Prolog规则中目标(语句)的顺序

Question

我最近开始学习Prolog，遇到了一个奇怪的问题。在这里，您可以看到一个代码示例(我使用SWI-Prolog7.2.3)，它给出了关系树和我的两个任务的解决方案。

/*   File:    ancestors.pl
 Author:  Dave Robertson
 Purpose: Relationships in a family tree

Suppose we have a family tree like this :

alan andrea   bruce betty      eddie elsie   fred  freda
 |     |        |     |          |     |       |     |
 |_____|        |_____|          |_____|       |_____|
    |              |                |             |
  clive        clarissa            greg         greta
   |  |__________|___|              |             |
   |__________|__|                  |_____________|
          |   |                            |
        dave doris                        henry

which is defined in Prolog by the following 3 sets of predicates:

*/

%   parent(Parent, Child).
%   Parent is the parent of Child.

parent(alan, clive).
parent(andrea, clive).
parent(bruce, clarissa).
parent(betty, clarissa).
parent(clive, dave).
parent(clarissa, dave).
parent(clive, doris).
parent(clarissa, doris).
parent(eddie, greg).
parent(elsie, greg).
parent(fred, greta).
parent(freda, greta).
parent(greg, henry).
parent(greta, henry).

%%   PROBLEM 1
%%   How do you find out if someone is the ancestor of someone else ?
ancestor(X,Y) :- parent(X,Y).
ancestor(X,Y) :- parent(X,Z), ancestor(Z,Y).

%%   PROBLEM 3
%%   How do you know if someone is related to someone else ?
relative(X,Y) :-  ancestor(X,Y).
relative(X,Y) :-  ancestor(Y,X).
relative(X,Y) :-  ancestor(Z,X), ancestor(Z,Y), X\==Y.

当我想去找戴夫的亲戚时，我会：

relative(dave,X).
X = clive ;
X = clarissa ;
X = alan ;
X = andrea ;
X = bruce ;
X = betty ;
X = doris ;
X = doris ;
X = clive ;
X = doris ;
X = clive ;
X = doris ;
X = clarissa ;
X = doris ;
X = clarissa ;
X = doris ;
false.

然后我改变了我对相对关系的定义

relative(X,Y) :-  ancestor(X,Y).
relative(X,Y) :-  ancestor(Y,X).
relative(X,Y) :-  X\==Y, ancestor(Z,X), ancestor(Z,Y).

我只是在最后一句中改变了目标的顺序。现在我有了以下输出：

relative(dave,X).

X = clive ;
X = clarissa ;
X = alan ;
X = andrea ;
X = bruce ;
X = betty ;
X = dave ;
X = doris ;
X = dave ;
X = doris ;
X = clive ;
X = dave ;
X = doris ;
X = clive ;
X = dave ;
X = doris ;
X = clarissa ;
X = dave ;
X = doris ;
X = clarissa ;
X = dave ;
X = doris ;
false.

我在输出中看到戴夫了！怎么会出这事？我写的X \== Y..。有人能给我一个很好的解释吗？

还有一个问题。我如何使我的程序不写相同的答案？

谢谢!

Answer 1

特别是作为一个新手，尽量避免使用不纯的构造和持之以恒的逻辑纯度！

如何使用prolog-dif!？使用

而不是X \== Y，只需编写dif(X, Y)。