Type constructors such as List or Option or DStream take other types as parameters to eventually produce concrete types. This reminds us of functions or value constructors, which take values as parameters to produce new values. Similarities do not end here; type constructors can be partially applied in the same fashion that functions can:
// Partially apply a value constructor.
val func: String => Int => String = ???
val funcA: Int => String = func("A")
// Partially apply a type constructor.
trait Either[L, R] { ??? }
type StringOrA[A] = Either[String, A]But what types are ? They are little labels that values carry so that we can reason about them. Types themselves have their own labels called kinds. A kind is more or less the type of a type.
scala> :k -v String
String's kind is A
*
This is a proper type.
scala> :k -v Function2
Function2's kind is F[-A1,-A2,+A3]
* -(-)-> * -(-)-> * -(+)-> *
This is a type constructor: a 1st-order-kinded type.
scala> :k -v Monad
Monad's kind is X[F[A]]
(* -> *) -> *
This is a type constructor that takes type constructor(s): a higher-kinded type.
A star * stands for a type that does not take any type parameters: a concrete type. The only type that can be assigned to a value is a concrete type.
// Valid
val x: String = ???
// Invalid
val x: Function2 = ???
// Invalid
val x: Monad = ???Now, Function2 type constructor is more interesting; ignoring variance annotations it has kind * -> * -> * -> *. It takes as input a concrete type * and returns a type constructor that takes two type parameters and returns a concrete type * -> * -> *. Put differently, it takes as input three type parameters and returns a concrete type. Monad takes as input a type of kind * -> * and returns a concrete type. We cannot pass a Function2 to a Monad, because the kinds do not match. Instead we can pass Option type constructor which has kind * -> *.
- [1] - Generics of a higher kind.