在Prolog中将立方体从位置A移动到位置B

Question

I have the following problem: there are some towers build with cubes on a table.

       a 
       b d
       c e
------------------- <- table

Now I want to move the cubes to another situation, like this one:

        c e
      a b d
------------------- 

The Prolog program should print the steps to get to this situation, for example: move cube a onto the table , and so on. I have the first situation represented in Prolog:

clean(t). % t is the table, you can always put things there
clean(X) :- \+ on(_,X). % X is the top element, if there is nothing above it

on(a,b). % a is on b
on(b,c). % b on c
on(d,e). % d on e
on(c,t). % c on the table
on(e,t). % and e on the table

Now my problem is to find an solution to make Prolog print the steps to the new situation. My first problem is, how to tell Prolog how the new situation looks like. I tried it with some lists, but I didn´t succeed until now.

Does anyone have an idea how to solve this?

Answer 1

If you encode the problem state by having facts of the on/2 predicate in your database, then you can only change that state by using assert and retract to change the database (and your predicates probably have to be declared dynamic as well). This is unwieldy. A nicer solution is to pass around the problem state as a parameter to your solving predicates (and maybe hide that structure behind a wrapper predicate). This works much better with the backtracking that you're probably going to need to implement the search for the solution.