So say you wanted to define a sort of "equality" on pairs (this isn't made up -- I wanted to do just this, recently). Say you have:
data MyPair a b = MyPair a b
instance (Eq a, Eq b) => Eq (MyPair a b) where (MyPair a b) == (MyPair a' b') = (a == a') && (b == b')
this is obvious. but supposing you wanted a sort of relaxed equality wherein, if you chould check for equality on the second element, you would, otherwise you would just check the first element. i would want to say something like:
instance (Eq a, Not (Eq b)) => Eq (MyPair a b) where (MyPair a _) == (MyPair a' _) = a == a'
But from what you're telling me there's no way to have both of these instances, correct?
How about using overlapping instances? Add: instance Eq a where _ == _ = True to your program: if there isn't a ``better'' instance, it'll use this one. Such an instance declaration is inherently dangerous to have around, but might help your particular situation. Some tests: Main> MyPair 1 2 == MyPair 1 3 False Main> MyPair 1 (\x -> x+1) == MyPair 1 (\x -> x+12) True Don't forget to start hugs with flags: +o -98 (I haven't tried with ghc.) Admittedly, this is quite horrible and should be avoided if possible. -Levent.