Fun with types! Resolving multiple instance declarations

Question

I\'m trying to write some Haskell code in which there are multiple data types, each of which can have multiple implementations. To do this, I define each data type as a

Answer 1

There's no really good way to do this; the best practice is to define some constants like

plusA, minusA :: (A a, Num x) => a x -> a x -> a x

which makes writing the Num instances more mechanical after you have an A instance:

instance A Foo where ...
instance Num x => Num (Foo x) where
    (+) = plusA
    (-) = minusA