Maybes and Functors

A brief introductions to functors and the Maybe type, briefly touching on a tiny bit of Monadic properties

Transcript

1. Maybes and Functors
   Matthew Machuga
   
   Laracon 2017 Science Fair
   

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2. You know what’s
   awful?
   

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3. null
   

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4. 1 billion dollar
   mistake
   

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5. We’ll get to that in a
   bit
   

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6. You know what’s
   awesome?
   

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7. Collection Operations
   

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8. Contrived Ex 1:
   $users = collect([
   new User([“id” => 1, “name” => “Rommie Bot”]),
   new User([“id” => 2, “name” => “Liona Bot”])
   ]);
   $getInfo = function($user) { return $user->name; };
   $toSlug = function($name) {
   return strtolower(str_replace(“ “, “-“, $name));
   };
   $slugifiedNames = $collection->map($getInfo)->map($toSlug);
   $slugifiedNames // => [“rommie-bot”, “liona-bot”]
   

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9. Interesting Properties
   Hidden Here
   

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10. Or: Functor Laws
    

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11. Interesting Properties /
    Functor Laws
    // map won’t run if the collection is empty
    $c->map(function($x) { return $x; }) == $c
    

    $c->map($getInfo)->map($toSlug) ==
    $c->map(function($user) {
    return $toSlug($getInfo($user));
    });
    // map(f).map(g) == map(g o f)
    

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12. Collection Operations
    are Pipelines
    

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13. But what about
    single values?
    

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14. You could do this..
    $value = collect([$something])
    ->map($doSomething)
    ->map($doSomethingElse)
    ->first();
    

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15. Let’s create a type
    

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16. Let's create a type
    • Follow the two rules
    
    • c.map(g).map(f) == c.map(g o f)
    
    • c.map(id) == c
    
    • Define a way to get to our raw value
    

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17. We’ll call it Box
    

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18. Box
    class Box {
    protected $value;
    public static function of($value) {
    return new static($value);
    }
    public function __construct($value) {
    $this->value = $value;
    }
    public function map(callable $f) : Box {
    return new static($f($this->value));
    }
    public function fold(callable $f = null) {
    return $f ? $f($this->value) : $this->value;
    }
    }
    

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19. Collection Example
    $users = collect([
    new User([“id” => 1, “name” => “Rommie Bot”]),
    new User([“id” => 2, “name” => “Liona Bot”])
    ]);
    $getInfo = function($user) { return $user->name; };
    $toSlug = function($name) {
    return strtolower(str_replace(“ “, “-“, $name));
    };
    $slugifiedNames = $collection->map($getInfo)->map($toSlug);
    $slugifiedNames // => [“rommie-bot”, “liona-bot”]
    

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20. Box Example
    $box = Box::of(new User([“id” => 1, “name” => “Liona
    Bot”]));
    $getInfo = function($user) { return $user->name; };
    $toSlug = function($name) {
    return strtolower(str_replace(“ “, “-“, $name));
    };
    $slugifiedNameInBox = $box->map($getInfo)->map($toSlug);
    $slugifiedNameInBox // => Box(“liona-bot”)
    $slugifiedNameInBox->fold() // => “liona-bot”
    

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21. Cool. What about
    null?
    

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22. Collections
    

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23. What happens?
    $value = collect([null])->map(function($v) {
    return $v->byebye();
    });
    

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28. You start getting this
    $value = collect([somethingThatMightBeNull()])
    ->map(function($v) {
    if ($v && method_exists($v, ‘byebye’)) {
    return $v->byebye();
    }
    });
    

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29. You can technically do this..
    $value = collect([somethingThatMightBeNull()])
    ->filter(function($el) {
    return !!$el;
    })
    ->map(function($v) {
    return $v->byebye();
    });
    

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30. Or possibly this..
    $value = collect([somethingThatMightBeNull()])
    ->filter(function($el) {
    return !!$el;
    })
    ->map(function($v) {
    return $v->byebye();
    });
    // map(f).map(g) == map(g o f)
    

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31. Okay how about box?
    $value = Box::of(null)->map(function($v) {
    return $v->byebye();
    });
    

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33. null
    

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34. Null
    $collection = collect([1, null]);
    $value = $collection->find(function($el) {
    return $el === 42;
    });
    $value // => null
    

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35. What does null mean?
    1. The element was not found
    
    2. The element was found and is null
    

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36. What does null mean?
    1. Presence
    
    2. Value
    

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37. PHP Conventions
    Didn’t find something?
    
    • Return false
    
    • Throw exception
    

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38. How to represent
    both?
    

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39. Maybe
    

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40. Maybe
    • Just(value) [Some(value)]
    • Nothing [None]
    

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41. Contrived Ex 1:
    $collection = collect([1, null]);
    $value = $collection->find(function($el) {
    return $el === 42;
    });
    $value // => null
    

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42. Contrived Ex 1 Reprise:
    $collection = collect([
    Maybe::just(1),
    Maybe::nothing()
    ]);
    $value = $collection->find(function($el) {
    return $el->fold() === 42;
    });
    $value // => Nothing
    

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43. Contrived Ex 1 Reprise:
    $collection = collect([
    Maybe::just(1),
    Maybe::nothing()
    ]);
    $value = $collection->find(function($el) {
    return $el->fold() === null;
    });
    $value // => Just(null)
    

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44. Contrived Ex 1 Reprise:
    $collection = collect([
    Maybe::just(1),
    Maybe::nothing()
    ]);
    $value = $collection->find(function($el) {
    return $el->fold() === 1;
    });
    $value // => Just(1)
    

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45. But is this better than
    null?
    

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46. One way...
    $maybeUser = Eloquent::maybeFind(9001);
    if ($maybeUser->isNothing()) {
    // Do nothing
    } else {
    $something = $maybeUser->fold();
    }
    

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47. Don’t do this
    Without pattern matching ¯\_(ツ)_/¯
    

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48. A better way...
    $maybeUser = Eloquent::maybeFind(9001);
    $something = $maybeUser->map(function($user) {
    return SomeObject::someTransformationLol($user);
    });
    // Later in the app
    function process(Maybe $maybeData) {
    return $maybeData->fold(function($value) {
    // If you need to do a side-effect dependent on if null
    // Could finally check here
    });
    }
    

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50. Nested Types
    $value = Box::of(1)->map(function($id) {
    return Eloquent::maybeFind(9001);
    }); // Box(Just(User)) | Box(Nothing)
    $value->map(function($user) {
    // $user is a Just(User)
    return $user->profile;
    }); // Box(Just(Maybe(User)))
    

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51. map
    public function map(callable $f) : Functor
    {
    return new static($f($this->value));
    }
    

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52. chain | flatMap
    public function chain(callable $f) : Monad
    {
    return $f($this->value);
    }
    

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53. chain | flatMap
    public function chain(callable $f) : Monad
    {
    return $f($this->value);
    }
    

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55. Nested Types
    $value = Box::of(1)->chain(function($id) {
    return Eloquent::maybeFind(9001);
    }); // Just(User) | Nothing
    $value->chain(function($user) {
    return $user->profile;
    }); // Maybe(Profile)
    

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56. You don’t like closures?
    class ConvertToTruth {
    public function convert($text) {
    return str_replace('the cloud',
    “someone else's computer", $text);
    }
    public function __invoke($text) {
    return $this->convert($text);
    }
    }
    

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57. You don’t like closures?
    collect(["My data lives in the cloud"])
    ->map(new ConvertToTruth)
    ->toArray()
    // [“My data lives in someone else’s machine”]
    // or
    Box::of(“My data lives in the cloud")
    ->map(new ConvertToTruth)
    ->fold() // My data lives in someone else’s machine
    

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58. Will this improve
    CRUD?
    

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59. Eloquent::findMaybe($id)
    

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60. Will this improve complex
    data transformations?
    

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61. Composable &
    Reusable functions
    

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62. Resources
    • Professor Frisby’s Mostly Adequate Guide to Functional Programming -
    Brian Lonsdorf (@drboolean)
    
    • https://drboolean.gitbooks.io/mostly-adequate-guide/content/
    
    • https://egghead.io/courses/professor-frisby-introduces-composable-
    functional-javascript
    
    • Elm
    
    • http://elm-lang.org
    
    • Maybe Haskell - Pat Brisbin:
    
    • https://gumroad.com/l/maybe-haskell/
    

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