How to print on the screen unbound variables? Prolog

Question

I want to print the list of unbound variables to the screen.

12 ?- write([X, Y]).

[_G1696,_G1699]

But I want to see in fact [X, Y] on the screen instead of [_G1696,_G1699]. Is it possible? How?

See [this answer](http://stackoverflow.com/a/7948525/772868) how to write and read variable names in a general manner. — false, Mar 03 '16 at 17:24

score 1 · Answer 1 · answered Mar 03 '16 at 11:24

I assume that you want to pretty print terms with unbound variables? You can use the de facto standard numbervars/3 predicates to accomplish it. For example:

pretty_print_term(Stream, Term) :-
    \+ \+ (
        numbervars(Term, 0, _),
        write_term(Stream, Term, [numbervars(true)])).

pretty_print_term_quoted(Stream, Term) :-
    \+ \+ (
        numbervars(Term, 0, _),
        write_term(Stream, Term, [numbervars(true), quoted(true)])).

In these snippets, double negation is used to pretty print a term while discarding the instantiation of the variables by the numbervars/3 predicate.

An usage example:

?- current_output(Stream), pretty_print_term_quoted(Stream, [X, Y]).
[A,B]
Stream = <stream>(0x10bebcf18).

Thank you! But can't I force numbervars/3 to select same variables name? (exactly X, Y instead of A, B ) — Svetlana, Mar 03 '16 at 13:57

How to print on the screen unbound variables? Prolog

1 Answers1