On 06.03.2010, at 18:19, Bertram Felgenhauer wrote:
Axel Simon wrote:
Any easy way of comparing pointers? I mean, if I have something like Tree a = N | B (Tree a) a (Tree a) and have (l::Tree a) and (r::Tree a), can I ask about the "physical equality"?
You can! Despite the names appearing in the following code, the following is safe:
-- | Equality on pointers. ptrEqual :: a -> a -> Bool ptrEqual x y = unsafePerformIO $ do nx <- makeStableName x ny <- makeStableName y return (nx==ny)
{-# LANGUAGE MagicHash #-} import GHC.Exts
ptrEqual' :: a -> a -> Bool ptrEqual' x y = case reallyUnsafePtrEquality# x y of 0# -> False 1# -> True
This is actually a pointer comparison. I believe it can produce false negatives though, because indirections are not followed. If that's correct, a false negative may be turned into a positive by garbage collection. So use with care.
Interesting. But if reallyUnsafePtrEquality# is a primitive, it is evaluated either before or after GC, so it can't compare a pointer from one generation with the next. Having ptrEqual return False if, in fact, the two values are identical is not a problem for the considered data structure. It merely implies that a structural comparison is performed which is slower. Cheers, Axel.
regards,
Bertram _______________________________________________ Libraries mailing list Libraries@haskell.org http://www.haskell.org/mailman/listinfo/libraries