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 ----------------------------------------------------------------- Paulo Jorge Lopes de Moura, PhD Assistant Professor Dep. of Computer Science, University of Beira Interior 6201-001 Covilhã, Portugal Office 3.18 Ext. 3276 Phone: +351 275 242081 Fax: +351 275 319899 Email: <mailto:pmo...@di.ubi.pt> Home page: <http://www.di.ubi.pt/~pmoura> Research: <http://logtalk.org/> Blog: <http://blog.logtalk.org/> ----------------------------------------------------------------- _______________________________________________ Bug-prolog mailing list Bug-prolog@gnu.org https://lists.gnu.org/mailman/listinfo/bug-prolog
