Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!watmath!clyde!burl!mgnetp!ihnp4!houxm!houxz!vax135!cornell!uw-beaver!tektronix!hplabs!sri-unix!decvax!mulga!munnari!Lee@Ucb-Vax From: Lee%Ucb-Vax@munnari.UUCP Newsgroups: net.lang.prolog Subject: BF search of SLD tree: Solution Message-ID: <756@sri-arpa.UUCP> Date: Wed, 6-Jun-84 22:12:38 EDT Article-I.D.: sri-arpa.756 Posted: Wed Jun 6 22:12:38 1984 Date-Received: Wed, 13-Jun-84 01:48:23 EDT Lines: 78 I didn't pay much attention to the recent discussion about breadth first traversals and that news has been removed from our system. However, I believe the original question concerned the BF traversal of the SLD (proof) tree for a given goal. The advantage of BF is that it will find all solutions eventually (depth first can get lost down an infinite branch). I have just been working on an algorithm which needs a fair search strategy for SLD trees so, as a first step, I wrote the following program. It has been briefly tested on MU-Prolog and C-Prolog. % Breadth first search of the SLD tree of a goal. % Returns all solutions. % The main data structure is a list of 'buds' (starts of % branches which have not been explored). Each one has % an instance of the initial goal and the current goal % for that branch. A difference list (X-Y) is used to % implement a queue of buds. % -- Lee Naish 6/6/84 ?- op(400, xfx, :). % used for initial_goal:current_goal ?- op(490, xfy, '.'. % so W:X.Y-Z is ((W:X).Y)-Z % return all solutions to G, using bf solve(G, Solns) :- bf(G:G.X-X, Solns). % do bf search of SLD tree bf(X-Y, []) :- X == Y, % d-list of buds is empty !. bf(G:true.X-Y, G.S) :- % found solution - put it on list !, bf(X-Y, S). bf(G:(B1,B).X-Y, S) :- !, clauses(B1, Cl), newbuds(G, B1, B, Cl, Y-Z), bf(X-Z, S). bf(G:B.X-Y, S) :- clauses(B, Cl), newbuds(G, B, true, Cl, Y-Z), bf(X-Z, S). % expand a bud into a d-list of new buds newbuds(_, _, _, [], X-X). newbuds(G, B1, B, (H:-T).C, CG:B2.X-Y) :- copy(G:(B1,B), CG:(H,CB)), % copy so branches dont cappend(T, CB, B2), % interfere with each other newbuds(G, B1, B, C, X-Y). % copy a term copy(X, Y) :- assert(tmp(X)), retract(tmp(Y)), !. % get list of clauses for call clauses(true, [(true:-true)]) :- !. clauses((X;Y), [(X;Y:-X), (X;Y:-Y)]) :- !. % handle other system preds here clauses(H, Cl) :- all((H:-T), clause(H, T), Cl). % append for conjunctions (with ',') cappend(A, true, A) :- !. cappend(true, A, A) :- !. cappend((A,B), C, (A,D)) :- !, cappend(B, C, D). cappend(A, B, (A,B)). ?- lib all. % all solutions predicate from library % (could also use bagof)