Kind
From HaskellWiki
(Difference between revisions)
(Include Wikipedia introduction, which is clearer (to me) and link to TaPL) 

(2 intermediate revisions by one user not shown) 
Revision as of 19:44, 26 August 2012
Wikipedia says, "In type theory, a kind is the type of a type constructor or, less commonly, the type of a higherorder 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 have kind *. Type constructors have kind P > Q, where P and Q are kinds. For instance:
Int :: * Maybe :: * > * Maybe Bool :: * a > a :: * [] :: * > * (>) :: * > * > *
In Haskell 98, * is the only inhabited kind, that is, all values have types of kind *. GHC introduces another inhabited kind, #, for unboxed types.