Difference between revisions of "Kind"
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| + | [http://en.wikipedia.org/wiki/Kind_%28type_theory%29 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." |
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| − | '''Kinds''' classify [[type]]s. Kinds are to types and type-constructors what types are to values. |
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Ordinary types have kind <TT>*</TT>. Type constructors have kind <TT>P -> Q</TT>, where <TT>P</TT> and <TT>Q</TT> are kinds. For instance: |
Ordinary types have kind <TT>*</TT>. Type constructors have kind <TT>P -> Q</TT>, where <TT>P</TT> and <TT>Q</TT> are kinds. For instance: |
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(->) :: * -> * -> * |
(->) :: * -> * -> * |
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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 [[ |
+ | 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 = |
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* [[GHC/Kinds]] |
* [[GHC/Kinds]] |
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| + | * [http://hackage.haskell.org/trac/ghc/wiki/IntermediateTypes#KindsareTypes Kinds ?, ??, # and (#)] |
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| + | * [[Books#Foundations|Pierce, Benjamin. ''Types and Programming Languages'']]. |
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[[Category:Language]] |
[[Category:Language]] |
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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.