Can GHC warn if class instance is a loop?

Question

Real World Haskell has this example:

class BasicEq3 a where
    isEqual3 :: a -> a -> Bool
    isEqual3 x y = not (isNotEqual3 x y)

    isNotEqual3 ::         


        

Answer 1

I think it's perfectly fine for GHC to issue a warning in case of an "unbroken" cyclic dependency. There's even a ticket along those lines: http://hackage.haskell.org/trac/ghc/ticket/6028

Just because something is "undecidable" doesn't mean no instance of the problem can be solved effectively. GHC (or any other Haskell compiler) already has quite a bit of the information it needs, and it'd be perfectly possible for it to issue a warning if the user is instantiating a class without "breaking" the cyclic dependency. And if the compiler gets it wrong in the rare cases as exemplified in previous posts, then the user can have a -nowarnundefinedcyclicmethods or a similar mechanism to tell GHC to be quiet. In nearly every other case, the warning will be most welcome and would add to programmer productivity; avoiding what's almost always a silly bug.