% Figure 11.10 An implementation of breadth-first search.
:- op( 900, fy, not).
% not Goal): negation as failure;
% Note: This is often available as a built-in predicate,
% often written as prefix operator "\+", e.g. \+ likes(mary,snakes)
not Goal :-
Goal, !, fail
;
true.
% solve( Start, Solution):
% Solution is a path (in reverse order) from Start to a goal
solve( Start, Solution) :-
breadthfirst( [ [Start] ], Solution).
% breadthfirst( [ Path1, Path2, ...], Solution):
% Solution is an extension to a goal of one of paths
breadthfirst( [ [Node | Path] | _], [Node | Path]) :-
goal( Node).
breadthfirst( [Path | Paths], Solution) :-
extend( Path, NewPaths),
conc( Paths, NewPaths, Paths1),
breadthfirst( Paths1, Solution).
extend( [Node | Path], NewPaths) :-
bagof( [NewNode, Node | Path],
( s( Node, NewNode), not member( NewNode, [Node | Path] ) ),
NewPaths),
!.
extend( Path, [] ). % bagof failed: Node has no successor