Difference between revisions of "Kind"

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(Include Wikipedia introduction, which is clearer (to me) and link to TaPL)
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(->) :: * -> * -> *
 
(->) :: * -> * -> *
   
In Haskell 98, <TT>*</TT> is the only '''inhabited kind''', that is, all values have types of kind <TT>*</TT>. GHC introduces another inhabited kind, <TT>#</TT>, for [[unboxed type]]s.
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In Haskell 98, <TT>*</TT> is the only '''inhabited kind''', that is, all values have types of kind <TT>*</TT>. GHC introduces another inhabited kind, <TT>#</TT>, for [[unlifted type]]s.
   
 
= See also =
 
= See also =

Revision as of 15:41, 31 December 2015

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 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 unlifted types.

See also