Marvelous Agents of Yield

Generators and thus yield are part of PHP since version 5.5. Most of the time it is used as a substitution for a full iterator implementation.
But there is way more power to generators! This talk will try to unveil the hidden superpowers of interruptible loops, signals and cooporative multitasking. Coincidental movie analogies included.

Transcript

1. AUGUST 25, 2018
   #FROSCON-PHP-TRACK
   

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3. github.com/hollodotme
   @hollodotme
   EXECUTIVE PRODUCER
   holger woltersdorf
   

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4. github.com/hollodotme
   @hollodotme@phpc.social
   EXECUTIVE PRODUCER
   holger woltersdorf
   

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5. CHAPTER 1: GENERATORS
   @hollodotme
   

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6. FITZ'PLANATION
   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.
   php.net
   @hollodotme
   

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7. FITZ'PLANATION
   BOY, WHAT
   THE HELL
   ARE YOU
   TALKING
   ABOUT?
   @hollodotme
   

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8. JEMMA'LOC
   function generate()
   {
   yield 1;
   }
   $generator = generate();
   echo 'Type: ', gettype($generator), PHP_EOL;
   echo 'Class: ', get_class($generator), PHP_EOL;
   echo 'Is ',
   (is_callable($generator) ? 'callable' : 'not callable'),
   PHP_EOL;
   $generatorClone = clone $generator;
   @hollodotme
   

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9. JEMMA'LOC
   function generate()
   {
   yield 1;
   }
   $generator = generate();
   echo 'Type: ', gettype($generator), PHP_EOL;
   echo 'Class: ', get_class($generator), PHP_EOL;
   echo 'Is ',
   (is_callable($generator) ? 'callable' : 'not callable'),
   PHP_EOL;
   $generatorClone = clone $generator;
   —
   Type: object
   Class: Generator
   Is not callable
   Fatal error: Uncaught Error: Trying to clone an uncloneable
   object of class Generator
   @hollodotme
   

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10. JEMMA'LOC
    function range( int $start, int $limit ) : array
    {
    $elements = [];
    for ( $i = $start; $i !<= $limit; $i!++ )
    {
    $elements[] = $i;
    }
    return $elements;
    }
    # vs.
    function xrange( int $start, int $limit ) : Generator
    {
    for ( $i = $start; $i !<= $limit; $i!++ )
    {
    yield $i;
    }
    }
    @hollodotme
    

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11. JEMMA'LOC
    foreach ( range( 1, 10000 ) as $i )
    {
    print $i . '.';
    }
    # 2 MiB
    foreach ( range( 1, 100000 ) as $i )
    {
    print $i . '.';
    }
    # 6 MiB
    foreach ( range( 1, 1000000 ) as $i )
    {
    print $i . '.';
    }
    # 34 MiB
    @hollodotme
    

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12. JEMMA'LOC
    foreach ( xrange( 1, 10000 ) as $i )
    {
    print $i . '.';
    }
    # 2 MiB
    foreach ( xrange( 1, 100000 ) as $i )
    {
    print $i . '.';
    }
    # 2 MiB
    foreach ( xrange( 1, 1000000 ) as $i )
    {
    print $i . '.';
    }
    # 2 MiB
    @hollodotme
    

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13. JEMMA'LOC
    # OPCODES OF RANGE()
    ~$ php -d vld.active=1 -d vld.execute=0 -f range.func.php
    number of ops: 22
    compiled vars:
    !0 = $start,
    !1 = $limit,
    !2 = $step,
    !3 = $elements,
    !4 = $i
    # OPCODES OF XRANGE()
    ~$ php -d vld.active=1 -d vld.execute=0 -f xrange.func.php
    number of ops: 16
    compiled vars:
    !0 = $start,
    !1 = $limit,
    !2 = $step,
    !3 = $i
    @hollodotme
    

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14. JEMMA'LOC
    function generate()
    {
    print 'generate - start' . PHP_EOL;
    for ( $i = 1; $i !<= 5; $i!++ )
    {
    print 'generate - yielding!!...' . PHP_EOL;
    yield $i;
    print 'generate - continued' . PHP_EOL;
    }
    print 'generate - end' . PHP_EOL;
    }
    $generator = generate();
    print 'Generator created' . PHP_EOL;
    while ( $generator!->valid() )
    {
    print 'Loop gets current value' . PHP_EOL;
    print $generator!->current() . PHP_EOL;
    $generator!->next();
    }
    @hollodotme
    

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15. JEMMA'LOC
    function generate()
    {
    print 'generate - start' . PHP_EOL;
    for ( $i = 1; $i !<= 5; $i!++ )
    {
    print 'generate - yielding!!...' . PHP_EOL;
    yield $i;
    print 'generate - continued' . PHP_EOL;
    }
    print 'generate - end' . PHP_EOL;
    }
    $generator = generate();
    print 'Generator created' . PHP_EOL;
    while ( $generator!->valid() )
    {
    print 'Loop gets current value' . PHP_EOL;
    print $generator!->current() . PHP_EOL;
    $generator!->next();
    }
    Generator created
    generate - start
    generate - yielding!!...
    Loop gets current value
    1
    generate - continued
    generate - yielding!!...
    Loop gets current value
    2
    generate - continued
    generate - yielding!!...
    Loop gets current value
    3
    generate - continued
    generate - yielding!!...
    Loop gets current value
    4
    generate - continued
    generate - yielding!!...
    Loop gets current value
    5
    generate - continued
    generate - end
    @hollodotme
    

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16. JEMMA'LOC
    function generate()
    {
    print 'generate - start' . PHP_EOL;
    for ( $i = 1; $i !<= 5; $i!++ )
    {
    print 'generate - yielding!!...' . PHP_EOL;
    yield $i;
    print 'generate - continued' . PHP_EOL;
    }
    print 'generate - end' . PHP_EOL;
    }
    $generator = generate();
    print 'Generator created' . PHP_EOL;
    while ( $generator!->valid() )
    {
    print 'Loop gets current value' . PHP_EOL;
    print $generator!->current() . PHP_EOL;
    $generator!->next();
    }
    Generator created
    generate - start
    generate - yielding!!...
    Loop gets current value
    1
    generate - continued
    generate - yielding!!...
    Loop gets current value
    2
    generate - continued
    generate - yielding!!...
    Loop gets current value
    3
    generate - continued
    generate - yielding!!...
    Loop gets current value
    4
    generate - continued
    generate - yielding!!...
    Loop gets current value
    5
    generate - continued
    generate - end
    @hollodotme
    

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17. JEMMA'LOC
    function generate()
    {
    print 'generate - start' . PHP_EOL;
    for ( $i = 1; $i !<= 5; $i!++ )
    {
    print 'generate - yielding!!...' . PHP_EOL;
    yield $i;
    print 'generate - continued' . PHP_EOL;
    }
    print 'generate - end' . PHP_EOL;
    }
    $generator = generate();
    print 'Generator created' . PHP_EOL;
    while ( $generator!->valid() )
    {
    print 'Loop gets current value' . PHP_EOL;
    print $generator!->current() . PHP_EOL;
    $generator!->next();
    }
    Generator created
    generate - start
    generate - yielding!!...
    Loop gets current value
    1
    generate - continued
    generate - yielding!!...
    Loop gets current value
    2
    generate - continued
    generate - yielding!!...
    Loop gets current value
    3
    generate - continued
    generate - yielding!!...
    Loop gets current value
    4
    generate - continued
    generate - yielding!!...
    Loop gets current value
    5
    generate - continued
    generate - end
    @hollodotme
    

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18. JEMMA'LOC
    function generate()
    {
    print 'generate - start' . PHP_EOL;
    for ( $i = 1; $i !<= 5; $i!++ )
    {
    print 'generate - yielding!!...' . PHP_EOL;
    yield $i;
    print 'generate - continued' . PHP_EOL;
    }
    print 'generate - end' . PHP_EOL;
    }
    $generator = generate();
    print 'Generator created' . PHP_EOL;
    while ( $generator!->valid() )
    {
    print 'Loop gets current value' . PHP_EOL;
    print $generator!->current() . PHP_EOL;
    $generator!->next();
    }
    Generator created
    generate - start
    generate - yielding!!...
    Loop gets current value
    1
    generate - continued
    generate - yielding!!...
    Loop gets current value
    2
    generate - continued
    generate - yielding!!...
    Loop gets current value
    3
    generate - continued
    generate - yielding!!...
    Loop gets current value
    4
    generate - continued
    generate - yielding!!...
    Loop gets current value
    5
    generate - continued
    generate - end
    @hollodotme
    

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19. JEMMA'LOC
    function generate()
    {
    print 'generate - start' . PHP_EOL;
    for ( $i = 1; $i !<= 5; $i!++ )
    {
    print 'generate - yielding!!...' . PHP_EOL;
    yield $i;
    print 'generate - continued' . PHP_EOL;
    }
    print 'generate - end' . PHP_EOL;
    }
    $generator = generate();
    print 'Generator created' . PHP_EOL;
    while ( $generator!->valid() )
    {
    print 'Loop gets current value' . PHP_EOL;
    print $generator!->current() . PHP_EOL;
    $generator!->next();
    }
    Generator created
    generate - start
    generate - yielding!!...
    Loop gets current value
    1
    generate - continued
    generate - yielding!!...
    Loop gets current value
    2
    generate - continued
    generate - yielding!!...
    Loop gets current value
    3
    generate - continued
    generate - yielding!!...
    Loop gets current value
    4
    generate - continued
    generate - yielding!!...
    Loop gets current value
    5
    generate - continued
    generate - end
    @hollodotme
    

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20. AND…
    GENERATORS
    IMPLEMENT
    IT…
    @hollodotme
    

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21. AND…
    GENERATORS
    IMPLEMENT
    IT…
    ITE…
    @hollodotme
    

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22. AND…
    GENERATORS
    IMPLEMENT
    IT…
    ITE…
    ITERATOR!
    @hollodotme
    

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23. FITZ'PLANATION
    The primary advantage of generators
    is their simplicity. Much less
    boilerplate code has to be written
    compared to implementing an
    Iterator class, and the code is
    generally much more readable.
    php.net
    @hollodotme
    

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24. JEMMA'LOC
    $generator = xrange( 1, 100 );
    while ( $generator!->valid() )
    {
    echo $generator!->current() . '.';
    $generator!->next();
    }
    # The Iterator interface
    interface Iterator extends Traversable
    {
    public function current();
    public function next();
    public function key();
    public function valid();
    public function rewind();
    }
    @hollodotme
    

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25. OH…
    GOOD TO SEE
    SOMETHING
    FAMILIAR

    -

    SOMETHING I
    CAN COUNT ON!
    @hollodotme
    

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26. BUT YOU CAN’T,
    PHIL.
    @hollodotme
    

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27. IT’S NOT
    COUNTABLE!
    @hollodotme
    

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28. JEMMA'LOC
    $generator = xrange( 1, 100 );
    count($generator);
    —
    PHP Warning: count(): Parameter must be an array or an object
    that implements Countable
    @hollodotme
    

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29. FITZ'PLANATION
    A generator is a highly dynamic
    construct with an iterator API, but an
    unknown number of elements.
    It’s not a data structure like an array
    or a common object.
    hw
    @hollodotme
    

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30. JEMMA'LOC
    $generator = xrange( 1, 100 );
    $count = count( iterator_to_array( $generator ) );
    echo 'Count: ', $count, PHP_EOL;
    $count = 0;
    foreach ( $generator as $item )
    {
    $count!++;
    }
    echo 'Count: ', $count, PHP_EOL;
    @hollodotme
    

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31. JEMMA'LOC
    $generator = xrange( 1, 100 );
    $count = count( iterator_to_array( $generator ) );
    echo 'Count: ', $count, PHP_EOL;
    $count = 0;
    foreach ( $generator as $item )
    {
    $count!++;
    }
    echo 'Count: ', $count, PHP_EOL;
    —
    Count: 100
    Exception: Cannot traverse an already closed generator
    @hollodotme
    

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32. JEMMA'LOC
    $generator = xrange( 1, 100 );
    $count = count( iterator_to_array( $generator ) );
    echo 'Count: ', $count, PHP_EOL;
    $generator!->rewind();
    $count = 0;
    foreach ( $generator as $item )
    {
    $count!++;
    }
    echo 'Count: ', $count, PHP_EOL;
    @hollodotme
    

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33. JEMMA'LOC
    $generator = xrange( 1, 100 );
    $count = count( iterator_to_array( $generator ) );
    echo 'Count: ', $count, PHP_EOL;
    $generator!->rewind();
    $count = 0;
    foreach ( $generator as $item )
    {
    $count!++;
    }
    echo 'Count: ', $count, PHP_EOL;
    —
    Count: 100
    Exception: Cannot rewind a generator that was already run
    @hollodotme
    

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34. What the
    F***?!
    @hollodotme
    

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35. FITZ'PLANATION
    […] generators are forward-only
    iterators, and cannot be rewound
    once iteration has started. This also
    means that the same generator can't
    be iterated over multiple times: the
    generator will need to be rebuilt by
    calling the generator function again.
    php.net
    @hollodotme
    

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36. YOU KNOW,
    IT'S LIKE ...
    @hollodotme
    

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37. GONE.
    @hollodotme
    

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38. JEMMA'LOC
    $generator = xrange( 1, 100 );
    $count = count( iterator_to_array( $generator ) );
    echo 'Count: ', $count, PHP_EOL;
    # Create new generator
    $generator = xrange( 1, 100 );
    $count = 0;
    foreach ( $generator as $item )
    {
    $count!++;
    }
    echo 'Count: ', $count, PHP_EOL;
    —
    Count: 100
    Count: 100
    @hollodotme
    

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39. YIELD FROM …
    @hollodotme
    

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40. JEMMA'LOC
    function countToTen() : Generator {
    yield 1;
    for ( $i = 2; $i < 4; $i!++ ) {
    yield $i;
    }
    yield from [4, 5];
    yield from json_decode( '[6,7]' );
    yield from eightNine();
    yield 10;
    }
    function eightNine() : Generator {
    yield '8 ' !=> 9;
    }
    foreach ( countToTen() as $key !=> $number )
    print (is_string($key) ? $key : '') . $number . ' ';
    @hollodotme
    

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41. JEMMA'LOC
    function countToTen() : Generator {
    yield 1;
    for ( $i = 2; $i < 4; $i!++ ) {
    yield $i;
    }
    yield from [4, 5];
    yield from json_decode( '[6,7]' );
    yield from eightNine();
    yield 10;
    }
    function eightNine() : Generator {
    yield '8 ' !=> 9;
    }
    foreach ( countToTen() as $key !=> $number )
    print (is_string($key) ? $key : '') . $number . ' ';
    —
    1 2 3 4 5 6 7 8 9 10
    @hollodotme
    

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42. YIELD ASS=>OC
    @hollodotme
    

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43. JEMMA'LOC
    function gen()
    {
    $key = new stdClass();
    $key!->property = 'I am an object!';
    yield $key !=> '& I am a simple string!';
    }
    $generator = gen();
    echo $generator!->key()!->property, ' ', $generator!->current();
    @hollodotme
    

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44. JEMMA'LOC
    function gen()
    {
    $key = new stdClass();
    $key!->property = 'I am an object!';
    yield $key !=> '& I am a simple string!';
    }
    $generator = gen();
    echo $generator!->key()!->property, ' ', $generator!->current();
    —
    I am an object! & I am a simple string!
    @hollodotme
    

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45. JEMMA'LOC
    $key = new stdClass();
    $key!->property = 'I am an object!';
    $array = [$key !=> '& I am a simple string.'];
    echo key($array)!->property, ' ', current($array);
    @hollodotme
    

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46. JEMMA'LOC
    $key = new stdClass();
    $key!->property = 'I am an object!';
    $array = [$key !=> '& I am a simple string.'];
    echo key($array)!->property, ' ', current($array);
    —
    PHP Warning: Illegal offset type in line 4
    PHP Notice: Trying to get property 'property'
    of non-object in line 6
    NO OUTPUT!!!
    @hollodotme
    

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47. YIELD & RETURN
    @hollodotme
    

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48. JEMMA'LOC
    function genFibonacci( int $loops ) : Generator {
    $fibonacci = '';
    for ( $i = 0; $i < $loops; $i!++ ) {
    yield $i;
    $fibonacci !.= round(
    pow( (sqrt( 5 ) + 1) / 2, $i ) / sqrt( 5 )
    );
    }
    return $fibonacci;
    }
    $generator = genFibonacci( 10 );
    while ( $generator!->valid() ) {
    print $generator!->current();
    $generator!->next();
    }
    print PHP_EOL . 'Fibonacci: ' . $generator!->getReturn() . PHP_EOL;
    @hollodotme
    

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49. JEMMA'LOC
    function genFibonacci( int $loops ) : Generator {
    $fibonacci = '';
    for ( $i = 0; $i < $loops; $i!++ ) {
    yield $i;
    $fibonacci !.= round(
    pow( (sqrt( 5 ) + 1) / 2, $i ) / sqrt( 5 )
    );
    }
    return $fibonacci;
    }
    $generator = genFibonacci( 10 );
    while ( $generator!->valid() ) {
    print $generator!->current();
    $generator!->next();
    }
    print PHP_EOL . 'Fibonacci: ' . $generator!->getReturn() . PHP_EOL;
    @hollodotme
    

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50. JEMMA'LOC
    function genFibonacci( int $loops ) : Generator {
    $fibonacci = '';
    for ( $i = 0; $i < $loops; $i!++ ) {
    yield $i;
    $fibonacci !.= round(
    pow( (sqrt( 5 ) + 1) / 2, $i ) / sqrt( 5 )
    );
    }
    return $fibonacci;
    }
    $generator = genFibonacci( 10 );
    while ( $generator!->valid() ) {
    print $generator!->current();
    $generator!->next();
    }
    print PHP_EOL . 'Fibonacci: ' . $generator!->getReturn() . PHP_EOL;
    —
    0123456789
    Fibonacci: 0112358132134
    @hollodotme
    

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51. PSA:
    PHP >=7.0
    ONLY
    @hollodotme
    

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52. UPGRADE
    YOUR
    STUFF!
    @hollodotme
    

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53. SERIOUSLY
    @hollodotme
    

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54. RECAP
    • A yield in a function or
    method always creates
    a Generator object
    • yield can provide a
    value to the caller of
    the Generator and
    pauses execution of its
    local context
    • Generator implements
    the Iterator interface,
    but can be consumed
    only once and
    forward-only
    • Generator can return
    a value when its
    execution stops
    @hollodotme
    

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55. WELL, NICE!
    @hollodotme
    

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56. ANY MORE FEATURES?
    @hollodotme
    

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57. CHAPTER 2: COROUTINES
    @hollodotme
    

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58. FITZ'PLANATION
    Coroutines and concurrency are
    largely orthogonal. Coroutines are a
    general control structure whereby
    flow control is cooperatively passed
    between two different routines
    without returning.
    stackoverflow.com
    @hollodotme
    

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59. JEMMA'LOC
    function queue( PDO $pdo ) : Generator
    {
    $pdo!->query( 'CREATE TABLE queue (message TEXT)' );
    $statement = $pdo!->prepare(
    'INSERT INTO queue (message) VALUES (:message)'
    );
    while ( true )
    {
    $message = yield;
    $statement!->execute( ['message' !=> $message] );
    print 'Sent: ' . $message . PHP_EOL;
    }
    }
    $queue = queue( new PDO( 'sqlite!::memory:' ) );
    $queue!->send( 'Foo' );
    $queue!->send( 'Bar' );
    @hollodotme
    

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60. JEMMA'LOC
    function queue( PDO $pdo ) : Generator
    {
    $pdo!->query( 'CREATE TABLE queue (message TEXT)' );
    $statement = $pdo!->prepare(
    'INSERT INTO queue (message) VALUES (:message)'
    );
    while ( true )
    {
    $message = yield;
    $statement!->execute( ['message' !=> $message] );
    print 'Sent: ' . $message . PHP_EOL;
    }
    }
    $queue = queue( new PDO( 'sqlite!::memory:' ) );
    $queue!->send( 'Foo' );
    $queue!->send( 'Bar' );
    @hollodotme
    

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61. JEMMA'LOC
    function queue( PDO $pdo ) : Generator
    {
    $pdo!->query( 'CREATE TABLE queue (message TEXT)' );
    $statement = $pdo!->prepare(
    'INSERT INTO queue (message) VALUES (:message)'
    );
    while ( true )
    {
    $message = yield;
    $statement!->execute( ['message' !=> $message] );
    print 'Sent: ' . $message . PHP_EOL;
    }
    }
    $queue = queue( new PDO( 'sqlite!::memory:' ) );
    $queue!->send( 'Foo' );
    $queue!->send( 'Bar' );
    —
    Sent: Foo
    Sent: Bar
    @hollodotme
    

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62. JEMMA'LOC
    function queue( PDO $pdo ) : Generator
    {
    $pdo!->query( 'CREATE TABLE queue (message TEXT)' );
    $statement = $pdo!->prepare(
    'INSERT INTO queue (message) VALUES (:message)'
    );
    $counter = 0;
    while ( true )
    {
    $message = yield $counter!++;
    $statement!->execute( ['message' !=> $message] );
    print 'Sent: ' . $message;
    }
    }
    $queue = queue( new PDO( 'sqlite!::memory:' ) );
    $queue!->send( 'Foo' );
    print ' (' . $queue!->current() . ')' . PHP_EOL;
    $queue!->send( 'Bar' );
    print ' (' . $queue!->current() . ')' . PHP_EOL;
    @hollodotme
    

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63. JEMMA'LOC
    function queue( PDO $pdo ) : Generator
    {
    $pdo!->query( 'CREATE TABLE queue (message TEXT)' );
    $statement = $pdo!->prepare(
    'INSERT INTO queue (message) VALUES (:message)'
    );
    $counter = 0;
    while ( true )
    {
    $message = yield $counter!++;
    $statement!->execute( ['message' !=> $message] );
    print 'Sent: ' . $message;
    }
    }
    $queue = queue( new PDO( 'sqlite!::memory:' ) );
    $queue!->send( 'Foo' );
    print ' (' . $queue!->current() . ')' . PHP_EOL;
    $queue!->send( 'Bar' );
    print ' (' . $queue!->current() . ')' . PHP_EOL;
    @hollodotme
    

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64. JEMMA'LOC
    Sent: Foo (1)
    Sent: Bar (2)
    @hollodotme
    

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65. @hollodotme
    

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66. WHAT
    HAPPENED
    TO ZERO?
    @hollodotme
    

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67. JEMMA'LOC
    Sent: Foo (1)
    Sent: Bar (2)
    —
    $queue = queue( new PDO( 'sqlite!::memory:' ) );
    print 'Hidden first value: ' . $queue!->current() . PHP_EOL;
    $queue!->send( 'Foo' );
    print ' (' . $queue!->current() . ')' . PHP_EOL;
    $queue!->send( 'Bar' );
    print ' (' . $queue!->current() . ')' . PHP_EOL;
    @hollodotme
    

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68. JEMMA'LOC
    Sent: Foo (1)
    Sent: Bar (2)
    —
    $queue = queue( new PDO( 'sqlite!::memory:' ) );
    print 'Hidden first value: ' . $queue!->current() . PHP_EOL;
    $queue!->send( 'Foo' );
    print ' (' . $queue!->current() . ')' . PHP_EOL;
    $queue!->send( 'Bar' );
    print ' (' . $queue!->current() . ')' . PHP_EOL;
    —
    Hidden first value: 0
    Sent: Foo (1)
    Sent: Bar (2)
    @hollodotme
    

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69. JEMMA'LOC
    function gen()
    {
    yield 'foo';
    yield 'bar';
    }
    $gen = gen();
    print $gen!->send( '' ) . PHP_EOL;
    @hollodotme
    

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70. JEMMA'LOC
    function gen()
    {
    yield 'foo';
    yield 'bar';
    }
    $gen = gen();
    print $gen!->send( '' ) . PHP_EOL;
    —
    bar
    @hollodotme
    

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71. JEMMA'LOC
    function gen()
    {
    yield 'foo';
    yield 'bar';
    }
    $gen = gen();
    $gen!->rewind();
    print $gen!->send( '' ) . PHP_EOL;
    @hollodotme
    

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72. JEMMA'LOC
    function gen()
    {
    yield 'foo';
    yield 'bar';
    }
    $gen = gen();
    $gen!->rewind();
    print $gen!->send( '' ) . PHP_EOL;
    —
    bar
    @hollodotme
    

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73. JEMMA'LOC
    function gen()
    {
    yield 'foo';
    yield 'bar';
    }
    $gen = gen();
    $gen!->rewind(); # can be omitted
    print $gen!->current() . PHP_EOL;
    print $gen!->send( '' ) . PHP_EOL;
    @hollodotme
    

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74. JEMMA'LOC
    function gen()
    {
    yield 'foo';
    yield 'bar';
    }
    $gen = gen();
    $gen!->rewind(); # can be omitted
    print $gen!->current() . PHP_EOL;
    print $gen!->send( '' ) . PHP_EOL;
    —
    foo
    bar
    @hollodotme
    

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75. JEMMA'LOC
    function gen()
    {
    yield 'foo';
    yield 'bar';
    }
    $gen = gen();
    $gen!->rewind(); # can be omitted
    print $gen!->current() . PHP_EOL;
    print $gen!->send( '' ) . PHP_EOL;
    —
    foo
    bar
    COPY THAT.
    @hollodotme
    

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76. JEMMA'LOC
    interface Traversable
    {
    # Marker interface, cannot be implemented directly
    }
    interface Iterator extends Traversable
    {
    public function rewind() : void;
    public function key();
    public function current();
    public function next() : void;
    public function valid() : bool;
    }
    interface Generator extends Iterator
    {
    public function getReturn();
    public function send( $value ) : void;
    public function throw( Throwable $throwable ) : void;
    public function !__wakeup() : void;
    }
    @hollodotme
    

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77. JEMMA'LOC
    interface Traversable
    {
    # Marker interface, cannot be implemented directly
    }
    interface Iterator extends Traversable
    {
    public function rewind() : void;
    public function key();
    public function current();
    public function next() : void;
    public function valid() : bool;
    }
    interface Generator extends Iterator
    {
    public function getReturn();
    public function send( $value ) : void;
    public function throw( Throwable $throwable ) : void;
    public function !__wakeup() : void;
    }
    @hollodotme
    

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78. JEMMA'LOC
    function gen() {
    yield 1;
    }
    $generator = gen();
    $generator!->!__wakeup();
    @hollodotme
    

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79. JEMMA'LOC
    function gen() {
    yield 1;
    }
    $generator = gen();
    $generator!->!__wakeup();
    —
    PHP Fatal error: Uncaught Exception:
    Unserialization of 'Generator' is not allowed
    @hollodotme
    

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80. JEMMA'LOC
    function gen() {
    yield 1;
    }
    $generator = gen();
    $generator!->!__wakeup();
    —
    PHP Fatal error: Uncaught Exception:
    Unserialization of 'Generator' is not allowed
    @hollodotme
    DON’T!
    

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81. JEMMA'LOC
    function gen() : Generator {
    while ( true ) {
    try {
    echo "\n", yield;
    }
    catch ( LogicException $e ) {
    echo 'Exception: ' . $e!->getMessage();
    }
    }
    }
    $generator = gen();
    $generator!->send( 'Hello ' );
    $generator!->throw( new LogicException( 'Raised inside the generator.' ) );
    $generator!->send( 'World' );
    $generator!->send( 'of tomorrow' );
    @hollodotme
    

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82. JEMMA'LOC
    function gen() : Generator {
    while ( true ) {
    try {
    echo "\n", yield;
    }
    catch ( LogicException $e ) {
    echo 'Exception: ' . $e!->getMessage();
    }
    }
    }
    $generator = gen();
    $generator!->send( 'Hello ' );
    $generator!->throw( new LogicException( 'Raised inside the generator.' ) );
    $generator!->send( 'World' );
    $generator!->send( 'of tomorrow' );
    —
    Hello
    Exception: Raised inside the generator.
    World
    of tomorrow
    @hollodotme
    

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83. JEMMA'LOC
    function gen() : Generator {
    while ( true ) {
    try {
    echo "\n", yield;
    }
    catch ( RuntimeException $e ) {
    echo $e!->getMessage();
    break;
    }
    }
    }
    $generator = gen();
    $generator!->send( 'Hello ' );
    $generator!->send( 'World' );
    $generator!->throw( new RuntimeException( 'Stopping coroutine.' ) );
    $generator!->send( 'of tomorrow' );
    @hollodotme
    

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84. JEMMA'LOC
    function gen() : Generator {
    while ( true ) {
    try {
    echo "\n", yield;
    }
    catch ( RuntimeException $e ) {
    echo $e!->getMessage();
    break;
    }
    }
    }
    $generator = gen();
    $generator!->send( 'Hello ' );
    $generator!->send( 'World' );
    $generator!->throw( new RuntimeException( 'Stopping coroutine.' ) );
    $generator!->send( 'of tomorrow' );
    —
    Hello
    World
    Stopping coroutine.
    @hollodotme
    

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85. RECAP
    • Through yield a value
    can be sent to the
    generator to continue
    its execution
    • Through yield an
    exception can be
    raised in a generator
    • yield provides first
    and receives second
    • You can start a
    generator by calling
    any of its Iterator
    methods
    • rewind ignores the
    first provided value
    @hollodotme
    

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86. TAHITI?
    @hollodotme
    

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87. CHAPTER 3:
    COOPERATIVE MULTITASKING
    @hollodotme
    

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88. FITZ'PLANATION
    Co-operative multitasking, also
    k n o w n a s n o n - p r e e m p t i v e
    multitasking, is a style of computer
    multitasking in which the operating
    system never initiates a context
    switch from a running process to
    another process. Instead, processes
    voluntarily yield control periodically
    or when idle or logically blocked in
    order to enable multiple applications
    to be run concurrently.
    wikipedia.org
    @hollodotme
    

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89. THE FRAMEWORK
    (Task Scheduler)
    AGENTS OF YIELD
    (Coroutines)
    SystemCalls
    (Hooks into the
    Coroutines)
    Ammunition
    (Limits)
    

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90. final class TheFramework {
    private $backup, $agents = [];
    public function !__construct() {
    $this!->backup = new SplQueue();
    }
    public function newAgent( string $agentName, Generator $coroutine ) : void {
    $agent = new Agent( $agentName, $coroutine );
    $this!->agents[ $agentName ] = $agent;
    $this!->sendToBattle( $agent );
    }
    public function sendToBattle( Agent $task ) : void {
    $this!->backup!->enqueue( $task );
    }
    public function fight() : void {
    while ( !$this!->backup!->isEmpty() ) {
    $agent = $this!->backup!->dequeue();
    $retval = $agent!->fight();
    if ( $retval instanceof SystemCall ) {
    $retval( $agent, $this ); continue;
    }
    if ( $agent!->outOfMunitions() ) {
    unset( $this!->agents[ $agent!->getName() ] ); continue;
    }
    $this!->sendToBattle( $agent );
    }
    }
    }
    @hollodotme
    

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91. final class Agent {
    private $name, $coroutine, $sendValue;
    private $beforeFirstYield = true;
    public function !__construct( string $name, Generator $coroutine ) {
    $this!->name = $name;
    $this!->coroutine = $coroutine;
    }
    public function getName() : string {
    return $this!->name;
    }
    public function setSendValue( $sendValue ) : void {
    $this!->sendValue = $sendValue;
    }
    public function fight() {
    if ( $this!->beforeFirstYield )
    {
    $this!->beforeFirstYield = false;
    return $this!->coroutine!->current();
    }
    $retval = $this!->coroutine!->send( $this!->sendValue );
    $this!->sendValue = null;
    return $retval;
    }
    public function outOfMunitions() : bool {
    return !$this!->coroutine!->valid();
    }
    }
    @hollodotme
    

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92. final class SystemCall
    {
    private $callback;
    public function !__construct( callable $callback )
    {
    $this!->callback = $callback;
    }
    public function !__invoke( Agent $agent, TheFramework $battleGround )
    {
    $callback = $this!->callback;
    return $callback( $agent, $battleGround );
    }
    }
    @hollodotme
    

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93. # System call corouting
    function getAgentName()
    {
    return new SystemCall(
    function ( Agent $agent, TheFramework $battleGround )
    {
    $agent!->setSendValue( $agent!->getName() );
    $battleGround!->sendToBattle( $agent );
    }
    );
    }
    # Agent of yield
    function agent( $shots )
    {
    $agentName = yield getAgentName();
    for ( $i = 1; $i !<= $shots; !++$i )
    {
    print "Agent {$agentName} fires shot {$i}." . PHP_EOL;
    yield;
    }
    print "Agent {$agentName} is out of munitions." . PHP_EOL;
    }
    @hollodotme
    

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94. THE FINAL BATTLE
    @hollodotme
    

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95. $battleGround = new TheFramework();
    $battleGround!->newAgent( 'Coulson', agent( 12 ) );
    $battleGround!->newAgent( 'Mack', agent( 5 ) );
    $battleGround!->fight();
    print 'Game Over' . PHP_EOL;
    @hollodotme
    

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96. $battleGround = new TheFramework();
    $battleGround!->newAgent( 'Coulson', agent( 12 ) );
    $battleGround!->newAgent( 'Mack', agent( 5 ) );
    $battleGround!->fight();
    print 'Game Over' . PHP_EOL;
    @hollodotme
    Agent Coulson fires shot 1.
    Agent Mack fires shot 1.
    Agent Coulson fires shot 2.
    Agent Mack fires shot 2.
    Agent Coulson fires shot 3.
    Agent Mack fires shot 3.
    Agent Coulson fires shot 4.
    Agent Mack fires shot 4.
    Agent Coulson fires shot 5.
    Agent Mack fires shot 5.
    Agent Coulson fires shot 6.
    Agent Mack is out of munitions.
    Agent Coulson fires shot 7.
    Agent Coulson fires shot 8.
    Agent Coulson fires shot 9.
    Agent Coulson fires shot 10.
    Agent Coulson fires shot 11.
    Agent Coulson fires shot 12.
    Agent Coulson is out of munitions.
    Game Over
    

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97. FITZ'PLANATION
    When I first heard about all this 

    I f o u n d t h i s co n ce p t to t a l l y
    awesome and that’s what motivated
    me to implement it in PHP. At the
    same time I find coroutines really
    scary. There is a thin line between
    awesome code and a total mess and I
    think coroutines sit exactly on that
    line. It’s hard for me to say whether
    writing async code in the way
    outlined is really beneficial.
    Nikita Popov
    @hollodotme
    THE REAL
    FITZ
    

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98. THE END
    @hollodotme
    

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99. github.com/hollodotme
    @hollodotme
    THANK YOU!
    holger woltersdorf
    

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100. RESOURCES
     • PHP Generator Documentation: https://secure.php.net/manual/en/language.generators.php
     • VLD extension for PHP: https://derickrethans.nl/projects.html#vld
     • «Introduction to generators» by Niklas Keller: https://blog.kelunik.com/2017/09/14/an-introduction-to-generators-in-php.html
     • PHP 5.5 migration guide: https://secure.php.net/manual/en/migration55.new-features.php
     • «Cooperative multitasking using coroutines (in PHP!)» by Nikita Popov

     https://nikic.github.io/2012/12/22/Cooperative-multitasking-using-coroutines-in-PHP.html
     • Repo with executable code of this talk:

     https://github.com/hollodotme/talk-preps/tree/master/agents-of-yield
     • Definition of Coroutines on Stack Overflow:

     https://stackoverflow.com/questions/553704/what-is-a-coroutine
     • Definition of Cooperative Multitasking on wikipedia.org: 

     https://en.wikipedia.org/wiki/Cooperative_multitasking
     @hollodotme
     

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