Prolog if 语句 [英] Prolog if-statement

问题描述

我正在尝试实现一个如下工作的谓词:

I'm trying to implement a predicate that works as follows:

pred :-
   % do this always

   % if-statement
      %do this only, when if-statement is true

   % do this also always, independent if if-statement where true or false.

我需要一个程序的这个功能，它可以选择是否有 gui (XPCE).你可以调用它

I need this functionality for a program, which has optionality a gui (XPCE) or not. You can call it with

start(true) % with gui

或

start(false) % without gui

因为我不想用相同的逻辑编写两个不同的谓词，但一次使用 gui，另一次没有，我想要一个谓词，仅当 start(true) 被调用时才调用 gui 代码调用.

Because I don't want to write two different predicates with the same logic, but one time with gui and another time without, I want to have one one predicate, that invokes the gui-code only if start(true) were invoked.

感谢您的帮助！

推荐答案

标准的Prologif-statement"是:

The standard Prolog "if-statement" is:

(If -> Then; Else)

其中 If、Then 和 Else 是目标.您可以轻松地在谓词的定义上使用 if 来打开谓词 start/1 的参数:

where If, Then, and Else are goals. You can use if easily on the definition of your predicate to switch on the argument of the predicate start/1:

pred :-
    % common code
    (   start(true) ->
        % gui-only code
    ;   % non-gui code
    ),
    % common code

当没有Else目标时，可以用目标true代替.当 If 目标失败时，目标 (If -> Then) 失败.IE.(If -> Then) 目标等价于 (If -> Then; fail)，而不是 (If -> Then; true).

When there's no Else goal, you can replace it with the goal true. The goal (If -> Then) fails when the If goal fails. I.e. the (If -> Then) goal is equivalent to (If -> Then; fail), not to (If -> Then; true).

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