On 4 Jan 2009, at 18:08, Aaron Tomb wrote:
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.
Of note, you probably don't need to do this. It's usually safer to
associate data with a key, using Data.Map, or just pairing objects
with a unique id.
Bob
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