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 class <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());
}

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());
}

The require extends constraints should be taken literally: the class that uses the trait must be a strict sub-class of that in the require extends constraint.

A require class <class name>; constraint in a trait specifies that the trait can only be used by the _non-generic, final, class <class name>. This contrasts with the require extends t; constraints that allow the trait to be used by an arbitrary strict subtype of t. By relaxing the strict subtype constraint of require extends, require class constraints allow splitting the implementation of a class into a class and one (or multiple) traits, as in the following:

trait T {
  require class C;
  public function foo(): void {
    $this->bar();
  }
}

final class C {
  use T;
  public function bar(): void {}
}

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());
}

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());
}

NOTE: trait cannot be used as a type, comparing to some other languages. Only class and interface are types. For example,

trait T {}
class C { use T; }
$a = new C();
$j = ($a is C);
$k = ($a is T); # error!

leads to error

Hit fatal : "is" and "as" operators cannot be used with a trait
    #0 at [:1]
    #1 include(), called at [:1]
    #2 include(), called at [:0]
Hit fatal : "is" and "as" operators cannot be used with a trait
Failed to evaluate expression