Prolog 中的 Switch 语句 [英] Switch statements in Prolog

问题描述

在 Prolog 谓词中，我经常写这样的重复条件语句，但我希望它们可以写得更简洁:

In Prolog predicates, I often write repetitive conditional statements like this one, but I wish they could be written more concisely:

output(Lang, Type, Output) :-   
    (Lang = javascript ->
        Output = ["function", Type];
    Lang = ruby ->
        Output = ["def", Type];
    Lang = java ->
        Output = [Type]).

是否可以用更简洁的 switch 语句替换这一系列的条件语句?

Would it be possible to replace this series of conditional statements with a more concise switch-statement?

推荐答案

在 Prolog 中，使用元谓词(将目标或谓词作为参数的谓词)定义您自己的控制结构非常容易.

In Prolog it is quite easy to define your own control structures, using meta-predicates (predicates that take goals or predicates as arguments).

例如，您可以实现像

switch(X, [
    a : writeln(case1),
    b : writeln(case2),
    c : writeln(case3)
])

通过定义

switch(X, [Val:Goal|Cases]) :-
    ( X=Val ->
        call(Goal)
    ;
        switch(X, Cases)
    ).

如有必要，可以通过许多 Prolog 系统支持的编译时转换来提高效率(inline/2 在 ECLiPSe 中，或在其他几个系统中的目标扩展).

If necessary, this can then be made more efficient by compile-time transformation as supported by many Prolog systems (inline/2 in ECLiPSe, or goal expansion in several other systems).

并且通过运算符声明，您可以将语法调整为您喜欢的任何内容.

And via operator declarations you can tweak the syntax to pretty much anything you like.

这篇关于Prolog 中的 Switch 语句的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！