PHP Manual: Generator syntax

Generator syntax

A generator function looks just like a normal function, except that instead of returning a value, a generator yields as many values as it needs to.

When a generator function is called, it returns an object that can be iterated over. When you iterate over that object (for instance, via a foreach loop), PHP will call the generator function each time it needs a value, then saves the state of the generator when the generator yields a value so that it can be resumed when the next value is required.

Once there are no more values to be yielded, then the generator function can simply exit, and the calling code continues just as if an array has run out of values.

Note:
A generator cannot return a value: doing so will result in a compile error. An empty return statement is valid syntax within a generator and it will terminate the generator.

yield keyword

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.

Example #1 A simple example of yielding values


<?php
function gen_one_to_three() {
    for ($i = 1; $i <= 3; $i++) {
        // Note that $i is preserved between yields.
        yield $i;
    }
}

$generator = gen_one_to_three();
foreach ($generator as $value) {
    echo "$value\n";
}
?>

The above example will output:

1
2
3

Note:
Internally, sequential integer keys will be paired with the yielded values, just as with a non-associative array.

Caution

If you use yield in an expression context (for example, on the right hand side of an assignment), you must surround the yield statement with parentheses. For example, this is valid:


$data = (yield $value);

But this is not, and will result in a parse error:


$data = yield $value;

This syntax may be used in conjunction with the Generator::send method.

Yielding values with keys

PHP also supports associative arrays, and generators are no different. In addition to yielding simple values, as shown above, you can also yield a key at the same time.

The syntax for yielding a key/value pair is very similar to that used to define an associative array, as shown below.

Example #2 Yielding a key/value pair


<?php
/*
 * The input is semi-colon separated fields, with the first
 * field being an ID to use as a key.
 */

$input = <<<'EOF'
1;PHP;Likes dollar signs
2;Python;Likes whitespace
3;Ruby;Likes blocks
EOF;

function input_parser($input) {
    foreach (explode("\n", $input) as $line) {
        $fields = explode(';', $line);
        $id = array_shift($fields);

        yield $id => $fields;
    }
}

foreach (input_parser($input) as $id => $fields) {
    echo "$id:\n";
    echo "    $fields[0]\n";
    echo "    $fields[1]\n";
}
?>

The above example will output:

1:
    PHP
    Likes dollar signs
2:
    Python
    Likes whitespace
3:
    Ruby
    Likes blocks

Caution

As with the simple value yields shown earlier, yielding a key/value pair in an expression context requires the yield statement to be parenthesised:


$data = (yield $key => $value);

Yielding null values

Yield can be called without an argument to yield a NULL value with an automatic key.

Example #3 Yielding NULLs


<?php
function gen_three_nulls() {
    foreach (range(1, 3) as $i) {
        yield;
    }
}

var_dump(iterator_to_array(gen_three_nulls()));
?>

The above example will output:

array(3) {
  [0]=>
  NULL
  [1]=>
  NULL
  [2]=>
  NULL
}

Yielding by reference

Generator functions are able to yield values by reference as well as by value. This is done in the same way as returning references from functions: by prepending an ampersand to the function name.

Example #4 Yielding values by reference


<?php
function &gen_reference() {
    $value = 3;

    while ($value > 0) {
        yield $value;
    }
}

/*
 * Note that we can change $number within the loop, and
 * because the generator is yielding references, $value
 * within gen_reference() changes.
 */
foreach (gen_reference() as &$number) {
    echo (--$number).'... ';
}
?>

The above example will output:

2... 1... 0...

Generator objects

When a generator function is called for the first time, an object of the internal Generator class is returned. This object implements the Iterator interface in much the same way as a forward-only iterator object would.