Prolog 中的逻辑“非"是什么? [英] What is the logical 'not' in Prolog?

问题描述

我面临的问题有点微不足道.我想在 Prolog 中使用逻辑 not，但似乎 not/1 不是我想要的东西:

The problem that I face, is a bit trivial. I want to use logical not in Prolog, but it seems that not/1 is not the thing that I want:

course(ai).
course(pl).
course(os).

have(X,Y) :- course(X),course(Y),not(X = Y).

我查询:

have(X,Y), write(X-Y), nl , fail.

我没有得到我想要的结果:(

And I do not get the result I want :(

推荐答案

代替 not(X = Y) 你需要写 \+ X = Y 或X \= Y.但考虑使用 dif(X,Y) 代替.dif/2 存在于 B、SWI、YAP、SICStus 中.查看差异:

In place of not(X = Y) you need to write \+ X = Y or X \= Y. But consider to use dif(X,Y) instead. dif/2 is present in B, SWI, YAP, SICStus. To see the difference:

?- X = b, dif(a, X).
X = b.

?- X = b, \+ a = X.
X = b.

所以到目前为止一切似乎都很好.但是，如果我们只是交换两个目标的顺序?

So up to now everything seems to be fine. But what, if we simply exchange the order of the two goals?

?- \+ a = X, X = b.
false.

?- dif(a, X), X = b.
X = b.

(\+)/1 现在给了我们不同的结果，因为有答案对于 a = X，目标 \+ a = X 将失败.

(\+)/1 now gives us a different result, because there is an answer for a = X, the goal \+ a = X will fail.

(\+)/1 因此不是否定，但意味着 此时不可证明及时.

(\+)/1 is thus not negation, but means not provable at this point in time.

dif/2 的安全近似在 ISO Prolog 中也是可能的.

A safe approximation of dif/2 is possible in ISO Prolog, too.

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