meta-predicate

This is the code that i am trying to understand. co(X) :- co(X,[],L). co([],A,A):- write(A). co([X|Xs], A, L) :- p(X-Z,A,R), !, Z1 is Z+1, co(Xs, [X-Z1|R], L). co([X|Xs], A, L) :- co(Xs, [X-1|A], L). p(X-Y,[X-Y|R],R):- !. p(X,[H|Y], [H|Z]) :- p(X,Y,Z). What is the use of '!' and predicate p(,,) in the above code. OR Can anybody just add comments in every step of the above code so that i can able to understand . Thanks. mat Beginners tend to use !/0 because they are not aware of its negative consequences. This is because most Prolog textbooks that are popular among beginners are quite bad and

The following higher order predicate succeeds if all pairs of the list's elements are true for a given relation. Is there a common or better, more intention revealing name for this relation? My original motivation for this name was that in clpfd , there is often a constraint all_different/1 which is described as being true, iff the elements are pairwise different . In fact, rather preferred to say the elements are all different, but I have been frequently corrected (by fellow Prolog programmers) to use pairwise different. In fact, this constraint can now most naturally be expressed as pairwise

So the univ operator. I don't exactly understand it. For example this: foo(PredList,[H|_]) :- bar(PredList,H). foo(PredList,[_|T]) :- foo(PredList,T),!. bar([H|_],Item) :- G =.. [H,Item],G. bar([_|T],Item) :- bar(T,Item). What is this doing? This looks to see if another predicate is true. I don't understand what the ".." does. How would you rewrite this without the univ operator? Univ ( =.. ) breaks up a term into a list of constituents, or constructs a term from such a list. Try: ?- f(x,y) =.. L. L = [f, x, y]. ?- f(x,y,z) =.. [f|Args]. Args = [x, y, z]. ?- Term =.. [g,x,y]. Term = g(x, y).

Given the following facts in a database: foo(a, 3). foo(b, 2). foo(c, 4). foo(d, 3). foo(e, 2). foo(f, 6). foo(g, 3). foo(h, 2). I want to collect all first arguments that have the smallest second argument, plus the value of the second argument. First try: find_min_1(Min, As) :- setof(B-A, foo(A, B), [Min-_|_]), findall(A, foo(A, Min), As). ?- find_min_1(Min, As). Min = 2, As = [b, e, h]. Instead of setof/3 , I could use aggregate/3 : find_min_2(Min, As) :- aggregate(min(B), A^foo(A, B), Min), findall(A, foo(A, Min), As). ?- find_min_2(Min, As). Min = 2, As = [b, e, h]. NB This only gives the

I'm currently working on a very short project on Prolog, and just got stuck trying to apply a "filter" I have created to a list. I have what you could call the filter ready, but I can't apply it. It'd be better if I illustrate: filter(A, B) ...outputs 'true' if certain conditions are met. filterList(A, [X, Y, Z]) ...outputs a list which includes all elements from the second argument that make the filter output false . (So if filter(A, X) is true, the output is [Y, Z] ). I have the "filter" function ready, but now I need to apply it to a list as shown on the second example, excluding all