How to remove repetitions using retract in this particular situation?

Question

    %Examples:
    %days([saturday,sunday,monday,tuesday,wednesday,thursday]).
    %slots([1,2,3,4,5]).
    %course_meetings(csen402,tutorial,t07,nehal,'tutorial for t07').
    %course_meetings(comm401,lecture,all_group_4,dr_amr_talaat,'lecture 1')
%tutorialrooms([c6301,b4108,c2201,c2301,c2202,c2203]).



        day_tut(Day,Slot,Place,Course,Group,Instructor,Descr):-
            days(X),member(Day,X),
            tutorialrooms(X1),member(Place,X1),
            course_meetings(Course,tutorial,Group,Instructor,Descr),
            slots(X2),member(Slot,X2),
            assert(day(Day,Slot,tutorial,Place,Course,Group,Instructor,Descr)).

I would like to find a way to remove certain facts after asserting for example every (day) fact has to have only one room for each day and slot example: we can have day(sat,1,_,c6301,_,_,_,_) and day(sat,1,_,c6302,_,_,_,_) but we can't have another occurrence of day(sat,1,_,c6301,_,_,_,_).

Answer 1

If you simply want to remove redundant solutions of a Goal – this is what you probably mean with removing repetitions – simply replace Goal by setof(t,Goal,_). This works as long as there are only ground solutions for Goal and as long as Goal terminates universally. Thus, there is no need for any data base manipulation to remove redundant solutions.

?- member(X, [a,b,a,c]).
   X = a
;  X = b
;  X = a  % redundant!
;  X = c.
?- setof(t,member(X, [a,b,a,c]),_).
   X = a
;  X = b
;  X = c.