php - "yield"覆盖 "yield from"项

Question

<?php
function generator1(): \Traversable
{
    yield 'alpha';
    yield 'beta';
    yield 'delta';
}

function generator2(): \Traversable
{
    yield from generator1();
    yield 'upsilon';
    yield 'omega';
}

function resolve(iterable $generator) {
    foreach ($generator as $index => $item) 
        echo "$index $item" . PHP_EOL;
}

resolve(generator1());
echo  PHP_EOL;

resolve(generator2());
die;

产生这个结果:

0 alpha
1 beta
2 delta

0 alpha
1 beta
2 delta
0 upsilon
1 omega

最后一个流应该是:

0 alpha
1 beta
2 delta
3 upsilon
4 omega

这里发生了什么？这是 PHP 中的错误吗(我在 Ubuntu 中运行 v7.4.10。)如何在不使用数组或其他形式的收集器的情况下获得正确的结果？

Answer 1

结果是 iterator_to_array() 函数的特长。第二个参数必须设置为 false，以便记录所有值，如注释中所述。
这在PHP manual中也有特别指出。 .

function generator1(): \Traversable
{
    yield 'alpha';
    yield 'beta';
    yield 'delta';
    yield 'gamma';
    yield 'epsilon';
    yield 'eta';
}

function generator2(): \Traversable
{
    yield from generator1();
    yield 'upsilon';
    yield 'omega';
}

var_dump(iterator_to_array(generator2()));

var_dump(iterator_to_array(generator2(), false));

结果:

array(6) {
  [0]=>
  string(7) "upsilon"
  [1]=>
  string(5) "omega"
  [2]=>
  string(5) "delta"
  [3]=>
  string(5) "gamma"
  [4]=>
  string(7) "epsilon"
  [5]=>
  string(3) "eta"
}
array(8) {
  [0]=>
  string(5) "alpha"
  [1]=>
  string(4) "beta"
  [2]=>
  string(5) "delta"
  [3]=>
  string(5) "gamma"
  [4]=>
  string(7) "epsilon"
  [5]=>
  string(3) "eta"
  [6]=>
  string(7) "upsilon"
  [7]=>
  string(5) "omega"
}

Try for yourself in the sandbox.
当使用 foreach 输出键和值时，这一点就变得清楚了。

foreach(generator2() as $key => $value){
  var_dump($key,$value);
}

输出:

int(0) string(5) "alpha"
int(1) string(4) "beta"
int(2) string(5) "delta"
int(3) string(5) "gamma"
int(4) string(7) "epsilon"
int(5) string(3) "eta"
int(0) string(7) "upsilon"
int(1) string(5) "omega"