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
2. You know what’s awful?
3. null
4. 1 billion dollar mistake
5. We’ll get to that in a bit
6. You know what’s awesome?
7. Collection Operations
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”]
9. Interesting Properties Hidden Here
10. Or: Functor Laws
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)
12. Collection Operations are Pipelines
13. But what about single values?
14. You could do this.. $value = collect([$something]) ->map($doSomething) ->map($doSomethingElse) ->first();
15. Let’s create a type
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
17. We’ll call it Box
18. Box <?php 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; } }
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”]
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”
21. Cool. What about null?
22. Collections
23. What happens? $value = collect([null])->map(function($v) { return $v->byebye(); });
28. You start getting this $value = collect([somethingThatMightBeNull()]) ->map(function($v) { if ($v && method_exists($v, ‘byebye’)) { return $v->byebye(); } });
29. You can technically do this.. $value = collect([somethingThatMightBeNull()]) ->filter(function($el) { return !!$el; }) ->map(function($v) { return $v->byebye(); });
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)
31. Okay how about box? $value = Box::of(null)->map(function($v) { return $v->byebye(); });
33. null
34. Null $collection = collect([1, null]); $value = $collection->find(function($el) { return $el === 42; }); $value // => null
35. What does null mean? 1. The element was not found 2. The element was found and is null
36. What does null mean? 1. Presence 2. Value
37. PHP Conventions Didn’t find something? • Return false • Throw exception
38. How to represent both?
39. Maybe
40. Maybe • Just(value) [Some(value)] • Nothing [None]
41. Contrived Ex 1: $collection = collect([1, null]); $value = $collection->find(function($el) { return $el === 42; }); $value // => null
42. Contrived Ex 1 Reprise: $collection = collect([ Maybe::just(1), Maybe::nothing() ]); $value = $collection->find(function($el) { return $el->fold() === 42; }); $value // => Nothing
43. Contrived Ex 1 Reprise: $collection = collect([ Maybe::just(1), Maybe::nothing() ]); $value = $collection->find(function($el) { return $el->fold() === null; }); $value // => Just(null)
44. Contrived Ex 1 Reprise: $collection = collect([ Maybe::just(1), Maybe::nothing() ]); $value = $collection->find(function($el) { return $el->fold() === 1; }); $value // => Just(1)
45. But is this better than null?
46. One way... $maybeUser = Eloquent::maybeFind(9001); if ($maybeUser->isNothing()) { // Do nothing } else { $something = $maybeUser->fold(); }
47. Don’t do this Without pattern matching ¯\_(ツ)_/¯
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 }); }
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)))
51. map public function map(callable $f) : Functor { return new static($f($this->value)); }
52. chain | flatMap public function chain(callable $f) : Monad { return $f($this->value); }
53. chain | flatMap public function chain(callable $f) : Monad { return $f($this->value); }
55. Nested Types $value = Box::of(1)->chain(function($id) { return Eloquent::maybeFind(9001); }); // Just(User) | Nothing $value->chain(function($user) { return $user->profile; }); // Maybe(Profile)
56. You don’t like closures? <?php class ConvertToTruth { public function convert($text) { return str_replace('the cloud', “someone else's computer", $text); } public function __invoke($text) { return $this->convert($text); } }
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
58. Will this improve CRUD?
59. Eloquent::findMaybe($id)
60. Will this improve complex data transformations?
61. Composable & Reusable functions
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/