Specifying Kinds of Types
Fri, 8 Feb 2002 12:36:56 +0100 (MET)
Ashley Yakeley wondered:
| I'd like to be able to declare the kinds of new types
| and synonyms, because sometimes Haskell can't infer
| them. For instance:
| data CMap0 p q = MkCMap0;
| Actually, I wanted p and q to both have kind '* -> *'.
The following workaround might be useful in this case:
data CMap0 p q = MkCMap0
| DummyConstructor (p Int) (q Int)
But that doesn't help with the following example:
| type Composer c = forall x y z. (c y z) -> (c x y) -> (c x z);
| [..x..y..z..] But I wanted them to have kind '* -> *'.
Here, you might do the following trick:
type HasKind_Help x dummy = x
type HasKind_Star_To_Star x = HasKind_Help x (x Int)
type C c x y = c (HasKind_Star_To_Star x)
type Composer c =
forall x y z . C c y z -> C c x y -> C c x z
(not tested!) There might be an easier workaround, too, but
you get the idea.
Chalmers University, Gothenburg, Sweden.