[Haskell-cafe] Issues(Bugs?) with GHC Type Families

[Ryan Ingram]

2008/3/3 Hugo Pacheco <hpacheco at gmail.com>:
> 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