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

[Rudy Matela]

> 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