Covariance, Contravariance & Diamond

Talk given at ForumPHP 2024

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1. COvariance, contravariance and diamond Alexandre Daubois
2. WHO AM I Alexandre Daubois @alexdaubois Expert Symfony Developer Expert Certified Dev Top 10 contributors of Symfony Author of “Clean Code in PHP”
3. Let’s say you create... ... a virtual machine management software.
4. Initial example <?php abstract class OperatingSystem { abstract public function createAndCopyFrom(OperatingSystem $os): OperatingSystem; } class Linux extends OperatingSystem { public function createAndCopyFrom(OperatingSystem $os): OperatingSystem { // ... } } class MacOS extends OperatingSystem { public function createAndCopyFrom(OperatingSystem $os): OperatingSystem { // ... } }
5. are you allowed to...?
6. <?php abstract class OperatingSystem { abstract public function createAndCopyFrom(OperatingSystem $os): OperatingSystem; } class Linux extends OperatingSystem { public function createAndCopyFrom(OperatingSystem $os): OperatingSystem|string { // ... } }
7. No, you cannot.
8. <?php abstract class OperatingSystem { abstract public function createAndCopyFrom(OperatingSystem $os): OperatingSystem; } class Linux extends OperatingSystem { public function createAndCopyFrom(OperatingSystem $os): Linux { // ... } }
9. Yes, you can.
10. <?php abstract class OperatingSystem { abstract public function createAndCopyFrom(OperatingSystem $os): OperatingSystem; } class Linux extends OperatingSystem { public function createAndCopyFrom(Linux $os): OperatingSystem { // ... } }
11. No, you cannot.
12. <?php abstract class OperatingSystem { abstract public function createAndCopyFrom(OperatingSystem $os): OperatingSystem; } class Linux extends OperatingSystem { public function createAndCopyFrom(OperatingSystem|string $os): OperatingSystem { // ... } }
13. Yes, you can.
14. Let’s recap what you can do. But also what you cannot. Widen argument type Yes Narrow argument type No Widen return type No Narrow return type Yes
15. Ever heard about s.o.l.i.d.?
16. S.O.L.I.D.
17. S.O.L.I.D. Base of modern clean code
18. D Dependency Inversion Principle
19. (What if cURL is missing?) <?php use Symfony\Component\HttpClient\CurlHttpClient; class OperatingSystemIsoDownloader { public function __construct(private CurlHttpClient $client) { } public function download(string $url) { return $this->client->request(/* ... */); } }
20. <?php use Symfony\Contracts\HttpClient\HttpClientInterface; class OperatingSystemIsoDownloader { public function __construct(private HttpClientInterface $client) { } public function download(string $url) { return $this->client->request(/* ... */); } }
21. HttpClientInterface CurlHttpClient Uses cURL internally NativeHttpClient Uses PHP streams AmpHttpClient Uses Amp concurrency
22. Why does dependency inversion actually work?
23. L Liskov Substitution Principle
24. Let f(x) be a property provable about objects x of type T. Then f(y) should be true for objects y of type S where S is a subtype of T. Barbara Liskov A behavioral notion of subtyping - 1994 Sales Deck Name Wikimedia commons
25. <?php class User implements \Stringable { public function __toString(): string { return '...'; } } class Admin extends User {} $user = new User(); $admin = new Admin(); assert($user instanceof \Stringable); assert($admin instanceof \Stringable); // Liskov!
26. <?php interface UserInterface {} class User { public function getReferral(): UserInterface {} } class Admin extends User {} $user = new User(); $admin = new Admin(); assert($user->getReferral() instanceof UserInterface); assert($admin->getReferral() instanceof UserInterface); // Liskov!
27. <?php interface UserInterface {} interface AdminInterface extends UserInterface {} class User { public function getReferral(): UserInterface {} } class Admin extends User { public function getReferral(): AdminInterface {} } $user = new User(); $admin = new Admin(); assert($user->getReferral() instanceof UserInterface); assert($admin->getReferral() instanceof UserInterface); // Boom, Liskov again!
28. Return type has been narrowed.
29. <?php interface UserInterface {} interface AdminInterface extends UserInterface {} class User { public function getReferral(): UserInterface {} } class Admin extends User { public function getReferral(): AdminInterface|string {} } $user = new User(); $admin = new Admin(); assert($user->getReferral() instanceof UserInterface); assert($admin->getReferral() instanceof UserInterface); // PHP error
30. <?php interface UserInterface {} interface AdminInterface extends UserInterface {} class User { public function getReferral(): AdminInterface {} } class Admin extends User { public function getReferral(): UserInterface {} } $user = new User(); $admin = new Admin(); assert($user->getReferral() instanceof UserInterface); assert($admin->getReferral() instanceof UserInterface); // PHP error
31. Return type cannot be widen.
32. This is covariance.
33. co-, like “together in the same direction”.
34. <?php function act(User $user) { $user->setIdentifier(1); } class User { public function setIdentifier(int $i) { /* ... */ } } class Admin extends User {} act(new User()); act(new Admin());
35. <?php function act(User $user) { $user->setIdentifier(1); } class User { public function setIdentifier(int $i) { /* ... */ } } class Admin extends User { public function setIdentifier(int|string $i) { /* ... */ } } act(new User()); act(new Admin());
36. Parameter type has been widen.
37. <?php // Simplified SPL Reminder interface Traversable {} class ArrayObject implements Traversable {} // Back to our code function act(User $user) { $user->doSomething(new \IteratorIterator(/* ... */)); } class User { public function attachReferrals(\Traversable $refs) { /* ... */ } } class Admin extends User { public function attachReferrals(\ArrayObject $refs) { /* ... */ } } act(new User()); act(new Admin()); // Boom, PHP error!
38. Parameter type cannot be narrowed.
39. This is contravariance.
40. contra-, like “in the counter direction”.
41. Let’s recap what you can do. But also what you cannot. Widen argument type Yes Narrow argument type No Widen return type No Narrow return type Yes
42. D Dependency Inversion Principle
43. L Liskov Substitution Principle
44. <?php use Symfony\Contracts\HttpClient\HttpClientInterface; class OperatingSystemIsoDownloader { public function __construct(private HttpClientInterface $client) { } public function download(string $url) { return $this->client->request(/* ... */); } }
45. What about covariance in PHPStan?
46. <?php interface UserInterface {} class User implements UserInterface {} /** * @template T */ class TypedArray { /** * @param array<T> $elements */ public function __construct(private array $elements) {} /** * @return T */ public function at(int $index): mixed { return $this->elements[$index]; } } /** @var TypedArray<UserInterface> $array */ $array = new TypedArray([new User(), new User()]); $user = $array->at(0); // IDEs and static analyzers should know that $user is of type UserInterface
47. <?php interface UserInterface {} class User implements UserInterface {} /** * @template T */ class TypedArray { /** * @param array<T> $elements */ public function __construct(private array $elements) {} /** * @return T */ public function at(int $index): mixed { return $this->elements[$index]; } } /** @var TypedArray<UserInterface> $array */ $array = new TypedArray([new User(), new User()]); $user = $array->at(0); // IDEs and static analyzers should know that $user is of type UserInterface
48. <?php interface UserInterface {} class User implements UserInterface {} /** * @template T */ class TypedArray { /** * @param array<T> $elements */ public function __construct(private array $elements) {} /** * @return T */ public function at(int $index): mixed { return $this->elements[$index]; } } /** @var TypedArray<UserInterface> $array */ $array = new TypedArray([new User(), new User()]); $user = $array->at(0); // IDEs and static analyzers should know that $user is of type UserInterface
49. <?php class MyDynamicClass extends \stdClass {} /** * @template-covariant T of \stdClass */ class TypedArrayOfDynamicObjects { // ... /** * @param T value */ public function push(mixed $value): void { // ... } } /** @var TypedArrayOfDynamicObjects<MyDynamicClass> $array */ $array = new TypedArrayOfDynamicObjects([ new MyDynamicClass(), new MyDynamicClass(), ]); $array->push(new \stdClass()); // IDEs and static analyzers should warn here
50. <?php class MyDynamicClass extends \stdClass {} /** * @template-covariant T of \stdClass */ class TypedArrayOfDynamicObjects { // ... /** * @param T value */ public function push(mixed $value): void { // ... } } /** @var TypedArrayOfDynamicObjects<MyDynamicClass> $array */ $array = new TypedArrayOfDynamicObjects([ new MyDynamicClass(), new MyDynamicClass(), ]); $array->push(new \stdClass()); // IDEs and static analyzers should warn here
51. <?php class MyDynamicClass extends \stdClass {} /** * @template-covariant T of \stdClass */ class TypedArrayOfDynamicObjects { // ... /** * @param T value */ public function push(mixed $value): void { // ... } } /** @var TypedArrayOfDynamicObjects<MyDynamicClass> $array */ $array = new TypedArrayOfDynamicObjects([ new MyDynamicClass(), new MyDynamicClass(), ]); $array->push(new \stdClass()); // IDEs and static analyzers should warn here
52. <?php class TypedArrayOfDynamicObjects { // ... /** * @param TypedArrayOfDynamicObjects<covariant \stdClass> $array */ public function merge(TypedArrayOfDynamicObjects $array): static { // ... } }
53. breathe.
54. Multiple inheritance.
55. <?php interface Button { public function render(): void; } interface Link { public function render(): void; } class ButtonLink implements Button, Link { public function render(): void { echo '<button>Click me</button>'; } }
56. UIElement Link Button ButtonLink
57. <?php interface UIElement { public function render(); } class Button implements UIElement { public function render(): void { echo '<button>Click me</button>'; } } class Link implements UIElement { public function render(): void { echo '<a href="#">Click me</a>'; } } class ButtonLink extends Button, Link {}
58. Which implementation should be used? <?php interface UIElement { public function render(); } class Button implements UIElement { public function render(): void { echo '<button>Click me</button>'; } } class Link implements UIElement { public function render(): void { echo '<a href="#">Click me</a>'; } } class ButtonLink extends Button, Link {}
59. This is the diamond problem.
61. Must use full “namespace” Crashes otherwise C++ Scala Priority algorithm Python C3 linearization What about other languages?
62. C# Ada Java Ruby Obj-C Delphi The “PHP Way”
63. There’s always a way.
64. <?php trait Button { public function render(): void { echo '<button>Click me</button>'; } } trait Link { public function render(): void { echo '<a href="#">Click me</a>'; } } class ButtonLink { use Button, Link; // But fatal error! }
65. <?php trait Button { public function render(): void { /* ... */ } } trait Link { public function render(): void { /* ... */ } } class ButtonLink { use Button, Link { Button::render insteadof Link; Button::render as renderButton; Link::render as renderLink; } public function renderElement(): void { $this->renderButton(); // calls Button::render $this->renderLink(); // calls Link::render } }
66. <?php trait Button { public function render(): void { /* ... */ } } trait Link { public function render(): void { /* ... */ } } class ButtonLink { use Button, Link { Button::render insteadof Link; Button::render as renderButton; Link::render as renderLink; } public function renderElement(): void { $this->renderButton(); // calls Button::render $this->renderLink(); // calls Link::render } }
67. <?php trait Button { public function render(): void { /* ... */ } } trait Link { public function render(): void { /* ... */ } } class ButtonLink { use Button, Link { Button::render insteadof Link; Button::render as renderButton; Link::render as renderLink; } public function renderElement(): void { $this->renderButton(); // calls Button::render $this->renderLink(); // calls Link::render } }
68. Y ou probably don’t need this.
69. After-show questions? LinkedIn : Alexandre Daubois X : @alexdaubois Symfony+AFUP Slacks
70. THANK YOU! Covariance, Contravariance and Diamond. Alexandre Daubois @alexdaubois