Prolog Tree Traversal

Question

Good Day,

I am trying to write a Prolog program that given a tree with a functor of a:

start(a(f,2,9), X).

I want it to square any values inside so that it yields:

X = a(f,4,81).

I have code that squares numbers in a list already that works. Here's what I have so far:

start([],[]).
start(Tree, []) :- Tree =.. [P|C], write(P), nl, write(C), nl, squareMe([P|C], []).
squareMe([X|T], [Y|Result]) :-   % I think the problem is here
    atom(X),
    Y=X,
    squareMe(T, Result).
squareMe([X|T], [Y|Result]) :- 
    number(X),
    Y is X * X,
    squareMe(T, Result).
squareMe([], []).

when the code writes P and C, I do get the correct values via the UNIV operator, but it seems to be failing inside squareMe.

When I invoke squareMe([P|C], []), my understanding is the P = a and C = [f, 2, 9]. So shouldn't atom(a) be true? It doesn't appear to be the case and I'm not sure why?

I've tried using trace/notrace to track my path, but it would sure be nice to see the values that were passed into squareMe. Is that possible? I'm using SWI-Prolog.

TIA, coson

Answer 1

This is an answer instead of a comment only because it is too long; I don't fully understand your question though.

To get the solution to the query you show at the top, it would be enough to write:

start(a(f, X, Y), a(f, XX, YY)) :-
    XX is X*X,
    YY is Y*Y.

That's it:

?- start(a(f, 2, 9), X).
X = a(f, 4, 81).

This is way too easy, and more importantly, I don't see any tree structure here, which is why I am certain I am misunderstanding the question. I definitely have trouble following the code you have shown. You should edit your question to explain:

• where is the tree structure you are traversing?
• are you using lists, flat terms, nested terms (a tree?)
• does your predicate have to work both ways, so, should you be able to ask: ?- start(X, Y). for example.