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listprologpredicate

Way to check if dynamic predicate contains a value

Very new to Prolog. Goal is to check if dynamic predicate contains a specific value. Doing it in the following way but does not work:

:- dynamic storage/1. 

not(member(Z,storage)), // checking if Z is already a member of storage - this does not work. 
assert(storage(Z)). // asserting Z if above is true

Could you please explain how this should be done?

Solution

  • Like this?

    :- dynamic storage/1. 

append_to_database_but_only_once(Z) :- 
   must_be(nonvar,Z),      % Z must be bound variable (an actual value)
   \+ storage(Z),          % If "storage(Z)" is true already, fail!
   assertz(storage(Z)).    % ...otherwise append to the database

    And so:

    ?- append_to_database_but_only_once(foo).
true.

?- storage(X).
X = foo.

?- append_to_database_but_only_once(foo).
false.

    Note that you don't need to query any datastructure or list with member/2. With assertz/1, you are changing the Prolog database itself! So afterwards you just need to perform the corresponding query, storage(Z).