Does anyone know why Given an interface like: ```i...

# compiler

[Charlie Tapping]

Does anyone know why
Given an interface like:

interface SynkAdapter<T : Any> : IDResolver<T>, MapEncoder<T>

An impl like this does not count as the aggregated type (SynkAdapter)

class FooSynkAdapter(
    private val fooResolver: IDResolver<Foo> = FooResolver(),
    private val fooMapEncoder: MapEncoder<Foo> = FooMapEncoder()
) : IDResolver<Foo> by fooResolver, MapEncoder<Foo> by fooMapEncoder

You have to do this??

class FooSynkAdapter(
    private val fooResolver: IDResolver<Foo> = FooResolver(),
    private val fooMapEncoder: MapEncoder<Foo> = FooMapEncoder()
) : SynkAdapter<Foo>, IDResolver<Foo> by fooResolver, MapEncoder<Foo> by fooMapEncoder

Surely if an interface is just a union of two other interfaces then implementing those interfaces should make it compatible? I’m sure there must be a good reason for having to be explicit?

[Sam]

Kotlin just isn’t duck-typed 🤷. To be a thing, you have to actually be the thing.

[Vampire]

Just that it has the same methods does not mean it should be the same.
Imho it is good that you need to be explicit, as Kotlin is not duck-typed.

[Charlie Tapping]

Well I get why duck typing can be a bit too flexible, but in this case it seems to be quite different. Its not simply that my impl contains a function which has a signature which happens to match

[Charlie Tapping]

I’m explicitly implementing the two interfaces which make up the composite

[Charlie Tapping]

feels like we could use a new keywork

[Charlie Tapping]

union interface

[Sam]

You want an intersection, not a union

[Sam]

Technically, you can make one, with generics

[Charlie Tapping]

or that yes 😛

[Sam]

fun <A, B> doSomethingWith(a: A) where A: IDResolver<B>, A: MapEncoder<B>

[Sam]

It’s not possible to denote the intersection type outside of a generic constraint, though 😞

[Charlie Tapping]

Well I’ll write the extra interface for now 😄, isn’t there work being done on union types? maybe this will extend to intersection types?

[Sam]

Yes, the ticket does cover both intersections and unions 😍

[Vampire]

While I wouldn't expect that it would automagically have the SnykAdapter type with that change either 🙂

[Sam]

You could get close though, with a type alias:

typealias SynkAdapter<T> = IDResolver<T> & MapEncoder<T>

Just that SynkAdapter wouldn’t be an actual type