如何传递一个空的生成器参数? [英] How to pass in an empty generator parameter?

问题描述

我有一个方法，该方法需要一个生成器以及一些其他参数，然后返回一个新的生成器:

I have a method which takes a generator plus some additional parameters and returns a new generator:

function merge(\Generator $carry, array $additional)
{
    foreach ( $carry as $item ) {
        yield $item;
    }
    foreach ( $additional as $item ) {
        yield $item;
    }
}

此功能的常用用例类似于此:

The usual use case for this function is similar to this:

function source()
{
    for ( $i = 0; $i < 3; $i++ ) {
        yield $i;
    }
}

foreach ( merge(source(), [4, 5]) as $item ) {
    var_dump($item);
}

但是问题是有时我需要将空源传递给merge方法.理想情况下，我希望能够执行以下操作:

But the problem is that sometimes I need to pass empty source to the merge method. Ideally I would like to be able to do something like this:

merge(\Generator::getEmpty(), [4, 5]);

这正是我在C#中的做法(有一个IEnumerable<T>.Empty属性).但我在手册中看不到任何empty生成器

Which is exactly how I would do in C# (there is a IEnumerable<T>.Empty property). But I don't see any kind of empty generator in the manual.

我已经使用此功能设法解决了这个问题(目前):

I've managed to work around this (for now) by using this function:

function sourceEmpty()
{
    if ( false ) {
        yield;
    }
}

这有效.代码:

foreach ( merge(sourceEmpty(), [4, 5]) as $item ) {
    var_dump($item);
}

正确输出:

int(4)
int(5)

但这显然不是理想的解决方案.将空生成器传递给merge方法的正确方法是什么?

But this is obviously not an ideal solution. What would be the proper way of passing an empty generator to the merge method?

推荐答案

我找到了解决方法:

由于\Generator扩展了\Iterator，因此我可以将方法签名更改为:

Since \Generator extends \Iterator I can just change the method signature to this:

function merge(\Iterator $carry, array $additional) 
{
    // ...

这是输入协方差，因此会破坏向后兼容性，但前提是有人确实扩展了merge方法.任何调用仍然有效.

This is input covariance thus it would break backward compatibility, but only if someone did extend the merge method. Any invocations will still work.

现在，我可以使用PHP的本机EmtpyIterator调用该方法:

Now I can invoke the method with PHP's native EmtpyIterator:

merge(new \EmptyIterator, [4, 5]);

通常的生成器也可以工作:

And the usual generator also works:

merge(source(), [4, 5])

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