Kinds and type constructors

We have already studied and used types extensively. They allow us to distinguish between different forms of values in a way that can be used to document and automatically check our code. Kinds do the same thing one level up: they are a way of distinguishing different forms of types.

Proper types

The simplest and most ubiquitous group of types are so-called proper types. These are what we meant when we used the word type earlier in this book. Probably all the types that come to your mind are proper types: Int, Bool, [Char], Float -> String, and so on.

The kind associated with proper types is written *. This kind is often pronounced type, and in recent years, GHC has provided Type as a synonym for * in the Data.Kind module.

We can check the kind of a type in GHCi with the :kind command (or :k for short):

*Main> :kind Int
Int :: *
*Main> :kind Bool -> (Char, Float)
Bool -> (Char, Float) :: *

Another term for proper types is...