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

On Wed, Aug 20, 2014 at 11:28 AM, Johan Tibell wrote:

On Wed, Aug 20, 2014 at 10:23 AM, Erik Hesselink wrote:

...

As I understood, the question was if GHC would first compare pointers, and only call the Eq instance if the pointers are not equal. I guess this would be safe, but I don't think GHC does such a thing.

I think the reason it isn't done is that it's not always an optimization. We do it manually in e.g. bytestring.

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 (Note: not actually tested.) It's difficult for me to imagine a case though where a timing attack could result on two data structures sharing the same pointer; usually we'd be talking about comparing two ByteStrings or Texts that come from very different sources. Michael