Re: I think I have found a bug in list processing

[Paulo Moura]

On 29/03/2012, at 16:24, phi16 wrote:

> I think the following dialogue is a bug:
> 
> 
> | ?- member(3,X),member(2,X),member(3,X).
> 
> X = [3,2|_] ? ;
> 
> X = [3,2,3|_] ? ;
> 
> X = [3|_] ? ;
> 
> X = [3,3|_] ? ;
> 
> X = [3,_,3|_] ? ;
> 
> X = [3,_,_,3|_] ? ;
> 
> X = [3,_,_,_,3|_] ?
> 
> 
> The input tells the list named X must contain 2.
> But the 3rd output shows there is possible that X does not contain 2.


You're correct. It's a bug. The first call to member/2 constructs an open list 
with 3 in the first position. The second call to member/2 adds 2 at the second 
position. The third call to member/2 (the one you are backtracking to until you 
will eventually run out of memory) will add 3 to the third position and, on 
backtracking to the fourth, fifth, ... positions. I.e. the expected/correct 
behavior is:

?- member(3,X),member(2,X),member(3,X).
X = [3,2|_A] ? ;

X = [3,2,3|_A] ? ;

X = [3,2,_A,3|_B] ? ;

X = [3,2,_A,_B,3|_C] ? ;

X = [3,2,_A,_B,_C,3|_D] ? ;

X = [3,2,_A,_B,_C,_D,3|_E] ? ;

X = [3,2,_A,_B,_C,_D,_E,3|_F] ? ;

X = [3,2,_A,_B,_C,_D,_E,_F,3|_G] ? ;

X = [3,2,_A,_B,_C,_D,_E,_F,_G,3|_H] ? ;

...

Cheers,

Paulo


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Paulo Jorge Lopes de Moura, PhD
Assistant Professor
Dep. of Computer Science, University of Beira Interior
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