如何在 Perl 6 中向现有类添加方法? [英] How do you add a method to an existing class in Perl 6?

问题描述

Int 类有一个方法 is_prime，所以我想通了，只是为了咯咯笑，我想为 Int 添加一些其他方法，用于我的一些从事数论工作的爱好项目.

The Int class has a method is_prime, so I figured, just for giggles, I'd like to add some other methods to Int for some of my hobby projects that do number theory stuff.

我以为我可以这样做:

class Int {
    method is-even (Int:D $number ) returns Bool:D {
        return False if $number % 2;
        return True;
        }
    }

say 137.is-even;

但这不起作用:

===SORRY!===
P6opaque: must compose before allocating

我不知道这是否意味着我不能这样做，或者我做错了.

I don't know if this means that I can't do that or that I'm doing it incorrectly.

我可以轻松创建一个继承自 Int 的新类，但这不是我感兴趣的:

I could easily make a new class that inherits from Int, but that's not what I'm interested in:

class MyInt is Int {
    method is-even () returns Bool:D {
        return False if self % 2;
        return True;
        }
    }

my $n = MyInt.new(138);
say $n.is-even;

我不是在寻找变通方法或替代解决方案.

I'm not looking for workarounds or alternate solutions.

推荐答案

这里有语法糖 - augment:

use MONKEY-TYPING;

augment class Int {
    method is-even() returns Bool:D {
        return False if self % 2;
        return True;
    }
}

增加一个类被认为是危险的，原因有两个:第一，远距离动作，第二，因为(据我所知)，有可能出现未定义行为 去优化 因为它可能会使各种方法缓存处于无效状态.

Augmenting a class is considered dangerous for two reasons: First, action at a distance, and second, because (as far as I'm aware), there's potential for undefined behaviour deoptimization as it might leave various method caches in an invalid state.

因此，需要在允许使用之前提供 MONKEY-TYPING 编译指示.

Thus, the requirement for providing the MONKEY-TYPING pragma before you're allowed to use it.

顺便说一下，is-even 可以更紧凑地写成 self %% 2.

As an aside, note that is-even could be written more compactly as self %% 2.

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