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
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