How to generate an assert that forces a false?

Question

First of all thanks for your help.

On to my problem: Let's say I have:

some_fact:- true.

and I want to asserta a rule on top of it that looks like this:

some_fact:- fail, !.

This is because I want to transform the "some_fact:- true." to force a false without removing the rule (I don't want to use abolish(some_fact,0). )

The problem is that I can't find the way to do it because I can't place the comma on the asserta/1. What I mean is that when I put:

asserta(some_fact:- fail, !).

the comma in between forces a call to asserta/2 instead of asserta/1 with the whole rule, and I cannot prevent that using quotes because it asserts a string.

Of course I can't just simply put asserta(some_fact:- fail). because prolog will search for the next some_fact which returns true.

Any ideas? Thanks again!

Answer 1

just add parenthesis:

?- asserta((some_fact:- fail, !)).
true.

Answer 2

To assert the fact:

asserta(zoo(zebra)).

When the fact is no longer true:

retract(zoo(zebra)).

More general:

asserting(Fact) :-
    asserta(Fact).

retracting(Fact) :-
    retract(Fact).

Example:

?- asserting(zoo(zebra)).
true.

?- zoo(zebra).
true.

?- retracting(zoo(zebra)).
true.

?- zoo(zebra).
false.