Generating Power with Yield

Generating
Power with Yield/Jason Myers @jasonamyers

Yield, a modern language love
story
Originally Proposed in 1995, the yield keyword became official via the
RFP on June 20th 2013 with PHP 5.5.Generators
Facebook said a Hip, Hop and Don't Stop, and did their own yield
generators in HipHop PHP

They are HEAVILY BASED off of Python, with a nod towards the
Mozilla JS implementation and the await C# concept.

Iterator
An object that lets us traverse a container

PHP Iterator Interface
Iterator extends Traversable {
/* Methods */
abstract public mixed current ( void )
abstract public scalar key ( void )
abstract public void next ( void )
abstract public void rewind ( void )
abstract public boolean valid ( void )
}
For example checkout classArrayIterator

ArrayIterator Example
$fruits = array(
"apple" => "yummy",
"orange" => "ah ya, nice",
"grape" => "wow, I love it!",
"plum" => "nah, not me"
);
$obj = new ArrayObject( $fruits );
$it = $obj->getIterator();
echo "Iterating over: " . $obj->count() . " valuesn";
while( $it->valid() )
{
echo $it->key() . "=" . $it->current() . "n";
$it->next();
}
Iterating over: 4 values
apple=yummy
orange=ah ya, nice
grape=wow, I love it!
plum=nah, not me

Generator
a special routine that can be used to control the iteration behavior of
a loop, and yields the values one at a time

TL;DR
A generator looks like a function but behaves like an iterator

Performant?
range(0, 1000000)
Uses over 100MB of RAM

Generator Version
function xrange($start, $limit, $step = 1) {
if ($start < $limit) {
if ($step <= 0) {
throw new LogicException('Step must be +ve');
}
for ($i = $start; $i <= $limit; $i += $step) {
yield $i;
}
} else {
if ($step >= 0) {
throw new LogicException('Step must be -ve');
}
for ($i = $start; $i >= $limit; $i += $step) {
yield $i;
}
}
}
uses less than 1KB!

TL; DR
function xrange($min, $max) {
for ($i = $min; $i < $max; $i++) {
yield $i;
}
}

Sequences
function collatz($val) {
yield $val;
while ($val != 1) {
if ($val%2 == 0) {
$val /= 2;
} else {
$val = 3*$val + 1;
}
yield $val;
}
}
foreach (collatz(11) as $c) {
echo $c," ";
}
11 34 17 52 26 13 40 20 10 5 16 8 4 2 1

Y U Only LOOPin?
It will work for any function that takes an Iterator or a Traversable as
argument
$arr = iterator_to_array(collatz(11));

Transformations
function multiply_sequence($a, $fac) {
foreach ($a as $val) {
yield $val*$fac;
}
}
function to_html_list($input) {
foreach ($input as $val) {
yield "<li>".$val."</li>";
}
}

Chaining
foreach (to_html_list(multiply_sequence(collatz(5),2)) as $val) {
echo $val,"n";
}
<li>10</li>
<li>32</li>
<li>16</li>
<li>8</li>
<li>4</li>
<li>2</li>

Selections
function select_pattern($input, $pattern) {
foreach ($input as $val) {
if (preg_match($pattern, $val)) {
yield $val;
}
}
}

Breath In
function getLines($file) {
$f = fopen($file, 'r');
if (!$f) {
throw new Exception();
}
while ($line = fgets($f)) {
yield $line;
}
fclose($f);
}
foreach (getLines("someFile") as $line) {
doSomethingWithLine($line);
}

Breath Out
function createLog($file) {
$f = fopen($file, 'a');
while (true) {
$line = yield;
fwrite($f, $line);
}
}
$log = createLog($file);
$log->send("First");
$log->send("Second");
$log->send("Third");

Bro Remote Me!
Fake the simultaneous processing of data

Green Threads
threads that are scheduled by a virtual machine (VM/interperter?)
instead of natively by the underlying operating system

function step1() {
$f = fopen("file.txt", 'r');
processLine($line);
yield true;
}
}

function step2() {
$f = fopen("file2.txt", 'r');
processLine($line);
yield true;
}
}

function step3() {
$f = fsockopen("www.example.com", 80);
stream_set_blocking($f, false);
$headers = "GET / HTTP/1.1rn";
$headers .= "Host: www.example.comrn";
$headers .= "Connection: Closernrn";
fwrite($f, $headers);
$body = '';
while (!feof($f)) {
$body .= fread($f, 8192);
yield true;
}
processBody($body);
}

ZOMG... THERE BE DRAGONS!
This relies on making sure we have no blocking IO

overREACTPHP much?
event based, non-blocking IO - ReActPHP

One More Thing
So if I can flip control, I can haz an Async?

class Buffer {
protected $reads, $data;
public function __construct() {
$this->reads = new SplQueue();
$this->data = new SplQueue();
}
public function read() {
if( $this->data->isEmpty() ) {
$deferred = new ReactPromiseDeferred();
$this->reads->enqueue($deferred->resolver());
return $deferred->promise();
} else {
return ReactPromiseWhen::resolve($this->data->dequeue());
}
}
public function write($str) {
if( $this->reads->isEmpty() ) {
$this->data->enqueue($str);
} else {
$this->reads->dequeue()->resolve($str);
}
}
}

function printer(Buffer $buffer) {
while( true ) {
$value = ( yield Util::async($buffer->read()) );
echo "Printer: ", $value, PHP_EOL;
yield Util::async(nested_printer($buffer));
}
}

function nested_printer(Buffer $buffer) {
for( $i = 0; $i < 5; $i++ ) {
// Yield a promise task and wait for the result - this is non-blocking
$value = ( yield Util::async($buffer->read()) );
echo "Nested printer: ", $value, PHP_EOL;
}
}

$buffer = new Buffer();
$scheduler = new AsyncScheduler();
$scheduler->add(new AsyncTaskGeneratorTask(printer($buffer)));
$i = 0;
$scheduler->add(new AsyncTaskRecurringTask(
function() use($buffer, &$i) { $buffer->write(++$i); }
));
$scheduler->run();
Printer: 1
Nested printer: 2
Nested printer: 3
Nested printer: 4
Nested printer: 5
Nested printer: 6
Printer: 7
Nested printer: 8
Nested printer: 9
Nested printer: 10
Nested printer: 11
Nested printer: 12
...

$loop = ReactEventLoopFactory::create();
$scheduler = new AsyncScheduler();
$scheduler->add(new AsyncTaskRecurringTask([$loop, 'tick']));
$scheduler->run();

Async
Created by Matt Pryor, on Bitbucket

Thanks
Huge thanks to Paul M. Jones and William Golden!

Generating Power with Yield

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