beginner prolog: data structures

Question

I'm trying to get my head around Prolog, and I am struggling somewhat with data structures.

I want to take a point(X,Y) and reflect it along the "diagonal" X=Y, so a point(-5,8) becomes point(8,-5).

/* the following line is what I've gotten so far, 
   but I don't know how to manipulate the data inside the structures. */
reflection(X,Y) :- =(Y,X).

test_answer :-
    reflection(point(-5,8), point(X,Y)),
    write(point(X, Y)).
test_answer :-
    write('Wrong answer!').

Should output point(8,-5).

Does this sort of thing even need data structures or am I over-thinking it?

Answer 1

The version of prolog I use (Strawberry Prolog) doesn't allow me to use the prefix notation for =, so unless you prolog has a different meaning for = then it appears that your code is this:

reflection(X,Y):- Y = X.

Which means that reflection\2 will only unify when the X & Y are unifiable.

So your code should result in point(-5,8) and not point(8,-5) as point(-5, 8) is unifiable with point(X, Y) when X = -5 & Y = 8. You don't say what you do get in your question.

You need to use this rule to make it work:

reflection(point(X,Y),point(Y,X)).

Answer 2

You can just write:

reflection(point(A,B), point(B,A)).

If that is in the file reflection.prolog, then:

$ gprolog --consult-file reflection.prolog
...
| ?- reflection( point(1,2), X).
X = point(2,1)
yes