (like in Agda or Arend)

lookup: MutableMap<K' : K, KClass<out K'>, (K' -> Foo<out K')>

[dmitriy.novozhilov]

Sorry, don't remember exact notation which makes such definitions clear

[eygraber]

Thanks, it's pretty clear now. I was hoping that using the class type parameter would help with this, but I guess it just doesn't work like that