Revision as of 18:53, 28 September 2017

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 such as Tree a have kinds of the form 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.

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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."
-Ordinary types, also called ''monotypes'' or ''nullary'' 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, also called ''monotypes'' or ''nullary'' type constructors, have kind <TT>*</TT>. Higher order type constructors such as <hask>Tree a</hask> have kinds of the form <TT>P -> Q</TT>, where <TT>P</TT> and <TT>Q</TT> are kinds. For instance:
   Int :: *

Kind: Difference between revisions

Revision as of 18:53, 28 September 2017

See also