Prolog 变量名以下划线开头，然后大写

Question

给出以下知识库:

cityinCountry(paris, france ) . /* Paris is a city in France*/
cityinCountry(berlin, germany).
cityinCountry(cairo, egypt).
cityinCountry(munich, germany). 

为什么cityinCountry(_City, germany).返回True，而cityinCountry(City, germany).返回berlin;慕尼黑？

我对变量的定义表明它们由以大写字母或下划线(_)开头的标识符表示。然而我的两个例子表现不同。有人可以解释一下吗？

Answer 1

它仍然是一个变量，但是通过在查询中使用这个名称，您就告诉 Prolog(或者至少是 Prolog 的实现)您不想显示它的值。例如。 cityInCountry(_City, Country) 将打印预期的四个结果，但仅显示 Country 值。

奇怪的是，我找不到这方面的明确文档，例如在 http://www.swi-prolog.org/pldoc/man?section=singleton 。它给出的唯一区别是

A singleton variable is a variable that appears only one time in a clause. It can always be replaced by _, the anonymous variable. In some cases, however, people prefer to give the variable a name. As mistyping a variable is a common mistake, Prolog systems generally give a warning (controlled by style_check/1) if a variable is used only once. The system can be informed that a variable is meant to appear once by starting it with an underscore, e.g., _Name.

我能找到的最接近的是 http://www.cse.unsw.edu.au/~billw/dictionaries/prolog/dontcare.html :

This indicates what the _ variable is actually representing, but has the feature that the binding of _Anybody typically doesn't get reported when Prolog finds a solution.

但是 _Anybody 位于规则主体内，无论如何都不会在现代 Prolog 实现中报告。