yield
?The yield
keyword returns data from a generator function:
The heart of a generator function is the yield keyword. In its simplest form, a yield statement looks much like a return statement, except that instead of stopping execution of the function and returning, yield instead provides a value to the code looping over the generator and pauses execution of the generator function.
A generator function is effectively a more compact and efficient way to write an Iterator. It allows you to define a function (your xrange
) that will calculate and return values while you are looping over it:
function xrange($min, $max) {
for ($i = $min; $i <= $max; $i++) {
yield $i;
}
}
[…]
foreach (xrange(1, 10) as $key => $value) {
echo "$key => $value", PHP_EOL;
}
This would create the following output:
0 => 1
1 => 2
…
9 => 10
You can also control the $key
in the foreach
by using
yield $someKey => $someValue;
In the generator function, $someKey
is whatever you want appear for $key
and $someValue
being the value in $val
. In the question's example that's $i
.
Now you might wonder why we are not simply using PHP's native range
function to achieve that output. And right you are. The output would be the same. The difference is how we got there.
When we use range
PHP, will execute it, create the entire array of numbers in memory and return
that entire array to the foreach
loop which will then go over it and output the values. In other words, the foreach
will operate on the array itself. The range
function and the foreach
only "talk" once. Think of it like getting a package in the mail. The delivery guy will hand you the package and leave. And then you unwrap the entire package, taking out whatever is in there.
When we use the generator function, PHP will step into the function and execute it until it either meets the end or a yield
keyword. When it meets a yield
, it will then return whatever is the value at that time to the outer loop. Then it goes back into the generator function and continues from where it yielded. Since your xrange
holds a for
loop, it will execute and yield until $max
was reached. Think of it like the foreach
and the generator playing ping pong.
Obviously, generators can be used to work around memory limits. Depending on your environment, doing a range(1, 1000000)
will fatal your script whereas the same with a generator will just work fine. Or as Wikipedia puts it:
Because generators compute their yielded values only on demand, they are useful for representing sequences that would be expensive or impossible to compute at once. These include e.g. infinite sequences and live data streams.
Generators are also supposed to be pretty fast. But keep in mind that when we are talking about fast, we are usually talking in very small numbers. So before you now run off and change all your code to use generators, do a benchmark to see where it makes sense.
Another Use Case for Generators is asynchronous coroutines. The yield
keyword does not only return values but it also accepts them. For details on this, see the two excellent blog posts linked below.
yield
?Generators have been introduced in PHP 5.5. Trying to use yield
before that version will result in various parse errors, depending on the code that follows the keyword. So if you get a parse error from that code, update your PHP.