Prolog减法值无效

Question

我有一个知识库：

bottle(b1).
bottle(b2).
bottle(b3).
bottle(b4).

full(bottle(b1),100).
full(bottle(b2),150).
full(bottle(b3),300).
full(bottle(b4),400).


consume(bottle(X),Milliliter) :-
   full(bottle(X),Y),
   Milliliter=<Y,
   Y-10.

因此，我想使用消费谓词，我希望减少分配给full的值，就像使用的值一样多。它是否允许从静态值中减去，如果还没有消耗瓶子，我如何才能解决这个问题才能达到真值。

Answer 1

如果要在调用“消费”时“更新”KB，则必须收回并断言事实，例如.

% Use this to add the initial facts (if you don;t have a clause to do this, prolog complains about modifying static clauses...)
addfacts :-
    asserta(full(bottle(b1),100)),
    asserta(full(bottle(b2),150)),
    asserta(full(bottle(b3),300)),
    asserta(full(bottle(b4),400)).

consume(bottle(X), Millis) :-
    % Retract the current state of the bottle
    retract(full(bottle(X), V)),
    % Calculate the new Millis after consumption
    Y is V - Millis,
    % Check it was possible (there should be 0 or more millis left after)
    Y >= 0,
    % Add the new fact
    asserta(full(bottle(X), Y)).

现在在prolog，你可以.

1 ?- addfacts.
true.

2 ?- full(bottle(b1), X).
X = 100.

3 ?- consume(bottle(b1), 10).
true.

4 ?- full(bottle(b1), X).
X = 90 .