[Haskell-cafe] How to check object's identity?
tom.davie at gmail.com
Sun Jan 4 12:33:03 EST 2009
On 4 Jan 2009, at 18:08, Aaron Tomb wrote:
> On Jan 3, 2009, at 7:28 AM, Xie Hanjian wrote:
>> I tried this in ghci:
>>> Prelude> 1:2: == 1:2:
>> 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#
> 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.
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