prolog - 实现一个 Prolog 谓词，说明一个元素是否属于一个列表。非数字列表的问题

Question

我正在为大学考试学习 Prolog，但我在这个练习中遇到了问题:

Implement the predicate not_member(X,L) that is TRUE if the element X does not belong to the list L.

如果我的推理是正确的，我已经找到了解决方案:

% FACT (BASE CASE): It is TRUE that X is not in the list if the list is empty.
not_member(_,[]).

% RULE (GENERAL CASE): If the list is non-empty, I can divide it in its Head
%   element and the sublist Tail. X does not belong to the list if it is different 
%   from the current Head element and if it does not belong to the sublist Tail.
not_member(X,[Head|Tail]) :-
   X =\= Head,
   not_member(X,Tail).

此代码适用于数字列表，如以下查询所示:

2 ?- not_member(4, [1,2,3]).
true.

3 ?- not_member(1, [1,2,3]).
false.

但是，对于具有一些非数字元素的列表，
它不起作用并报告错误:

4 ?- not_member(a, [a,b,c]).
ERROR: =\=/2: Arithmetic: `a/0' is not a function

为什么？

Answer 1

让我们检查文档!

(=\=)/2是算术运算符。

+Expr1 =\= +Expr2 True if expression Expr1 evaluates to a number non-equal to Expr2.

您必须使用 (\=)/2比较两个通用术语:

not_member(_, []) :- !.

not_member(X, [Head|Tail]) :-
     X \= Head,
    not_member(X, Tail).

和:

?- not_member(d, [a,b,c]).
true.