prolog variable not exist

Question

Here is my simple program to test if else and function call in swi prolog. What is wrong with below code which says rule does not exist

call_rule(Roll):-
(
    member(Roll,[123]),
    writeln('inside call rule'),
    nb_getval(rule, 'this is the rule')
).
print_roll(Roll) :-
    (   Roll < 2 ->
        writeln('not a roll')
    ;
        (  Roll > 1243 ->
           writeln('not a roll'),writeln('this is 2nd alternative'),writeln('this is third alternative')
        ;
           ( Roll =:= 12 ->
                writeln('boxcars')
            ;
                ( call_rule(Roll) ->
                    nb_getval(rule, RULE),
            writeln('snake eyes')
                ;
                    nb_getval(rule,SUBJECT),
            writeln(SUBJECT)    
                )
            )
        )
    ).

result:

3 ?- print_roll(123).  
inside call rule
ERROR: nb_getval/2: variable `rule' does not exist

generally, you should avoid global variables, in any language, and much more in Prolog — CapelliC
I second what CappeliC wrote! Avoid nb_setval/2 and all predicates like it: They cannot be used as true relations and you therefore do not benefit from the usual advantages (generality etc.) of declarative solutions. — mat
@CapelliC: I could not understand your point. I am beginner in prolog, I learned this way of writing looking at various articles. I appreciate if you can show me optimal version of this code. — user2129623
@mat: thanks for pointing, but still muddleing to get advantage of generality — user2129623
@user2129623: Try to think in terms of relations. In this case, you apparently want to define a relation between a roll and its name, so think about (for example) roll_name/2. Generality: You can use such a predicate in all directions, for example: "Which rolls and names exist?" (?- roll_name(R, N).) or "Which rolls correspond to a given name?". nb_setval/2 and other predicates like it usually impose a single direction: You may be able to determine a name for a given roll, but not vice versa. Also, you cannot use your predicates in isolation any more, as you have already seen. — mat

Willem Van Onsem Willem Van Onsem · Accepted Answer · 2014-10-28T12:49:13

Before you can get a value from the non-backtrackable store, you need to nb_setval the key:

Example:

?- nb_getval(a,X).
ERROR: nb_getval/2: variable `a' does not exist
?- nb_setval(a,foo).
true.

?- nb_getval(a,X).
X = foo.

prolog variable not exist

1 Answers