The yield keyword
Learn to write a function that yields values and preserves state.
What is the yield keyword?
If a function contains the yield keyword, it is a generator function. When called, the code does not run; instead a Generator object is returned.
Each yield produces a value and execution pauses there. On the next request it resumes from where it left off. This way values are produced lazily; they are not all computed at once.
Iterating with foreach and keys
foreach iterates a Generator automatically: each loop fetches the next yield value. The form foreach ($gen as $value) gives the value.
To also produce a key, write yield $key => $value; then foreach ($gen as $k => $v) gives both.
Exercise Write the countUp generator
Write the countUp(int $from, int $to): \Generator function. Yield each number from $from to $to with yield. Then iterate countUp(2, 6) with foreach and print each number on its own line.
Show example solution
take int $from and int $to, return \Generator. Use a for loop to yield each number from $from to $to. Then iterate with foreach (countUp(2, 6) as $n) and echo $n . "\n";
Example solution:
<?php
function countUp(int $from, int $to): \Generator {
for ($i = $from; $i <= $to; $i++) {
yield $i;
}
}
foreach (countUp(2, 6) as $n) {
echo $n . "\n";
}Test yourself
-
What happens when you call a function containing yield?
function gen() { yield 1; yield 2; } $g = gen();- The code runs immediately, the first value is returned
- A Generator object is returned, the code does not run yet
- An array is returned
- An error is thrown
Show answer
A Generator object is returned, the code does not run yet — Correct. Calling a generator function returns a Generator object; the code does not advance to the first yield.
-
yield in the countDown generator
<?php function countDown(int $n): \Generator { while ($n > 0) { ____ $n--; } }Show answer
<?php function countDown(int $n): \Generator { while ($n > 0) { yield $n; $n--; } } -
Match the generator concepts
- yield
- Produces a value and pauses
- Generator
- Object returned by a yield function
- foreach
- Iterates a generator automatically
- yield $k => $v
- Yields a key-value pair