Hi.
The definitions are accepted by GHC:
class Functor f where type FC f a :: Constraint type FC f a = ()
fmap :: (FC f a, FC f b) => (a -> b) -> f a -> f b
instance Functor [] where fmap = map
Yes. This is what I would have expected to work.
But I don't like the 'a' being an index parameter, and then the following expression:
fmap (+1) [1::Int]
Gives the error:
Could not deduce (FC [] Int) arising from a use of `fmap' In the expression: fmap (+ 1) [1 :: Int] In an equation for `it': it = fmap (+ 1) [1 :: Int]
gives the error:
Number of parameters must match family declaration; expected 1 In the type synonym instance default declaration for `FC' In the class declaration for `Functor'
I get the same error, but it looks like a bug to me: If I move the declaration type FC f a = () to the instance, then the example passes. Cheers, Andres