Reversing facts in prolog and display them

Question

If I have the following:

friends(tim,joe).

if i go:

?- friends(X,Y).

I would get:

X=tim
Y=joe

What would I have to print the following with out adding any new facts

X=tim
Y=joe
X=joe
Y=tim

Answer 1

You will have to add a new rule:

are_friends(X,Y):- friends(X,Y).
are_friends(X,Y):- friends(Y,X).

Then you ask:

?- are_friends(X,Y).

Prolog will answer

X=tim, Y=joe   _

and it will wait for your further command. If you press ;, then it will print the next solution:

X=tim, Y=joe   ;

X=joe, Y=tim   _

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To just show the results twice - as opposed to producing them in a proper Prolog fashion - we can write

show_friends :- 
  friends(X,Y),
  write('X='), write(...), write(', Y='), write(...), nl,
  write('X='), write(...), write(', Y='), write(...), nl,
  fail.

but this is really, really, really just faking it. Ughgh. You fill in the blanks.