下面是一些类型常量在尝试决定是使用它们还是泛型类型参数化时可能有用的示例。

引用类型常量

引用类型常量与引用静态类常量一样简单。

<?hh

namespace Hack\UserDocumentation\TypeConstants\Exampes\Examples\Referencing;

/*
 With generics, you reference the type parameter through explicit type
 parameterization declarations on all classes, functions, traits and interfaces
 using that type. In some cases that could be redundant because we know
 the type implicitly by knowing the type of the parent class.
 */
abstract class UserG<Tg as arraykey> {
  public function __construct(private Tg $id) {}
  public function getID(): Tg {
    return $this->id;
  }
}

class AppUserG<Tg as int> extends UserG<Tg> {}

// Still have to parameterize this function even though we know what Tg is.
function get_id_from_userg<Tg as arraykey>(AppUserG $ug): Tg {
  return $ug->getID();
}


/*
 With type constants, we can avoid having to declare the type parameter on a
 child class and function. The only thing needed in a child class is the
 concrete type binding on the abstract type constant of the parent. Otherwise,
 we just use reference the type constant as a normal property.
 */

abstract class UserTC {
  abstract const type Ttc as arraykey;
  public function __construct(private this::Ttc $id) {}
  public function getID(): this::Ttc {
    return $this->id;
  }
}

class AppUserTC extends UserTC {
  const type Ttc = int;
}

function get_id_from_userTC(AppUserTC $uc): AppUserTC::Ttc {
  return $uc->getID();
}

function run(): void {
  $aug = new AppUserG(-1);
  var_dump(get_id_from_userg($aug));
  $autc = new AppUserTC(-2);
  var_dump(get_id_from_userTC($autc));
}

run();

Output

int(-1)
int(-2)

覆盖类型常量

对于在类中声明的类型常量，可以提供约束和具体类型。当提供约束时，这允许类型常量被子类覆盖。接口不支持此功能。

<?hh

namespace Hack\UserDocumentation\TypeConstants\Exampes\Examples\Overriding;

abstract class BaseAbstract {
  abstract const type T;
}

class ChildWithConstraint extends BaseAbstract {
  // We can override this constraint in a child of this concrete class
  // since we provided an explicit as constraint.
  const type T as ?arraykey = ?arraykey;
}

class ChildOfChildWithNoConstraint extends ChildWithConstraint {
  // Cannot override this in a child of this class.
  const type T = arraykey;
}

class ChildOfChildOfChildWithNoConstraint extends ChildOfChildWithNoConstraint {
  // Type error here
  const type T = string;
}

function run(): void {
  echo "No real output!";
}

run();

Output

No real output!

类型常量和实例方法

您可以使用类型常量作为类实例方法的输入。

<?hh

namespace Hack\UserDocumentation\TypeConstants\Exampes\Examples\Instance;

abstract class Box {
  abstract const type T;
  public function __construct(private this::T $value) {}
  public function get(): this::T {
    return $this->value;
  }
  public function set(this::T $val): this {
    $this->value = $val;
    return $this;
  }
}

class IntBox extends Box {
  const type T = int;
}

function run(): void {
  $ibox = new IntBox(10);
  $ibox->set(11);
  var_dump($ibox);
  invariant($ibox instanceof Box, 'Upcast to Box');
  var_dump($ibox);
  // CHECK THIS -- THIS SHOULD (?) ERROR BUT THE TYPECHECKER IS NOT CATCHING IT
  // This will be an error because 'this::T' in '$box' may not be an int
  $ibox->set(1337);
  var_dump($ibox);
  // This is not an error because the type checker knows that the type
  // returned by $box->get(), is the same accepted by $box->set()
  $ibox->set($ibox->get());
  var_dump($ibox);
}

run();

Output

object(Hack\UserDocumentation\TypeConstants\Exampes\Examples\Instance\IntBox)#1 (1) {
  ["value":"Hack\UserDocumentation\TypeConstants\Exampes\Examples\Instance\Box":private]=>
  int(11)
}
object(Hack\UserDocumentation\TypeConstants\Exampes\Examples\Instance\IntBox)#1 (1) {
  ["value":"Hack\UserDocumentation\TypeConstants\Exampes\Examples\Instance\Box":private]=>
  int(11)
}
object(Hack\UserDocumentation\TypeConstants\Exampes\Examples\Instance\IntBox)#1 (1) {
  ["value":"Hack\UserDocumentation\TypeConstants\Exampes\Examples\Instance\Box":private]=>
  int(1337)
}
object(Hack\UserDocumentation\TypeConstants\Exampes\Examples\Instance\IntBox)#1 (1) {
  ["value":"Hack\UserDocumentation\TypeConstants\Exampes\Examples\Instance\Box":private]=>
  int(1337)
}