On Wed, Aug 20, 2014 at 11:28 AM, Johan Tibell <johan.tibell@gmail.com> wrote:

On Wed, Aug 20, 2014 at 10:23 AM, Erik Hesselink <hesselink@gmail.com> 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