Re: [Haskell-cafe] Does GHC compare pointers when eval'ing (==)

There are two cases I can think of where it would also change the semantics of the code:

1. An Eq instance that doesn't obey the reflective property (not recommended):

data BadEq = BadEq instance Eq BadEq where BadEq == BadEq = False

2. Eq instances intended to avoid timing attacks, by always comparing the entire data structure.

newtype SlowEq a = SlowEq [a] instance Eq a => Eq (SlowEq a) where SlowEq x == SlowEq y = slowAnd $ length x == length y : zipWith (==) x y

slowAnd = loop True where loop !x [] = x loop !x (!y:ys) = loop (x && y) ys

Third case that would change the semantics: 3. Non-terminating evaluation of a value: let x = [1..] in x == x