It's all bolted on.

Generics are to types what HKT are to type signatures. In functions, they usually map to arity. That is, an arity mismatch is are a higher-kinded type mismatch. An easy (if not totally accurate) way to view them is like doubly-generic types.

If you have some type A, it has the kind * . If you have some function from A -> B, it has the kind * -> * .

Higher Kinded Types

Higher-kinded types. Generics that take generics as parameters. They allow things like writing a function that both takes and returns any container type you can map over with different (defined) element types. Given how hard arity-0 generics have been to get into Go, I suspect that the chances of HKT making it are slim to none (but I think that the grandparent knows that and is just being funny).

I'll give the 'why' rather than the 'what'. In most mainstream languages you take a List<Int> and substitute for List<X> without changing the implementation of List. But there's another substitution you could have made, which is L<Int>. HKT allows you to parametrise L over Int, in the same way you were able to parametrise List over Int.