[Haskell-cafe] How to check object's identity?

Aaron Tomb atomb at galois.com
Sun Jan 4 12:08:41 EST 2009

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#


Take note of the "reallyUnsafe" prefix, though. :-) It's not something  
most programs should ever need to deal with.


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