2008/3/3 Hugo Pacheco <[EMAIL PROTECTED]>: > class C a where > type S a (k :: * -> *) :: * > instance C [a] where > type S [a] k = (a,k a) > > does not compile under the claim that the type variable k is not in scope.
It's not entirely syntactical sugar; I believe that when a type family is a member of a type-class it makes the assertion that only the class variables are part of the "function". In the class declaration: > class C a where > type S a (k :: * -> *) :: * there are two arguments to the type function S, but the class C only provides one. Contrast with the following > type Pair a k = (a, k a) > class C a where > type S a :: (* -> *) -> * > instance C [a] where > type S [a] = Pair a > datakind Nat = Zero > or > datakind Nat = Zero | Succ Nat datakinds are listed under "future work" in the papers I have read; they aren't implemented yet. -- ryan _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
