PHP equivalent to Python's yield operator - php

Question

In Python (and others), you can incrementally process large volumes of data by using the 'yield' operator in a function. What would be the similar way to do so in PHP?
For example, lets say in Python, if I wanted to read a potentially very large file, I could work on each line one at a time like so (this example is contrived, as it is basically the same thing as 'for line in file_obj'):
def file_lines(fname):
f = open(fname)
for line in f:
yield line
f.close()
for line in file_lines('somefile'):
#process the line
What I'm doing right now (in PHP) is I'm using a private instance variable to keep track of state, and acting accordingly each time the function is called, but it seems like there must be a better way.

Answer 1

There is a rfc at https://wiki.php.net/rfc/generators adressing just that, which might be included in PHP 5.5.
In the mean time, check out this proof-of-concept of a poor mans "generator function" implemented in userland.
namespace Functional;
error_reporting(E_ALL|E_STRICT);
const BEFORE = 1;
const NEXT = 2;
const AFTER = 3;
const FORWARD = 4;
const YIELD = 5;
class Generator implements \Iterator {
private $funcs;
private $args;
private $key;
private $result;
public function __construct(array $funcs, array $args) {
$this->funcs = $funcs;
$this->args = $args;
}
public function rewind() {
$this->key = -1;
$this->result = call_user_func_array($this->funcs[BEFORE],
$this->args);
$this->next();
}
public function valid() {
return $this->result[YIELD] !== false;
}
public function current() {
return $this->result[YIELD];
}
public function key() {
return $this->key;
}
public function next() {
$this->result = call_user_func($this->funcs[NEXT],
$this->result[FORWARD]);
if ($this->result[YIELD] === false) {
call_user_func($this->funcs[AFTER], $this->result[FORWARD]);
}
++$this->key;
}
}
function generator($funcs, $args) {
return new Generator($funcs, $args);
}
/**
* A generator function that lazily yields each line in a file.
*/
function get_lines_from_file($file_name) {
$funcs = array(
BEFORE => function($file_name) { return array(FORWARD => fopen($file_name, 'r')); },
NEXT => function($fh) { return array(FORWARD => $fh, YIELD => fgets($fh)); },
AFTER => function($fh) { fclose($fh); },
);
return generator($funcs, array($file_name));
}
// Output content of this file with padded linenumbers.
foreach (get_lines_from_file(__FILE__) as $k => $v) {
echo str_pad($k, 8), $v;
}
echo "\n";

Answer 2

PHP has a direct equivalent called generators.
Old (pre php 5.5 answer):
Unfortunately, there isn't a language equivalent. The easiest way is to either to what you're already doing, or to create a object that uses instance variables to maintain state.
There is however a good option if you want to use the function in conjunction with the foreach-statement: SPL Iterators. They can be used to achieve something quite similar to python generators.

Answer 3

I prototype everything in Python before implementing in any other languages, including PHP. I ended up using callbacks to achieve what I would with the yield.
function doSomething($callback)
{
foreach ($something as $someOtherThing) {
// do some computations that generates $data
call_user_func($callback, $data);
}
}
function myCallback($input)
{
// save $input to DB
// log
// send through a webservice
// etc.
var_dump($input);
}
doSomething('myCallback');
This way each $data is passed to the callback function and you can do what you want.

Answer 4

Extending #Luiz's answer - another cool way is to use anonymous functions:
function iterator($n, $cb)
{
for($i=0; $i<$n; $i++) {
call_user_func($cb, $i);
}
}
$sum = 0;
iterator(10,
function($i) use (&$sum)
{
$sum += $i;
}
);
print $sum;

Answer 5

There may not be an equivalent operator, but the following code is equivalent in function and overhead:
function file_lines($file) {
static $fhandle;
if ( is_null($fhandle) ) {
$fhandle = fopen($file, 'r');
if ( $fhandle === false ) {
return false;
}
}
if ( ($line = fgets($fhandle))!== false ) {
return $line;
}
fclose($fhandle);
$fhandle = null;
}
while ( $line = file_lines('some_file') ) {
// ...
}
That looks about right. Sorry, I haven't tested it.

Answer 6

The same sentence 'yield' exists now on PHP 5.5:
http://php.net/manual/en/language.generators.syntax.php