What does a variable in the right do in yield expression? - php

Question

According to PHP documentation, a variable passed in ::send() method of generator class is assigned to yield keyword.
it makes sense in following statement :
$v = yield;
But what about the statements like :
$v = yield $a;
yield keyword is already assigned and it shall return the variable passed into. Then what does $a do?
I've tried to figure this out and searched many posts and questions but none of them explain this.

Answer 1

Answering by myself...
$v = yield $a;
meaning, first yield $a just as generally it does, then assign a variable passed by send() method to $v.
Still can't get clear explanation. But I think this is somewhat an answer of this matter.

Answer 2

Using send() will define $v, so if you use $v = yield $a; and before the iteration, you sent a value with send() it will yield the value which you sent it.
It can be hard to get your head around so here is an example, which creates two parts, the foreach iterator which loops over the generator and then on iteration 10 it sends a stop to the while loop. Then using getReturn gets the last value of the yield, like your $v in $v = yield $a;
<?php
$engine = function($callback) {
$i = 0;
while (true) {
$state = (yield $callback($i++));
if ($state == 'stop') {
return $i;
}
}
};
$generator = $engine(function($i) {
return $i;
});
foreach ($generator as $value) {
echo "{$generator->key()} = {$value}\n";
if ($generator->key() == 10) {
$generator->send('stop');
}
}
echo 'Stopped on: '.$generator->getReturn();
https://3v4l.org/dY8rV
Result:
0 = 0
1 = 1
2 = 2
3 = 3
4 = 4
5 = 5
6 = 6
7 = 7
8 = 8
9 = 9
10 = 10
Stopped on: 11

Answer 3

The yield keyword can be used for three things:
On its own, to return control temporarily to calling code. This is the basis of all the other uses.
With a value after (yield $foo), to pass a variable to calling code.
With an assignment before ($bar = yield), to receive a variable from calling code.
These are sort of similar to how you can use a function call:
On its own, like doSomething(), to pass control temporarily to the definition of doSomething.
With a parameter, like doSomething($foo), to pass a variable to the function.
With an assignment before, like $var = doSomething(), to receive a variable from the function.
You are hopefully quite familiar with writing $bar = doSomething($foo); to pass $foo to a function, and get $bar out afterwards. $bar = yield $foo is similar, but the value is passed "out" to the calling code, and the new value received back from that calling code afterwards.
$foo and $bar are not connected in any way, they are just the input and output of that particular yield. In the calling code, the $foo part of yield $foo or $bar = yield $foo can be accessed by calling ->next() or ->current() or getting the value in a foreach. The calling code then calls ->send() with some value which becomes the $bar in $bar = yield or $bar = yield $foo.