Prolog STRIPS planner never completes - prolog

Question

Following examples by Ivan Bratko on Artificial Intelligence in Prolog through his book:
"Prolog Programming for Artificial Intelligence - 3rd Edition" (ISBN-13: 978-0201403756) (1st edition 1986 by Addison-Wesley, ISBN 0-201-14224-4)
I've noticed that a lot of the examples do not run to completion but instead seem to get stuck. I have tried several different implementations following it to the letter, but with no luck. Would anyone be willing to take a gander at the code to see if they can spot where there is faulty logic or if I made a mistake?
This is the complete program of a STRIPS style planner for a blocks world as illustrated in the book:
% This planner searches in iterative-deepening style.
% A means-ends planner with goal regression
% plan( State, Goals, Plan)
plan( State, Goals, []) :-
satisfied( State, Goals). % Goals true in State
plan( State, Goals, Plan) :-
append( PrePlan, [Action], Plan), % Divide plan achieving breadth-first effect
select( State, Goals, Goal), % Select a goal
achieves( Action, Goal),
can( Action, Condition), % Ensure Action contains no variables
preserves( Action, Goals), % Protect Goals
regress( Goals, Action, RegressedGoals), % Regress Goals through Action
plan( State, RegressedGoals, PrePlan).
satisfied( State, Goals) :-
delete_all( Goals, State, []). % All Goals in State
select( State, Goals, Goal) :- % Select Goal from Goals
member( Goal, Goals). % A simple selection principle
achieves( Action, Goal) :-
adds( Action, Goals),
member( Goal, Goals).
preserves( Action, Goals) :- % Action does not destroy Goals
deletes( Action, Relations),
not((member( Goal, Relations),
member( Goal, Goals))).
regress( Goals, Action, RegressedGoals) :- % Regress Goals through Action
adds( Action, NewRelations),
delete_all( Goals, NewRelations, RestGoals),
can( Action, Condition),
addnew( Condition, RestGoals, RegressedGoals). % Add precond., check imposs.
% addnew( NewGoals, OldGoals, AllGoals):
% OldGoals is the union of NewGoals and OldGoals
% NewGoals and OldGoals must be compatible
addnew( [], L, L).
addnew( [Goal | _], Goals, _) :-
impossible( Goal, Goals), % Goal incompatible with Goals
!,
fail. % Cannot be added
addnew( [X | L1], L2, L3) :-
member( X, L2), !, % Ignore duplicate
addnew( L1, L2, L3).
addnew( [X | L1], L2, [X | L3]) :-
addnew( L1, L2, L3).
% delete_all( L1, L2, Diff): Diff is set-difference of lists L1 and L2
delete_all( [], _, []).
delete_all( [X | L1], L2, Diff) :-
member( X, L2), !,
delete_all( L1, L2, Diff).
delete_all( [X | L1], L2, [X | Diff]) :-
delete_all( L1, L2, Diff).
can( move( Block, From, To), [clear(Block), clear(To), on(Block,From)]) :-
block(Block),
object(To),
To \== Block,
object( From),
From \== To,
Block \== From.
adds( move(X,From,To),[on(X,To),clear(From)]).
deletes( move(X,From,To),[on(X,From), clear(To)]).
object(X) :-
place(X)
;
block(X).
impossible( on(X,X), _).
impossible( on( X,Y), Goals) :-
member( clear(Y), Goals)
;
member( on(X,Y1), Goals), Y1 \== Y % Block cannot be in two places
;
member( on( X1, Y), Goals), X1 \== X. % Two blocks cannot be in same place
impossible( clear( X), Goals) :-
member( on(_,X), Goals).
block(a).
block(b).
block(c).
block(d).
block(e).
block(f).
block(g).
place(1).
place(2).
place(3).
place(4).
I added 7 blocks and 4 locations and tested it with a representation where all the blocks are alphabetically stacked from a through g on position 1, and the goal is to stack them in the same order on position 2.
To run the program call plan(StartState,GoalState, Sol).
plan([on(a,1), on(b,a), on(c,b), on(d,c), on(e,d), on(f,e), on(g,f),
clear(g), clear(2), clear(3)],
[clear(1), on(a,2), on(b,a), on(c,b), on(d,c), on(e,d), on(f,e),
on(g,f), clear(g), clear(3)],
P).
~ ~
g g
f f
e e
d ---> d
c c
b b
a ~ ~ ~ ~ a ~ ~
_ _ _ _ _ _ _ _
1 2 3 4 1 2 3 4
References:
Definition of move: http://media.pearsoncmg.com/intl/ema/ema_uk_he_bratko_prolog_3/prolog/ch17/fig17_2.txt
End means planner with goal regression: http://media.pearsoncmg.com/intl/ema/ema_uk_he_bratko_prolog_3/prolog/ch17/fig17_8.txt
Any advice would be greatly appreciated.

Answer 1

In the end, the code is correct but the combinatorial explosion kills it.
Data:
3 places, 3 blocks succeeds with 5 moves after 9'755 calls to plan/3.
4 places, 3 blocks succeeds with 5 moves after 98'304 calls to plan/3.
3 places, 4 blocks succeeds with 7 moves after 915'703 calls to plan/3.
3 places, 5 blocks succeeds with 9 moves after 97'288'255 calls to plan/3.
There is no sense trying with more, especially not with 4 places, 7 blocks. It is clear that heuristics, exploitation of symmetry, etc. are needed to go further. All of those need larger amounts of memory. Here, memory used remains small in all cases: only one path down the iteratively deepened (and stored on stack) search tree is live at any time. We don't remember any states visited or anything, it's a very simple search.
Below the updated code (LONG, 337 lines)
Changes (important ones marked with 'FIX' in the code)
library(list) predicates have been used where possible, getting rid of some code.
Debugging output generation using format/2 added.
Assertions (see here) using assertion/1 added to check that what happens is what I think happens.
Predicates and variables renamed to better reflect their intended meaning.
run/0 predicate added which initializes the State and Goal, calls plan/3 and prettyprints the Plan.
can/2 confusingly combined two separate aspects: instantiating an Action and determining its Preconditions. Separated into two predicates instantiate_action/1 and preconditions/2.
select_goal/2 looked like it depended on State, but really didn't. Cleaned up.
Note the trick for making this an "iterative deepening" search. It is very clever but on second thoughts, it is too clever by half as it is based on the predicate run/3 behaving differently when being called with Plan an unbound variable than with Plan being a bound variable. The first case occurs only at the very top node of the implied search tree. This may be further explained in the textbook, which I don't have, and it took some time to realize what is actually happening in this code.
If the pruning expression ((nonvar(Plan), Plan == []) -> fail ; true ) that I put at the start of the search branch of plan/3 irritates, then so should the iterative deepening trick. IMHO, better use tree depth counters and return the Plan via an accumulator. Especially if someone will be tasked to maintain such code in a production system (that is, a "system in production", not a "forward-chaining rule-based system").
% Based on
%
% Exercise 17.5 on page 429 of "Prolog Programming for Artificial Intelligence"
% by Ivan Bratko, 3rd edition
%
% The text says:
%
% "This planner searches through the state space in iterative-deepening style."
%
% See also:
%
% https://en.wikipedia.org/wiki/Iterative_deepening_depth-first_search
% https://en.wikipedia.org/wiki/Blocks_world
%
% The "iterative deepening" trick is all in the "Plan" list structure.
% If you remove it, the search becomes depth-first and no longer terminates!
% ----------
% Encapsulator to be called by user from the toplevel
% ----------
run :-
% Setting up
start_state(State),
final_state(Goals),
% TODO: Build predicates that verify that State and Goal are actually validly constructed
% Or choose better representations
nb_setval(glob_plancalls,0), % global variable for counting calls (non-backtrackable)
b_setval(glob_depth,0), % global variable for counting depth (backtrable)
% plan/3 is backtrackable and generates different/successively longer plans on backtrack
% it may however generate the same plan several times
plan(State, Goals, Plan),
dump_plan(Plan,1).
% ----------
% Writing out a solution found
% ----------
dump_plan([P|R],N) :-
% TODO: Verify that the plan indeed works!
format('Plan step ~w: ~w~n',[N,P]),
NN is N+1,
dump_plan(R,NN).
dump_plan([],_).
% The representation of the blocks world (see below) is a bit unfortunate as places and blocks
% have to be declared separately and relationships between places and blocks, as well
% as among blocks themselves have to declared explicitely and consistently.
% Additionally we have to specify which elements have a view of the sky (i.e. are clear/1)
% On the other hand, the final state and end state need not be specified fully, which is
% interesting (not sure what that means exactly regarding solution finding)
% The atoms used in describing places and blocks must be distinct due to program construction!
start_state([on(a,1), on(b,a), on(c,b), clear(c), clear(2), clear(3), clear(4)]).
final_state([on(a,2), on(b,a), on(c,b), clear(c), clear(1), clear(3), clear(4)]).
% ----------
% Representation of the blocks world
% ----------
% We have BLOCKs identified by atoms a,b,c, ...
% Each of those is identified by block/1 attribute.
% A block/1 is clear/1 if there is nothing on top of it.
% A block/1 is on(Block, Object) where Object is a block/1 or place/1.
block(a).
block(b).
block(c).
% We have PLACEs (i.e. columns of blocks) onto which to stack blocks.
% Each of these is identified by place/1 attribute.
% A place/1 can be clear/1 if there is nothing on top of it.
% (In fact these are like special immutable blocks and should be modeled as such)
place(1).
place(2).
place(3).
place(4).
% OBJECTs are place/1 or block/1.
object(X) :- place(X) ; block(X).
% ACTIONs are terms "move( Block, From, To)".
% "Block" must be block/1.
% "From" must be object/1 (i.e. block/1 or place/1).
% "To" must be object/1 (i.e. block/1 or place/1).
% Evidently constraints exist for a move/3 to be possible from or to any given state.
% STATEs are sets (implemented by lists) of "goal" terms.
% A goal term is "on( X, Y)" or "clear(Y)" where Y is object/1 and X is block/1.
% ----------
% plan( +State, +Goals, -Plan)
% Build a "Plan" get from "State" to "Goals".
% "State" and "Goals" are sets (implemented as lists) of goal terms.
% "Plan" is a list of action terms.
% The implementation works "backwards" from the "Goals" goal term list towards the "State" goal term list.
% ----------
% ___ Satisfaction branch ____
% This can only succeed if we are at the "end" of a Plan (the Plan must match '[]') and State matches Goal.
plan( State, Goals, []) :-
% Debugging output
nb_getval(glob_plancalls,P),
b_getval(glob_depth,D),
NP is P+1,
ND is D+1,
nb_setval(glob_plancalls,NP),
b_setval(glob_depth,ND),
statistics(stack,STACK),
format('plan/3 call ~w at depth ~d (stack ~d)~n',[NP,ND,STACK]),
% If the Goals statisfy State, print and succeed, otherwise print and fail
( satisfied( State, Goals) ->
(sort(Goals,Goals_s),
sort(State,State_s),
format(' Goals: ~w~n', [Goals_s]),
format(' State: ~w~n', [State_s]),
format(' *** SATISFIED ***~n'))
;
format(' --- NOT SATISFIED ---~n'),
fail).
% ____ Search branch ____
%
% Magic which generates the breath-first iterative deepening search:
%
% In the top node of the call tree (the node directly underneath "run"), "Plan" is unbound.
%
% At point "XXX" "Plan" is set to a list of as-yet-unbound actions of a given length.
% At each backtrack that reaches up to "XXX", "Plan" is bound to list longer by 1.
%
% In any other node of the call tree than the top node, "Plan" is bound to a list of fixed length
% becoming shorter by 1 on each recursive call.
%
% The length of that list determines how deep the search through the state space *must* go because
% satisfaction can only be happen if the "Plan" list is equal to [] and State matches Goal.
%
% So:
% On first activation of the top, build plans of length 0 (only possible if Goals passes satisfied/2 directly)
% On second activation of the top, build plans of length 1 (and backtrack over all possibilities of length 1)
% ...
% On Nth activation of the top, build plans of length N-1 (and backtrack over all possibilities of length N-1)
%
% A slight improvement is to fail the search branch immediately if Plan is a nonvar and is equal to []
% because append( PrePlan, [Action], Plan) will fail...
plan( State, Goals, Plan) :-
% The line below can be commented out w/o ill effects, it is just there to fail early
((nonvar(Plan), Plan == []) -> fail ; true ),
% Debugging output
nb_getval(glob_plancalls,P),
b_getval(glob_depth,D),
NP is P+1,
ND is D+1,
nb_setval(glob_plancalls,NP),
b_setval(glob_depth,ND),
statistics(stack,STACK),
format('plan/3 call ~w at depth ~d (stack ~d)~n',[NP,ND,STACK]),
format(' goals ~w~n',[Goals]),
% Even more debugging output
( var(Plan) -> format(' Top node of plan/3 call~n') ; true ),
( nonvar(Plan) -> (length(Plan,LP), format(' Low node of plan/3 call, plan length to complete: ~w~n',[LP])) ; true ),
% prevent runaway behaviour
% assertion(NP < 1000000),
% XXX
% append/3 is backtrackable.
% For the top node, it will generate longer completely uninstantiated PrePlans on backtracking:
% PrePlan = [], Plan = [Action] ;
% PrePlan = [_G981], Plan = [_G981, Action] ;
% PrePlan = [_G981, _G987], Plan = [_G981, _G987, Action] ;
% PrePlan = [_G981, _G987, _G993], Plan = [_G981, _G987, _G993, Action] ;
% For lower nodes, Plan is instantiated to a list of length N already, and PrePlan will therefore necessarily
% be the prefix list of length N-1
% XXX
append( PrePlan, [Action], Plan),
% Backtrackably select some concrete Goal from Goals
select_goal( Goals, Goal), % FIX: In the original this seems to depend on State, but it really doesn't
assert_goal(Goal),
format( ' Depth ~d, selected Goal: ~w~n',[ND,Goal]),
% Check whether Action achieves the Goal.
% As Action is free, what we actually do is instantiate Action backtrackably with something that achieves Goal
achieves( Action, Goal),
format( ' Depth ~d, selected Action: ~w~n', [ND,Action]),
% Fully instantiate Action backtrackably
% FIX: Passed "conditions", the precondition for a move, which is unused at this point: broken up into two calls
instantiate_action( Action),
format( ' Depth ~d, action instantiated to: ~w~n', [ND,Action]),
assertion(ground(Action)),
% Check that the Action does not clobber any of the Goals
preserves( Action, Goals),
% We now have a ground Action that "achieves" some goals in Goals while "preserving" all of them
% Work backwards from Goals to a "prior goals". regress/3 may fail to build a consistent GoalsPrior!
regress( Goals, Action, GoalsPrior),
plan( State, GoalsPrior, PrePlan).
% ----------
% Check
% ----------
assert_goal(X) :-
assertion(ground(X)),
assertion((X = on(A,B), block(A), object(B) ; X = clear(C), object(C))).
% ----------
% A State (a list) is satisfied by Goals (a list) if all the terms in Goals can also be found in State
% ----------
satisfied( State, Goals) :-
subtract( Goals, State, []). % Set difference yields empty list: [] = Goals - State
% ----------
% Backtrackably select a single Goal term from a set of Goals
% ----------
select_goal( Goals, Goal) :-
member( Goal, Goals).
% ----------
% When does an Action (move/2) achieve a Goal (clear/1, on/2)?
% This is called with instantiated Goal and free Action, so this actually instantiates Action
% with something (partially specified) that achieves Goal.
% ----------
achieves( Action, Goal) :-
assertion(var(Action)),
assertion(ground(Goal)),
would_add( Action, GoalsAdded),
member( Goal, GoalsAdded).
% ----------
% Given a ground Action and ground Goals, will Action from a State leading to Goals preserve Goals?
% ----------
preserves( Action, Goals) :-
assertion(ground(Action)),
assertion(ground(Goals)),
would_del( Action, GoalsDeleted),
intersection( Goals, GoalsDeleted, []). % "would delete none of the Goals"
% ----------
% Given existing Goals and an (instantiated) Action, compute the previous Goals
% that, when Action is applied, yield Goals. This may actually fail if no
% consistent GoalsPrior can be built!
% ** It is actually not at all self-evident that this is right and that we get a valid
% "GoalsPrior" via this method! ** (prove it!)
% FIX: "Condition" replaced by "Preconditions" which is what this is about.
% ----------
regress( Goals, Action, GoalsPrior) :-
assertion(ground(Action)),
assertion(ground(Goals)),
would_add( Action, GoalsAdded),
subtract( Goals, GoalsAdded, GoalsPriorPass), % from the "lists" library
preconditions( Action, Preconditions),
% All the Preconds must be fulfilled in Goals2, so try adding them
% Adding them may not succeed if inconsistencies appear in the resulting set of goals, in which case we fail
add_preconditions( Preconditions, GoalsPriorPass, GoalsPrior).
% ----------
% Adding preconditions to existing set of goals and checking for inconsistencies as we go
% Previously named addnew/3
% New we use union/3 from the "lists" library and the modified "consistent"
% ----------
add_preconditions( Preconditions, GoalsPriorIn, GoalsPriorOut) :-
add_preconditions_recur( Preconditions, GoalsPriorIn, GoalsPriorIn, GoalsPriorOut).
add_preconditions_recur( [], _, GoalsPrior, GoalsPrior).
add_preconditions_recur( [G|R], Goals, GoalsPriorAcc, GoalsPriorOut) :-
consistent( G, Goals),
union( [G], GoalsPriorAcc, GoalsPriorAccNext),
add_preconditions_recur( R, Goals, GoalsPriorAccNext, GoalsPriorOut).
% ----------
% Check whether a given Goal is consistent with the set of Goals to which it will be added
% Previously named "impossible/2".
% Now named "consistent/2" and we use negation as failure
% ----------
consistent( on(X,Y), Goals ) :-
\+ on(X,Y) = on(A,A), % this cannot ever happen, actually
\+ member( clear(Y), Goals ), % if X is on Y then Y cannot be clear
\+ ( member( on(X,Y1), Goals ), Y1 \== Y ), % Block cannot be in two places
\+ ( member( on(X1,Y), Goals), X1 \== X ). % Two blocks cannot be in same place
consistent( clear(X), Goals ) :-
\+ member( on(_,X), Goals). % if something is on X, X cannot be clear
% ----------
% Backtrackably instantiate a partially instantiated Action
% Previously named "can/2" and it also instantiated the "Condition", creating confusion
% ----------
instantiate_action(Action) :-
assertion(Action = move( Block, From, To)),
Action = move( Block, From, To),
block(Block), % will unify "Block" with a concrete block
object(To), % will unify "To" with a concrete object (block or place)
To \== Block, % equivalent to \+ == (but = would do here); this demands that blocks and places have disjoint sets of atoms
object(From), % will unify "From" with a concrete object (block or place)
From \== To,
Block \== From.
% ----------
% Find preconditions (a list of Goals) of a fully instantiated Action
% ----------
preconditions(Action, Preconditions) :-
assertion(ground(Action)),
Action = move( Block, From, To),
Preconditions = [clear(Block), clear(To), on(Block, From)].
% ----------
% would_del( Move, DelGoals )
% would_add( Move, AddGoals )
% If we run Move (assuming it is possible), what goals do we have to add/remove from an existing Goals
% ----------
would_del( move( Block, From, To), [on(Block,From), clear(To)] ).
would_add( move( Block, From, To), [on(Block,To), clear(From)] ).
Running the above produces lots of output and eventually:
plan/3 call 57063 at depth 6 (stack 98304)
Goals: [clear(2),clear(3),clear(4),clear(c),on(a,1),on(b,a),on(c,b)]
State: [clear(2),clear(3),clear(4),clear(c),on(a,1),on(b,a),on(c,b)]
*** SATISFIED ***
Plan step 1: move(c,b,3)
Plan step 2: move(b,a,4)
Plan step 3: move(a,1,2)
Plan step 4: move(b,4,a)
Plan step 5: move(c,3,b)
See also
STRIPS automatic planner
Iterative deepening depth-first search
Blocks World