Er, what I meant was

class Foo s t a b | s -> a, t -> b, s b -> t, t a -> s

That is what I get for dashing it off off the cuff. =)

Then s determines some type argument for it, t determines its type argument, but Using s and the other type argument I can determine the whole containing type 't' and vice versa.

instance Foo (Identity a) (Identity b) a b

On Wed, May 1, 2013 at 7:02 AM, Ian Lynagh <ian@well-typed.com> wrote:

On Tue, Apr 30, 2013 at 11:35:10PM -0400, Edward Kmett wrote:
>
> I have dozens of classes of forms like
>
> class Wrapped s t a b | a -> s, b -> t, a t -> s, b s -> t

Isn't this equivalent to just

class Wrapped s t a b | a -> s, b -> t

?

Thanks
Ian