On 21/07/2011, at 9:08 AM, Paul Johnson wrote:
I would have thought that the compiler, as a matter of optimisation, could insert a check to see if (==) is comparing an object with itself. The only way I can see this breaking is with perverse instances of Eq that would return False for "f == f".
== is a function with user-defined instances. It would be, to put it minimally, bad manners, but there's nothing to actually *stop* a programmer writing data Boojum = Plant | Snark instance Eq Boojum where Plant == Plant = True _ == _ = False f x = x == x main = print $ f Snark Presumably inside the body of f, x and x would be identical pointers, but the only right answer is False, not True. If you think this is a bit far fetched, consider the IEEE definition of equality for floating-point numbers: let x = 0.0/0.0 in x == x The answer is False, so the optimisation breaks down even with a system-defined type.