Not a general one that's interesting.

There are two Eq instances that'll compile for all functions (not that it's advisable):

instance Eq ((->) a b) where
         (==) _ _ = True

instance Eq ((->) a b) where

         (==) _ _ = False

You can't get more interesting in the general case, because you can't inspect the arguments.

If you are okay with distinguishing solely by application you can get a little more interesting:

instance (Bounded a, Enum a, Eq b) => Eq ((->) a b) where
    f == g = all (\ x -> f x == g x) [minBound .. maxBound]

*Main> (&&) == (&&)
True
*Main> (&&) == (||)
False

Though I'm still not sure I'd say it's a *good idea*...