Re: strange behavior of subset

Thanks for your answer.

Prolog users must be very careful with
this predicate (the name "subset" is
rather hazardous).

here is a simpler example :

?- subset([p(A), q(A)], [p(1), p(2), q(2)]).
false.

?- subset([p(A), q(A)], [p(2), p(1), q(2)]).
A = 2,
B = 3.

Cheers
Dominique


Le 07/01/2014 14:52, Jan Wielemaker a écrit :
> On 01/07/2014 02:21 PM, db wrote:
>> Ok I found the problem :
>>
>> subset is based on membercheck that succeeds only once.
>
> Yes.  It is one of these predicates that have been added to
> the library very long ago.  The documentation already suggests
> it is not a logical predicate.  You give a nice example.
>
>      Cheers --- Jan
>
>>
>> Bye.
>> Dominique
>>
>>
>> Le 07/01/2014 14:17, db a �crit :
>>> Hello,
>>>
>>> I used swi prolog Multi-threaded, 64 bits, Version 7.1.4
>>> under Ubuntu.
>>>
>>> I am suprise by the behavior of the subset predicate :
>>>
>>>
>>> ?- subset([p(A), q(A,B)], [p(1), p(2), q(2,3)]).
>>> false.
>>>
>>>
>>> ?- subset([p(A), q(A,B)], [p(2), p(1), q(2,3)]).
>>> A = 2,
>>> B = 3.
>>>
>>> Thus subset takes into account the order of the element
>>> in the sets, what is not expected when one deals with sets.
>>>   Is is normal ?
>>>
>>> Thanks a lot
>>>
>>> Dominique
>>>
>>>
>>>
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