1CSC 8520 Spring 2010. Paula Matuszek

• On the syllabus page there are links to two Prolog programs: a simple Prolog planner and an ADT library.

• Download them and consult planner.pl, which will in turn consult adts.pl.

?- consult('planner.pl').

% adts compiled 0.00 sec, 6,364 bytes

% planner.pl compiled 0.00 sec, 11,456 bytes

true.

Lab Exercise 1

2CSC 8520 Spring 2010. Paula Matuszek

• Test that it is working by typing “test.” at the prompt.• You should get a list of moves.

?- test.

moves are

pickup(a)

putdown(a)

pickup(b)

stack(b, c)

pickup(a)

stack(a, b)

true.

?-

3CSC 8520 Spring 2010. Paula Matuszek

Planner• Planner is a very simple Prolog implementation

of a planner using a STRIPS-like notation.

• A state is a list of conditions.

• A problem has an initial state and a goal state. For instance:– the initial state for a blocks world might be:

– [handempty, ontable(b), ontable(c), on(a, b), clear(c), clear(a)]

– the goal state for the blocks world might be: [handempty, ontable(c), on(a,b), on(b, c), clear(a)]

4CSC 8520 Spring 2010. Paula Matuszek

Moves• A move is an action, consisting of a name, a

list of preconditions, and an effect list of changes.

• Moves are included in the Prolog file. • For example:move(pickup(X), % move name

[handempty, clear(X), on(X, Y)], % preconditions

[del(handempty), del(clear(X)), % list of conditions

del(on(X, Y)), add(clear(Y)), % to be added and

add(holding(X))]). % deleted

5CSC 8520 Spring 2010. Paula Matuszek

plan• plan takes a current state, a goal state, a list of

states which have already been reached (Been_list), and a list of moves which have already been carried out. It then– checks to see if the current state = goal state. If so, print

out list of moves.

– finds an applicable move

– does the adds and deletes

– checks to be sure we haven’t already been there

– adds the current state to the list of states and the current move to the list of moves

– recurs.

6CSC 8520 Spring 2010. Paula Matuszek

Running it• go takes an initial state and a goal state

• it invokes plan with – current state = initial state– goal state = goal state– Been_list = initial state– Moves list empty.

• test is just there to give a way to test it, using the blocks world.

6

7CSC 8520 Spring 2010. Paula Matuszek

Spare tire problem• Problem: you have a flat. Develop a plan to

fix it. – Initial condition is that the flat is on the axle and

the spare is in the trunk.

– Goal condition is that the flat is in the trunk and the spare is on the axle.

– Some possible actions include removing something from something, installing something in something.

– Some possible conditions include at(a,b) and clear(axle)

8CSC 8520 Spring 2010. Paula Matuszek

Lab exercise 2.• Develop the appropriate initial state, goal

state and move definitions for the Spare Tire Problem.

• Add to planner.pl your moves and a new predicate test2 which solves this problem.