# Higher Kinded Types¶

Identity a = a
List Int
Array Int
Either String Int


Above snippet registers some types like Identity, List, Int, Array, Either, String, where Int, String are the concrete types and List, Array, Either, Identity are type constructors. Furthermore, Either stands apart from the other 3, for it can be applied twice( Either : (a : Type) -> (b : Type) -> Either a b ), while the other 3 could only be applied once.

To describe this internal property of types, a concept called Kind has been built and, you can regard a kind as the type of a type.

Each type has a kind ascription:

(the star * means it's the concrete type)

Int : *

Identity, Array, List : * -> *

Either : * -> * -> *