Traits And Interfaces: Trait And Interface Requirements

Trait and interface requirements allow you to restrict the use of these constructs by specifying what classes may actually use a trait or implement an interface. This can simplify long lists of abstract method requirements, and provide a hint to the reader as to the intended use of the trait or interface.

Syntax

To introduce a trait requirement, you can have one or more of the following in your trait:

require extends <class name>;
require implements <interface name>;

To introduce an interface requirement, you can have one or more of following in your interface:

require extends <class name>;

Traits

Here is an example of a trait that introduces a class and interface requirement, and shows a class that meets the requirement:

abstract class Machine {
  public function openDoors(): void {
    return;
  }
  public function closeDoors(): void {
    return;
  }
}
interface Fliers {
  public function fly(): bool;
}

trait Plane {
  require extends Machine;
  require implements Fliers;

  public function takeOff(): bool {
    $this->openDoors();
    $this->closeDoors();
    return $this->fly();
  }
}

class AirBus extends Machine implements Fliers {
  use Plane;

  public function fly(): bool {
    return true;
  }
}

<<__EntryPoint>>
function run(): void {
  $ab = new AirBus();
  \var_dump($ab);
  \var_dump($ab->takeOff());
}
Output
object(HHVM\UserDocumentation\Guides\Hack\TraitsAndInterfaces\TraitAndInterfaceRequirements\TraitGood\AirBus) (0) {
}
bool(true)

Here is an example of a trait that introduces a class and interface requirement, and shows a class that does not meet the requirement:

abstract class Machine {
  public function openDoors(): void {
    return;
  }
  public function closeDoors(): void {
    return;
  }
}
interface Fliers {
  public function fly(): bool;
}

trait Plane {
  require extends Machine;
  require implements Fliers;

  public function takeOff(): bool {
    $this->openDoors();
    $this->closeDoors();
    return $this->fly();
  }
}

// Having this will not only cause a typechecker error, but also cause a fatal
// error in HHVM since we did not meet the trait requirement.
class Paper implements Fliers {
  use Plane;

  public function fly(): bool {
    return false;
  }
}

<<__EntryPoint>>
function run(): void {
  // This code will not run in HHVM because of the problem mentioned above.
  $p = new Paper();
  \var_dump($p);
  \var_dump($p->takeOff());
}
Typechecker output
File "/home/example/trait-bad.hack", line 31, characters 7-11:
This class does not satisfy all the requirements of its traits or interfaces. (Typing[4111])
  File "/home/example/trait-bad.hack", line 32, characters 7-11:
  This requires to extend or implement HHVM\UserDocumentation\Guides\Hack\TraitsAndInterfaces\TraitAndInterfaceRequirements\TraitBad\Machine
  File "/home/example/trait-bad.hack", line 19, characters 19-25:
  Required here

NOTE: require extends should be taken literally. The class must extend the required class; thus the actual required class does not meet that requirement. This is to avoid some subtle circular dependencies when checking requirements.

Interfaces

Here is an example of an interface that introduces a class requirement, and shows a class that meets the requirement:

abstract class Machine {
  public function openDoors(): void {
    return;
  }
  public function closeDoors(): void {
    return;
  }
}
interface Fliers {
  require extends Machine;
  public function fly(): bool;
}

class AirBus extends Machine implements Fliers {
  public function takeOff(): bool {
    $this->openDoors();
    $this->closeDoors();
    return $this->fly();
  }

  public function fly(): bool {
    return true;
  }
}

<<__EntryPoint>>
function run(): void {
  $ab = new AirBus();
  \var_dump($ab);
  \var_dump($ab->takeOff());
}
Output
object(HHVM\UserDocumentation\Guides\Hack\TraitsAndInterfaces\TraitAndInterfaceRequirements\InterfaceGood\AirBus) (0) {
}
bool(true)

Here is an example of an interface that introduces a class requirement, and shows a class that does not meet the requirement:

abstract class Machine {
  public function openDoors(): void {
    return;
  }
  public function closeDoors(): void {
    return;
  }
}
interface Fliers {
  require extends Machine;
  public function fly(): bool;
}

// Having this will not only cause a typechecker error, but also cause a fatal
// error in HHVM since we did not meet the interface requirement (extending
// Machine).
class Paper implements Fliers {
  public function fly(): bool {
    return false;
  }
}

<<__EntryPoint>>
function run(): void {
  // This code will actually not run in HHVM because of the fatal mentioned
  // above.
  $p = new Paper();
  \var_dump($p);
  \var_dump($p->takeOff());
}
Typechecker output
File "/home/example/interface-bad.hack", line 22, characters 7-11:
This class does not satisfy all the requirements of its traits or interfaces. (Typing[4111])
  File "/home/example/interface-bad.hack", line 22, characters 24-29:
  This requires to extend or implement HHVM\UserDocumentation\Guides\Hack\TraitsAndInterfaces\TraitAndInterfaceRequirements\InterfaceBad\Machine
  File "/home/example/interface-bad.hack", line 15, characters 19-25:
  Required here

File "/home/example/interface-bad.hack", line 22, characters 7-11:
This type doesn't implement the method `closeDoors` (Typing[4054])
  File "/home/example/interface-bad.hack", line 22, characters 24-29:
  Which is required by this interface
  File "/home/example/interface-bad.hack", line 10, characters 19-28:
  As defined here

File "/home/example/interface-bad.hack", line 22, characters 7-11:
This type doesn't implement the method `openDoors` (Typing[4054])
  File "/home/example/interface-bad.hack", line 22, characters 24-29:
  Which is required by this interface
  File "/home/example/interface-bad.hack", line 7, characters 19-27:
  As defined here

File "/home/example/interface-bad.hack", line 34, characters 17-23:
No instance method `takeOff` in `HHVM\UserDocumentation\Guides\Hack\TraitsAndInterfaces\TraitAndInterfaceRequirements\InterfaceBad\Paper` (Typing[4053])
  File "/home/example/interface-bad.hack", line 23, characters 19-21:
  Did you mean `fly` instead?
  File "/home/example/interface-bad.hack", line 32, characters 12-16:
  This is why I think it is an object of type HHVM\UserDocumentation\Guides\Hack\TraitsAndInterfaces\TraitAndInterfaceRequirements\InterfaceBad\Paper
  File "/home/example/interface-bad.hack", line 22, characters 7-11:
  Declaration of `HHVM\UserDocumentation\Guides\Hack\TraitsAndInterfaces\TraitAndInterfaceRequirements\InterfaceBad\Paper` is here