Why "ERROR: Out of global stack"

%(SWI-Prolog)You can run the program by: ?- bestfs([8,1,3,7,0,2,6,5,4],P). 

initial([8,1,3,7,0,2,6,5,4]).

goal([1,2,3,8,0,4,7,6,5]).

operators([left, right, up, down]).

% 8-puzzle solution
% initial([8,1,3,7,0,2,6,5,4]).
% goal([1,2,3,8,0,4,7,6,5]).

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% Best-first Search %
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
bestfs(Start,Path):-
heuristic(Start, Distance),
bestfs_path([ node(Start, Distance, []) ], Path).
bestfs_path([node(Goal, _, Path)| _], Path):-
goal(Goal).
bestfs_path([node(Current, _, Path) | Queue],
GoalPath) :-
findall(node(Child, Distance, PathToChild),
(
  apply(Operator, Current, Child),
  heuristic(Child, Distance),
  append(Path, [Operator], PathToChild)
 ), ChildNodes),
add_to_list(ChildNodes, Queue, NewQueue),
bestfs_path(NewQueue, GoalPath).

% add_to_list adds a new list into an old (ordered) list
% by recursively adding members of the new list at their
% appropriate position in the old list. The returned list
% is also ordered.

add_to_list([], L, L).
add_to_list([H|T], OldList, NewList):-
insert_at_place(H, OldList, TempList),
add_to_list(T, TempList, NewList).

% insert_at_place simply adds a new element at the right
% place of an ordered list so as to return another
% ordered list (provisions have been made for the node
% datastructure that is used in out implementation of
% best-first search)
insert_at_place(X, [], [X]).
insert_at_place(node(NodeX, X, PathX),
        [node(NodeY, Y, PathY) | L],
             [node(NodeX, X, PathX),
           node(NodeY, Y, PathY) | L]) :-
X =< Y.

insert_at_place(node(NodeX, X, PathX),
                [node(Node, Y, PathY) | L],
                [node(Node, Y, PathY) | NewL]) :-
X > Y,
insert_at_place(node(NodeX, X, PathX), L, NewL).
 % Of course, we need to choose our heuristic
% change this to use any other heuristic, such as
% manhattan distance
heuristic(X, Y) :-
displaced(X, Y).
% manhattan(X,Y).

%=====================================================================

%Implementation of 8-Puzzle Operators

%====================================================================

% move_left in the top row
move_left([X1,0,X3, X4,X5,X6, X7,X8,X9], [0,X1,X3, X4,X5,X6, X7,X8,X9]).
move_left([X1,X2,0, X4,X5,X6, X7,X8,X9], [X1,0,X2, X4,X5,X6, X7,X8,X9]).

% move_left in the middle row
move_left([X1,X2,X3, X4,0,X6, X7,X8,X9], [X1,X2,X3, 0,X4,X6, X7,X8,X9]).
move_left([X1,X2,X3, X4,X5,0, X7,X8,X9], [X1,X2,X3, X4,0,X5, X7,X8,X9]).

% move_left in the bottom row
move_left([X1,X2,X3, X4,X5,X6, X7,0,X9], [X1,X2,X3, X4,X5,X6, 0,X7,X9]).
move_left([X1,X2,X3, X4,X5,X6, X7,X8,0], [X1,X2,X3, X4,X5,X6, X7,0,X8]).

% move_right in the top row
move_right([0,X2,X3, X4,X5,X6, X7,X8,X9], [X2,0,X3, X4,X5,X6, X7,X8,X9]).
move_right([X1,0,X3, X4,X5,X6, X7,X8,X9], [X1,X3,0, X4,X5,X6, X7,X8,X9]).

% move_right in the middle row
move_right([X1,X2,X3, 0,X5,X6, X7,X8,X9], [X1,X2,X3, X5,0,X6, X7,X8,X9]).
move_right([X1,X2,X3, X4,0,X6, X7,X8,X9], [X1,X2,X3, X4,X6,0, X7,X8,X9]).

% move_right in the bottom row
move_right([X1,X2,X3, X4,X5,X6, 0,X8,X9], [X1,X2,X3, X4,X5,X6, X8,0,X9]).
move_right([X1,X2,X3, X4,X5,X6, X7,0,X9], [X1,X2,X3, X4,X5,X6, X7,X9,0]).

% move_up from the middle row
move_up([X1,X2,X3, 0,X5,X6, X7,X8,X9], [0,X2,X3, X1,X5,X6, X7,X8,X9]).
move_up([X1,X2,X3, X4,0,X6, X7,X8,X9], [X1,0,X3, X4,X2,X6, X7,X8,X9]).
move_up([X1,X2,X3, X4,X5,0, X7,X8,X9], [X1,X2,0, X4,X5,X3, X7,X8,X9]).

% move_up from the bottom row
move_up([X1,X2,X3, X4,X5,X6, 0,X8,X9], [X1,X2,X3, 0,X5,X6, X4,X8,X9]).
move_up([X1,X2,X3, X4,X5,X6, X7,0,X9], [X1,X2,X3, X4,0,X6, X7,X5,X9]).
move_up([X1,X2,X3, X4,X5,X6, X7,X8,0], [X1,X2,X3, X4,X5,0, X7,X8,X6]).

% move_down from the top row 
move_down([0,X2,X3, X4,X5,X6, X7,X8,X9], [X4,X2,X3, 0,X5,X6, X7,X8,X9]).
move_down([X1,0,X3, X4,X5,X6, X7,X8,X9], [X1,X5,X3, X4,0,X6, X7,X8,X9]).
move_down([X1,X2,0, X4,X5,X6, X7,X8,X9], [X1,X2,X6, X4,X5,0, X7,X8,X9]).

% move_down from the middle row
move_down([X1,X2,X3, 0,X5,X6, X7,X8,X9], [X1,X2,X3, X7,X5,X6, 0,X8,X9]).
move_down([X1,X2,X3, X4,0,X6, X7,X8,X9], [X1,X2,X3, X4,X8,X6, X7,0,X9]).
move_down([X1,X2,X3, X4,X5,0, X7,X8,X9], [X1,X2,X3, X4,X5,X9, X7,X8,0]).

% Applying an operator
apply(left,S1,S2)  :- move_left(S1,S2).
apply(right,S1,S2) :- move_right(S1,S2).
apply(up,S1,S2)    :- move_up(S1,S2).
apply(down,S1,S2)  :- move_down(S1,S2).

%======================================================================

%Implementation of 8-Puzzle Heuristic Functions

%======================================================================


% displacement heuristic

displaced(State, Number) :-                                 
  goal(Goal),                                              
  misplaced(State,Goal,Number).                            

% misplaced returns the number of tiles found in the wrong position

misplaced([],[],0).                                         
misplaced([0|T1],[0|T2],Number) :- !,                       
  m开发者_运维技巧isplaced(T1,T2,Number).                                 
misplaced([H|T1],[H|T2],Number) :- !,                       
  misplaced(T1,T2,Number).                                 
misplaced([H1|T1],[H2|T2],Number) :- !,                     
  H1\==H2,                                                 
  misplaced(T1,T2,N),                                      
  Number is N+1.                                           


% Manhattan Distance heuristic

manhattan(State, Number) :-
  manh(State,State,0,Number).

manh([], _, X, X).

manh([H|T], State, Acc, Result) :-
  nth1(Position, State, H),
  NewPos is Position - 1,
  Xaux1 is NewPos mod 3,
  X1 is integer(Xaux1),
  Y1 is NewPos // 3,
  goal(Goal),
  nth1(GoalPosition, Goal, H),
  NewGPos is GoalPosition - 1,
  Xaux2 is NewGPos mod 3,
  X2 is integer(Xaux2),
  Y2 is NewGPos // 3,
  S1 is abs(X1-X2),
  S2 is abs(Y1-Y2),
  N is S1+S2,
  NewAcc is Acc+N,
  manh(T, State, NewAcc, Result).

The reason you're getting an error informing you that you're out of stack space is because...

Wait for it...

You're out of stack space.

This is because Prolog's solver makes it very easy to get very deep recursion going. Try a bigger stack or look to rein in your variables with more constraints to make the searching easier for poor ol' Prolog.