It Feels Great to Iterate

Iterators are an awesome and important feature of PHP, and PHP comes with a lot of them built in too. Let’s talk about what they are, how they’re used, and how to make your own. Then we'll talk about generators, which is really cool PHP 5.5+ feature that takes iterators to the next level. And… we can’t talk about iterators without also discussing the Decorator design pattern. After all, most of the SPL iterator classes are decorators too. It’s never felt so great to iterate! May the foreach be with you!

Transcript

1. It Feels Great
   to Iterate
   By Jeremy Lindblom • @jeremeamia
   ( see also: @seaphp • @awsforphp • @phpbard )
   

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2. WARNING
   This is not a talk about agile
   development, but, yes,
   “iterate” is a somewhat
   overloaded term, so I can
   see how you might have
   been confused.
   

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3. This talk
   is about
   iterators
   

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4. Actually,
   we'll be
   covering
   a lot of
   OOP
   

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5. ————————
   ★ Iterators ★
   ————————
   ★ Decorators ★
   ————————
   ★ Generators ★
   ————————
   

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6. Q. Why is PHP
   so friggin'
   awesome?
   

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7. array()  
   

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8. []  
   ( Since PHP 5.4 )
   

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9. Is it a(n)…
   a.  List
   b.  Map
   c.  Vector
   d.  Queue
   e.  Stack
   []  
   

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10. Is it a(n)…
    a.  List
    b.  Map
    c.  Vector
    d.  Queue
    e.  Stack
    f.  All of the above
    []  
    

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11. array_column(…);  
    array_diff(…);  
    array_filter(…);  
    array_keys(…);  
    array_map(…);  
    array_merge(…);  
    array_push(…);  
    array_slice(…);  
    array_unshift(…);  
    

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12. $speaker  =  [  
       'name'          =>  'Jeremy  Lindblom',  
       'twitter'    =>  '@jeremeamia',  
       'employer'  =>  'AWS  (Amazon)',  
       'bio'            =>  [  
           'Works  on  AWS  SDK  for  PHP',  
           'Is  president  of  @SeaPHP',  
           'Is  PHP-­‐FIG  rep  for  Guzzle',  
           'Writes  PHPoetry  as  @phpbard',  
       ]  
    ];  
    

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13. foreach  ($data  as  $item)  {  
           echo  $item  .  "\n";  
    }  
    Iterating arrays
    

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14. foreach  ($data  as  $key  =>  $value)  {  
         echo  $key  .  ':'  .  $value  .  "\n";  
    }  
    Iterating arrays (w/ keys)
    

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15. $data  =  new  DirectoryIterator(__DIR__);  
    foreach  ($data  as  $file)  {  
           echo  $file-­‐>getFilename()  .  "\n";  
    }  
    Iterating objects?
    

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16. View Slide

17. $data  =  new  DirectoryIterator(__DIR__);  
    foreach  ($data  as  $file)  {  
           echo  $file-­‐>getFilename()  .  "\n";  
    }  
    Iterating objects?
    

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18. View Slide

19. AppendIterator
    ArrayIterator
    CachingIterator
    CallbackFilterIterator
    DirectoryIterator
    EmptyIterator
    FilesystemIterator
    FilterIterator
    GlobIterator
    InfiniteIterator
    Iterator
    IteratorAggregate
    IteratorIterator
    LimitIterator
    MultipleIterator
    NoRewindIterator
    OuterIterator
    ParentIterator
    RecursiveArrayIterator
    RecursiveDirectoryIterator
    RecursiveFilterIterator
    RecursiveIteratorIterator
    RecursiveTreeIterator
    RegexIterator
    SimpleXMLIterator
    SplFileObject
    SplQueue
    SplStack
    

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20. View Slide

21. •  Iterator
    •  Iterate / Traverse / "foreach over"
    •  Iterable / Traversable
    •  Iteration
    •  Emit / Yield
    Quick Terms
    

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22. if  ($object  instanceof  Traversable)  {  
           echo  "It  can  be  used  in  a  foreach.";  
    }  
    Traversable objects
    

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23. TIP #1
    You can tell if a value is
    foreach-able by
    checking if it is an
    array or an instance of
    Traversable.
    

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24. class  Foo  implements  Traversable  {  
           …  
    }  
    Implement Traversable?
    

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25. class  Foo  implements  Traversable  {  
           …  
    }  
    >  PHP  Fatal  error:  Class  Foo  must    
    >  implement  interface  Traversable  as    
    >  part  of  either  Iterator  or  
    >  IteratorAggregate.  
    Implement Traversable?
    NOPE!
    

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26. View Slide

27.  
    Traversable  
     
    Iterator  
     
    IteratorAggregate  
    

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28. interface  Iterator  {  
         function  current();  //  mixed|false  
         function  key();          //  scalar|null  
         function  next();        //  void  
         function  rewind();    //  void  
         function  valid();      //  bool  
    }  
    Iterator Interface
    

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29. foreach  ($iterator  as  $key  =>  $value)  {  
           //  Do  stuff!  
    }  
    

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30. foreach  ($iterator  as  $key  =>  $value)  {  
           //  Do  stuff!  
    }  
     
    //  THIS  IS  THE  SAME  AS:  
     
    $iterator-­‐>rewind();  
    while  ($iterator-­‐>valid())  {  
           $key  =  $iterator-­‐>key();  
           $value  =  $iterator-­‐>current();  
           //  Do  stuff!  
           $iterator-­‐>next();  
    }  
    

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31. TIP #2
    foreach automatically
    performs a rewind on
    the iterator.
    Other loops do not.
    

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32. TIP #3
    Most iterators must be
    rewound before they
    work properly.
    

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33. $numbers  =  new  HundoIterator();  
    foreach  ($numbers  as  $number)  {  
           echo  $number  .  "\n";  
    }  
     
    >  1  
    >  2  
    >  …  
    >  99  
    >  100  
    Example Implementation
    

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34. class  HundoIterator  implements  Iterator  {  
       private  $num  =  1;  
       function  current()  {  
             return  $this-­‐>valid()  ?  $this-­‐>num  :  false;  
       }  
       function  key()  {  
           return  $this-­‐>valid()  ?  $this-­‐>num  –  1  :  null;  
       }  
       function  next()  {$this-­‐>num++;}  
       function  rewind()  {$this-­‐>num  =  1;}  
       function  valid()  {return  $this-­‐>num  <=  100;}  
    }  
    Example Implementation
    

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35. TIP #4
    To avoid bugs, you
    should make your
    Iterator's methods
    “idempotent” per
    iteration.
    

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36. class  HundoIterator  implements  Iterator  {  
       private  $num  =  1;  
       function  current()  {  
             return  $this-­‐>valid()  ?  $this-­‐>num  :  false;  
       }  
       function  key()  {  
           return  $this-­‐>valid()  ?  $this-­‐>num  –  1  :  null;  
       }  
       function  next()  {$this-­‐>num++;}  
       function  rewind()  {$this-­‐>num  =  1;}  
       function  valid()  {return  $this-­‐>num  <=  100;}  
    }  
    Idempotency?
    

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37. $numbers  =  new  HundoIterator();  
    foreach  ($numbers  as  $number)  {  
           echo  $number  .  "\n";  
    }  
     
    >  1  
    >  2  
    >  …  
    >  99  
    >  100  
    Example Implementation
    

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38. COOL!
    

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39. WAIT!
    

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40. WHY?
    

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41. Because…
    OOP?
    

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42. No, give
    me real
    reasons.
    

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43. OK…
    Fine!
    

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44. $numbers  =  range(1,  100);  
    foreach  ($numbers  as  $number)  {  
           echo  $number  .  "\n";  
    }  
     
    >  1  
    >  2  
    >  …  
    >  99  
    >  100  
    Array Implementation
    

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45. $numbers  =  range(1,  100);  
    foreach  ($numbers  as  $number)  {  
           echo  $number  .  "\n";  
    }  
     
    >  1  
    >  2  
    >  …  
    >  99  
    >  100  
    Array Implementation
    Yeah!
    See…?
    

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46. $numbers  =  range(1,  100);  
    foreach  ($numbers  as  $number)  {  
           echo  $number  .  "\n";  
    }  
     
    >  1  
    >  2  
    >  …  
    >  99  
    >  100  
    Array Implementation
    Wait
    for it…
    

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47. View Slide

48. $numbers  =  range(1,  1000000000);  
    foreach  ($numbers  as  $number)  {  
           echo  $number  .  "\n";  
    }  
    When arrays don't work
    

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49. $numbers  =  range(1,  1000000000);  
    foreach  ($numbers  as  $number)  {  
           echo  $number  .  "\n";  
    }  
     
    >  PHP  Fatal  error:    Allowed  memory  
    >  size  of  536870912  bytes  exhausted  
    When arrays don't work
    Oh noes!
    

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50. $numbers  =  range(1,  1000000000);  
    foreach  ($numbers  as  $number)  {  
           echo  $number  .  "\n";  
    }  
     
    >  PHP  Fatal  error:    Allowed  memory  
    >  size  of  536870912  bytes  exhausted  
    When arrays don't work
    Inorite?
    

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51. class  RangeIterator  implements  Iterator  {  
       private  $value  =  1;  
       private  $limit;  
       function  __construct($limit)  {  
           $this-­‐>limit  =  $limit;  
       }  
       //  ...  
       function  valid()  {  
           return  ($this-­‐>value  <=  $this-­‐>limit);  
       }  
    }  
    Will iterators work?
    

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52. $numbers  =  new  RangeIterator(1000000000);  
    foreach  ($numbers  as  $number)  {  
           echo  $number  .  "\n";  
    }  
     
    >  1  
    >  2  
    >  …  
    >  999999999  
    >  1000000000  
    Will iterators work?
    

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53. $numbers  =  new  RangeIterator(1000000000);  
    foreach  ($numbers  as  $number)  {  
           echo  $number  .  "\n";  
    }  
     
    >  1  
    >  2  
    >  …  
    >  999999999  
    >  1000000000  
    Will iterators work?
    Ah,
    I see.
    

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54. $numbers  =  new  RangeIterator(1000000000);  
    foreach  ($numbers  as  $number)  {  
           echo  $number  .  "\n";  
    }  
     
    >  1  
    >  2  
    >  …  
    >  999999999  
    >  1000000000  
    Will iterators work?
    Yep.
    Told ya.
    

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55. Why Iterators?
    1.  Reduced memory ✓
    

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56. Why Iterators?
    1.  Reduced memory ✓
    2.  Encapsulation
    3.  Lazy evaluation
    4.  Composition
    

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57. Encapsulation[0]
     
     
         =  $note-­‐>getMessage()  ?>  
     
     
    

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58. Encapsulation[1]
    $sql  =  'SELECT  *  FROM  notes';  
    $stmt  =  $pdo-­‐>prepare($sql);  
    $stmt-­‐>execute();  
     
    $notes  =  $stmt-­‐>fetchAll();  
    

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59. Encapsulation[1]
    $sql  =  'SELECT  *  FROM  notes';  
    $stmt  =  $pdo-­‐>prepare($sql);  
    $stmt-­‐>execute();  
     
    $notes  =  $stmt-­‐>fetchAll();  
    •  Loads all results into memory.
    •  Hard to transform data.
    

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60. Encapsulation[2]
    $sql  =  'SELECT  *  FROM  notes';  
    $stmt  =  $pdo-­‐>prepare($sql);  
    $stmt-­‐>execute();  
     
    $notes  =  [];  
    while  ($row  =  $stmt-­‐>fetch())  {  
     $notes[]  =  new  Note($row);  
    }  
    

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61. Encapsulation[2]
    $sql  =  'SELECT  *  FROM  notes';  
    $stmt  =  $pdo-­‐>prepare($sql);  
    $stmt-­‐>execute();  
     
    $notes  =  [];  
    while  ($row  =  $stmt-­‐>fetch())  {  
     $notes[]  =  new  Note($row);  
    }  
    •  Loads all results into memory.
    •  Easy to transform data.
    

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62. View Slide

63. Encapsulation[3]
    $sql  =  'SELECT  *  FROM  notes';  
    $stmt  =  $pdo-­‐>prepare($sql);  
    $stmt-­‐>execute();  
     
    $notes  =  [];  
    foreach  ($stmt  as  $row)  {  
     $notes[]  =  new  Note($row);  
    }  
    •  Loads all results into memory.
    •  Easy to transform data.
    

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64. Encapsulation[4]
    class  NoteIterator  extends  IteratorIterator  {  
           public  function  current()  {  
                   return  new  Note(parent::current());  
           }  
    }  
    

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65. TIP #5
    Be lazy. Delay doing
    anything with data until
    you absolutely need to.
    Iterators help with this.
    

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66. Encapsulation[5]
    $sql  =  'SELECT  *  FROM  notes';  
    $stmt  =  $pdo-­‐>prepare($sql);  
    $stmt-­‐>execute();  
     
    $notes  =  new  NoteIterator($stmt);  
    •  Uses memory wisely.
    •  Easy to transform data.
    •  Encapsulates logic.
    •  Notes created lazily.  
    

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67. Why Iterators?
    1.  Reduced memory ✓
    2.  Encapsulation ✓
    3.  Lazy evaluation ✓
    4.  Composition
    

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68. Why Iterators?
    1.  Reduced memory ✓
    2.  Encapsulation ✓
    3.  Lazy evaluation ✓
    4.  Composition ✓
    

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69. Composition
    When one object
    contains another object,
    combining their
    behaviors.
    

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70. Composition
    $notes  =  new  NoteIterator($stmt);  
    PDOStatement  
    (Traversable)  
    We "wrapped" one
    iterator with another,
    composing their
    behaviors together.
    

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71. AppendIterator
    ArrayIterator
    CachingIterator
    CallbackFilterIterator
    DirectoryIterator
    EmptyIterator
    FilesystemIterator
    FilterIterator
    GlobIterator
    InfiniteIterator
    Iterator
    IteratorAggregate
    IteratorIterator
    LimitIterator
    MultipleIterator
    NoRewindIterator
    OuterIterator
    ParentIterator
    RecursiveArrayIterator
    RecursiveDirectoryIterator
    RecursiveFilterIterator
    RecursiveIteratorIterator
    RecursiveTreeIterator
    RegexIterator
    SimpleXMLIterator
    SplFileObject
    SplQueue
    SplStack
    

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72. And now a
    poem about
    Decorators,
    by the
    @phpbard
    

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73. View Slide

74.  
    Component  
    Component   ComponentDecorator  
     
    

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75. Foo  extends  Bar  
    

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76. $nums  =  new  RangeIterator(100);  
    foreach  ($nums  as  $num)  {  
           echo  $num  .  "\n";  
    }  
     
    >  1  
    >  2  
    >  …  
    >  99  
    >  100  
    Iterators
    

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77. interface  Iterator  {  
         function  current();  //  mixed|false  
         function  key();          //  scalar|null  
         function  next();        //  void  
         function  rewind();    //  void  
         function  valid();      //  bool  
    }  
    Iterators
    

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78. LimitIterator  
    $nums  =  new  RangeIterator(100);  
    $nums  =  new  LimitIterator($nums,  10,  5);  
    foreach  ($nums  as  $num)  {  
           echo  $num  .  "\n";  
    }  
     
    >  10  
    >  11  
    >  12  
    >  13  
    >  14  
    

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79. CallbackFilterIterator  
    $nums  =  new  RangeIterator(100);  
    $nums  =  new  CallbackFilterIterator($nums,  
           function  ($n)  {return  $n  %  5  ===  0;}  
    );  
    foreach  ($nums  as  $num)  {  
           echo  $num  .  "\n";  
    }  
    >  5  
    >  10  
    >  …  
    >  95  
    >  100  
    

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80. LimitIterator   CallbackFilterItr  
     
    Iterator  
    OurCoolIterator  
    

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81. LimitIterator   CallbackFilterItr  
     
    Iterator  
    OurCoolIterator  
    ?
    

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82. LimitIterator   CallbackFilterItr  
     
    Iterator  
    OurCoolIterator  
    x  
    

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83. Decorate!
    $nums  =  new  RangeIterator(100);  
     
    $nums  =  new  CallbackFilterIterator($nums,  
           function  ($n)  {return  $n  %  5  ===  0;}  
    );  
     
    $nums  =  new  LimitIterator($nums,  10,  5);  
    

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84. Decorate!
    foreach  ($nums  as  $num)  {  
           echo  $num  .  "\n";  
    }  
     
    >  10  
    >  15  
    >  20  
    >  25  
    >  30  
    

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85. LimitIterator  
    CallbackFilterIterator  
    RangeIterator  
     
    current()  
    current()  
    current()  
    

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86. Thank you!
    @phpbard
    phpbard.tumblr.com
    

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87. TIP #6
    Use composition of
    iterators instead of
    implementing new
    classes when possible.
    

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88. Generators
    $stmt-­‐>execute();  
     
    $getNotes  =  function  ($stmt)  {  
         foreach  ($stmt  as  $row)  {  
               yield  new  Note($row);  
         }  
    };  
     
    $notes  =  $getNotes($stmt);  
    

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89. TIP #7
    Read the Manual.
    

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90. It Feels Great
    to Iterate
    By Jeremy Lindblom • @jeremeamia
    ( see also: @seaphp • @awsforphp • @phpbard )
    

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91. Vocabulary
    •  Iterator
    •  Traversable
    •  Interface
    •  Encapsulation
    •  Composition
    •  Decorator
    •  Generator
    •  Adapter
    •  Idempotent
    •  SOLID
    •  Inheritance
    •  Proxy
    

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