Prolog - Using dynamic with asserts

Question

I'm new to Prolog and I'm having a hard time using a dynamic predicate.

First, here's the code I'm executing

:- dynamic(list/1).

add(X, LL) :- asserta(list([])), asserta(list(X)), retract(list(LL)).

I know the code looks weird, but I'm simply looking for the right syntax to use.

Now, if I do :

add(2, LL).

Answer will be :

LL = 2 ;

LL = [].

But what I want to do is to add the X (2) INTO the array ([]). So..

LL = [2].

It looks simple (probably is to), but I can't get it work.

Thanks a lot.

Answer 1

If you want to add X to the front of the list:

add(X, LL) :-
    (   retract(list(Prev))
    ->  LL = [X|Prev]
    ;   LL = [X]
    ),
    asserta(list(LL)).

But I agree with @jschimpf's advice. Assert/retract should only be used under certain circumstances as may be quite in efficient in some applications.