Haskell/Kinds

Fun with Types Kinds for C++ users * is any concrete type, including functions. These all have kind *: type MyType = Int type MyFuncType = Int -> Int myFunc :: Int -> Int typedef int MyType; typedef int (*MyFuncType)(int); int MyFunc(int a); * -> * is a template that takes one type argument. It is like a function from types to types: you plug a type in and the result is a type. Confusion can arise from the two uses of MyData (although you can give them different names if you wish) - the first is a type constructor, the second is a data constructor. These are equivalent to a class template and a constructor respectively in C++. Context resolves the ambiguity - where Haskell expects a type (e.g. in a type signature) MyData is a type constructor, where a value, it is a data constructor. data MyData t -- type constructor with kind * -> * = MyData t -- data constructor with type a -> MyData a *Main> :k MyData MyData :: * -> * *Main> :t MyData MyData :: a -> MyData a template <typename t> class MyData { t member; }; * -> * -> * is a template that takes two type arguments data MyData t1 t2 = MyData t1 t2 template <typename t1, typename t2> class MyData { t1 member1; t2 member2; MyData(t1 m1, t2 m2) : member1(m1), member2(m2) { } }; (* -> *) -> * is a template that takes one template argument of kind (* -> *) data MyData tmpl = MyData (tmpl Int) template <template <typename t> class tmpl> class MyData { tmpl<int> member1; MyData(tmpl<int> m) : member1(m) { } }; Fun with Types