data A a = A a

amap::(Eq a, Eq b)=>(a->b)-> A a->A b
amap f (A a) = A $ f a

instance Functor A where
  fmap = amap

Error occurred
ERROR line 7 - Cannot justify constraints in instance member binding
*** Expression    : fmap
*** Type          : Functor A => (a -> b) -> A a -> A b
*** Given context : Functor A
*** Constraints   : (Eq b, Eq a)