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

No, I'm afraid GHC doesn't do anything like that. Also that isn't possible in general.

You see, the methods of a type class could be mutually recursive in a useful way. Here's a contrived example of such a type class. It's perfectly fine to not define either sumOdds or sumEvens, even though their default implementations are in terms of each other.

class Weird a where
    measure :: a -> Int

    sumOdds :: [a] -> Int
    sumOdds [] = 0
    sumOdds (_:xs) = sumEvens xs

    sumEvens :: [a] -> Int
    sumEvens [] = 0
    sumEvens (x:xs) = measure x + sumOdds xs