On Jan 3, 2009, at 7:28 AM, Xie Hanjian wrote:
Hi,
I tried this in ghci:
Prelude> 1:2:[] == 1:2:[] True
Does this mean (:) return the same object on same input, or (==) is not for identity checking? If the later is true, how can I check two object is the *same* object?
As others have explained, the == operator doesn't tell you whether two values are actually stored at the same location in memory. If you really need to do this, however, GHC does provide a primitive for comparing the addresses of two arbitrary values: reallyUnsafePtrEquality# :: a -> a -> Int# http://haskell.org/ghc/docs/latest/html/libraries/ghc-prim/GHC-Prim.html#22 Take note of the "reallyUnsafe" prefix, though. :-) It's not something most programs should ever need to deal with. Aaron