[plt-scheme] equal? operator

Eli Barzilay schreef:

>On Nov 20, Hans Oesterholt-Dijkema wrote:
>  
>
>>Is it possible to extend the equal? operator for new (structure)
>>types in mzscheme?
>>    
>>
>
>If you need this for objects in your OO system, then why don't you
>implement your own equality predicate using the class system?
>(Especially given your reasons for why the standard solutions don't
>work.)
>
>  
>
Hmm. I don't know If I follow you here. But no, I'm not interested in an 
equal?
predicate for my OO system currently. I am interested in an equal? 
predicate for
my datastruct library (sets, avl trees, etc).

>
>On Nov 20, Hans Oesterholt-Dijkema wrote:
>  
>
>>Oh, and is it maybe also possible to extend eq?  I'd like eq? to
>>work differently for persistent roos objects, comparing their Object
>>Identifiers instead of their "memory" locations.
>>    
>>
>
>You don't want to do that!  Making `eq?' do anything else is going to
>be confusing anyone who sees your code.
>  
>
I do want to do this, because the semantics are right. Look here:

 > (define store (list (list 2 3 4) (list 5 5 9)))
 > (define (search n)
     (define (in n L)
       (if (null? L)
          #f
          (if (= n (car L))
             #t
             (in n (cdr L)))))
     (define (f L)
        (if (null? L)
           #f
           (if (in n (car L))
               (car L)
               (f (cdr L)))))
     (f store))

 >(define a (search 4))
 >(define b (search 4))
 >(eq? a b)
#t

Now the equivalent operation in Persistent ROOS

 > (psearch (= (my-class my-attribute) 4))
(#roos #roos #roos)
 > (define a (car (psearch (= (my-class my-attribute) 4))))
 > a
#roos
 > (define b (car (psearch (= (my-class my-attribute) 4))))
 >b
#roos
 > (eq? a b)
#f
 > (peq? a b)
#t

Although the memory locations of the returned roos objects are different,
they are actually the same, in the backend system. The Object IDentifier
of a and b adhere (= a b).

Best whishes,

Hans



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