WTF is HKT

Higher order functions are functions that returns functions, higher order types are types that returns types. In a poor man's term:

class List<T> // "HKT"

List takes in a type T and returns List<T>. But, we know that poor Java doesn't have HKT:

<T> void foo(List<T> xs) {} // Good
<F, T> void bar(F<T> xs) {} // Bad

F is an HKT and Java does not allow it, we want:

<F<?>, T> void bar(F<T> xs) {}

This is doable in Scala:

def foo[F[_], A, B](xs: F[A]): F[B] = ???
// so
def map[F[_] <: Seq[?], A, B](xs: F[A], f: A => B): F[B] = ???

Notice that in Kotlin, all Iterable map methods must uniformly return a List, breaking the shape.

Scala win, QED.