I have a class, C, of * -> * types.
class C f where
foo :: f a -> a -> a
I also have two types, A and B, which are instances of C
instance C A where
foo a x = ...
instance C B where
foo b x = ...
For various reasons, I also want the type D, defined as follows, to be an instance of C.
type D x = (A x, B x)
According to the answer to this question, type synonyms can't be partially applied, and to define the instance as I would like, I either need to define it in steps (preferable) or define D as a newtype (less preferable). It seems to me like there should be some way to accomplish the former, but I haven't been able to figure one out so far. Any help is appreciated.