为什么这个 prolog 查询既是真的又是假的? [英] Why is this prolog query both true and false?

问题描述

我的 SWI-Prolog 知识库包含以下两个事实:

My SWI-Prolog knowledge base contains the following two facts:

f(a,b).
f(a,c).

现在如果我提出查询

?- f(a,c).
true.

但是

?- f(a,b).
true ;
false.

为什么 f(a,b) 既为真又为假?当知识库中有三个事实时，也会发生这种情况.如果我附加 f(a,d).到 KB，则 f(a,d) 为真(仅)，但 f(a,b) 和 f(a,c) 既为真又为假.发生了什么，我该怎么做才能使 Prolog(仅)对这些查询做出正确的回答?

Why is f(a,b) both true and false? This also happens when there are three facts in the KB. If I append f(a,d). to the KB, then f(a,d) is true (only), but f(a,b) and f(a,c) are both true and false. What's going on, and what can I do so that Prolog answers (only) true to these queries?

推荐答案

(注:这个答案有点猜测)

(Note: this answer is somewhat of a guess)

考虑 Prolog 如何确定 f(a,c) 是否为真.它检查第一条规则 f(a,b)，但没有找到匹配项，但第二条规则 f(a,c) 匹配.因此，f(a,c) 为真.此外，由于 f 没有更多规则，因此没有必要允许回溯发生——没有其他可能的解决方案.

Consider how Prolog determines whether f(a,c) is true or not. It checks the first rule, f(a,b), and doesn't find a match, but the second rule, f(a,c) matches. Therefore, f(a,c) is true. Furthermore, since there are no more rules for f, there is no point in allowing a backtrack to occur -- there are no other possible solutions.

现在考虑f(a,b).Prolog 将检查第一条规则，并找到匹配项.因此，f(a,b) 为真.但是，并非所有规则都已用尽.因此，Prolog 将允许继续搜索(如果您点击 ;).当你继续搜索和回溯时，它会发现剩余的规则，特别是 f(a,c)，与 f(a,b) 不匹配.因此，结果为假.

Now consider f(a,b). Prolog will check the first rule, and find a match. Therefore, f(a,b) is true. However, not all rules have been exhausted. Therefore, Prolog will allow the search to continue (if you hit ;). When you do continue the search and backtrack, it will discover that the remaining rules, specifically f(a,c), do not match f(a,b). Therefore, the result is false.

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