Kind
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Wikipedia says, "In type theory, a kind is the type of a type constructor or, less commonly, the type of a higher-order type operator. A kind system is essentially a simply typed lambda calculus 'one level up,' endowed with a primitive type, denoted * and called 'type', which is the kind of any (monomorphic) data type."
Ordinary types, also called monotypes or nullary type constructors, have kind *. Higher order type constructors have kinds of the form P -> Q, where P and Q are kinds. For instance:
Int :: * Maybe :: * -> * Maybe Bool :: * a -> a :: * [] :: * -> * (->) :: * -> * -> *
A type with a more complicated kind is the StateT monad transformer
newtype StateT s m a :: * -> (* -> *) -> * -> *
In Haskell 98, * is the only inhabited kind, that is, all values have types of kind *. GHC introduces another inhabited kind, #, for unlifted types.
See also
- GHC/Kinds
- Kinds on the GHC Commentary
- TypeType on the GHC Commentary
- Kinds ?, ??, # and (#)
- Pierce, Benjamin. Types and Programming Languages.