Prolog - print true or false in print_solution write method - prolog

Question

I want to print true or false based on some predicate predicate passing. For example, in Greek mythology Athena is not a descendant of Poseidon. I have all of the facts and rules correct to prove it as such, but cannot simply print in the print_solution method whether the predicate is true or not.
I have the rule descendant(athena, poseidon) that will print false in the interpreter as a single statement. E.g. false is printed if I:
$ prolog
['greek_mythology.pl'].
descendant(athena, poseidon).
false
I now want to write the same in my print_solution method:
print_solution :-
write('Athena is a descendant of Poseidon: '), write(?????), nl.
I tried assigning a variable in the solution and printing that:
print_solution :-
Z :- descendant(athena, poseidon),
write('Athena is a descendant of Poseidon: '), write(Z), nl.
but I get the error:
ERROR: /.../greek_mythology.pl:164:17: Syntax error: Operator priority clash
Out of desperation I tried checking to see if poseidon was in the list of descendant(athena, X):
setof(X, descendant(athena, X), Z), // [gaia,kronos,oranos,rheia,zeus]
member(poseidon, Z).
but that just gives back:
Z = [poseidon|_G3320] .
which I don't understand. Apparently member/2 only works on lists, and Z is apparently not a list:
is_list(Z).
false.
How can I simply print true or false in the write function if the predicate returns passes like it does in the interpreter console? Desired output: Athena is a descendant of Poseidon: false.

Answer 1

You can use if/then/else construct in Prolog also
print_solution :-
( descendant(athena, poseidon)
-> Z = true
; Z = false
),
write('Athena is a descendant of Poseidon: '), write(Z), nl.
seems also could be more useful if parametrized
print_solution(Desc, Anc) :-
( descendant(Desc, Anc)
-> Z = true
; Z = false
),
format('~w is a descendant of ~w: ~w~n', [Desc, Anc, Z]).
print_solution :- print_solution(athena, poseidon).

Answer 2

I think (not absolutely sure) that what you want is reification of the success or failure of a query to a truth value. A naive way to do it would be:
goal_success(Goal, true) :-
Goal.
goal_success(Goal, false) :-
\+ Goal.
(This has problems but read on)
Then, you can use it like this:
?- goal_success(descendant(athena, poseidon), S),
/* write whatever */, write(S).
However, what you are trying to do is basically write your own meta-interpreter, and I am not sure it is really that useful. After all, what you show in your question is exactly how you would expect to interact with the Prolog top level (the Prolog interactive prompt). As a programmer (a human) what you want from Prolog is answers to your questions. What you give Prolog, however, is queries, and you get solutions to these queries. Your question is:
Is Athena a descendant of Poseidon?
You evaluate the query:
?- descendant(athena, poseidon).
false.
There is no solution to this query (let's not go into the whole "closed world assumption" discussion now, it is enough to say that not being able to find a proof means that the opposite is true...)
So we interpret this as:
Athena is not a descendant of Poseidon.
Usually, this should be enough. This is why it seems that you want to extend the Prolog top level, and write your own interpreter on top of it, which is of course fine, but more involved. Once you go down that road, you would like to interact with this interpreter like this maybe?
??- Is Athena a descendant to Poseidon?
?>> No, she is not!
Anyway, if you search the internet for "Prolog meta interpreter" you will get enough to get you started. "The Art of Prolog" by Sterling & Shapiro has a whole chapter on that, too.