<@U88HXKLP3> forall support in Kotlin type system ...

# arrow

[elizarov]

@jacob forall support in Kotlin type system is limited. Generic functions in Kotlin have forall type signature like fun <T> foo(a: T): List<T> (the type of foo in Haskell terms would be forall t. t -> List t), but you cannot directly capture such a functional type in a plain variable. You’ll have to always capture both foo and T with another generic function like fun <T> bar(a: T) = foo(foo(a)) or with a generic class like class A<T> { fun bar(a: T) = foo(a) }

[elizarov]

In a sense, forall quantifier in Kotlin have to alway be on the “top level” of the type.

[elizarov]

So in Kotlin you cannot directly have List (forall t. t -> t). In order to model such a type in Kotlin you’ll have to define an interface with generic function interface IdFun { fun <T> id(a: T): T } and then List<IdFun> would be equivalent to List (forall t. t -> t)

[pakoito]

Nice! This is similar to HKT emulation in typeclasses: https://arrow-kt.io/docs/patterns/glossary/#using-higher-kinds-with-typeclasses

[jacob]

@elizarov thanks for the heads up 🙂 I'll give it a shot that way and see if I can make some traction