hack類型常量：Constraints

類型常量的約束（Constraints）類似于泛型類型參數(shù)的約束，因為它們限制類型常量可能被覆蓋的內(nèi)容。

抽象類

對抽象類型常量的約束要求任何覆蓋抽象類型常量的類必須是約束類型的子類型。

<?hh

namespace Hack\UserDocumentation\TypeConstants\Constraints\Examples\AbsClass;

abstract class AbsParent {
  abstract const type Foo as arraykey;
}

class FirstChild extends AbsParent {
  const type Foo = string; // good since string is arraykey.
}

class SecondChild extends AbsParent {
  const type Foo = float; // typechecker error since float is not an arraykey.
}

/***

abstract-type-errors.php:13:7,17: Class Hack\UserDocumentation\TypeConstants\Constraints\Examples\AbsClass\SecondChild does not correctly implement all required methods  (Typing[4110])
  abstract-type-errors.php:13:27,35: Some methods are incompatible with those declared in type Hack\UserDocumentation\TypeConstants\Constraints\Examples\AbsClass\AbsParent
Read the following to see why:
  abstract-type-errors.php:6:23,25: Unable to satisfy constraint on this type constant
  abstract-type-errors.php:6:30,37: This is an array key (int/string)
  abstract-type-errors.php:14:20,24: It is incompatible with a float

***/

Concrete Classes

一個具體的類型常量必須有一個約束，以便任何孩子覆蓋該類型的常量。

<?hh

namespace Hack\UserDocumentation\TypeConstants\Constraints\Examples\ConcClass;

class ParentWithConstraint {
  const type Foo as arraykey = arraykey;
}

class Child extends ParentWithConstraint {
  // good since string is arraykey and parent constrained to arraykey.
  const type Foo = string;
}

class ParentWithoutConstraint {
  const type Foo = arraykey;
}

class BadChild extends ParentWithoutConstraint {
  // Although int is an arraykey, parent not constrained to arraykey, so this
  // is a typechecker error.
  const type Foo = int;
}


/***

concrete-type-errors.php:18:7,14: Class Hack\UserDocumentation\TypeConstants\Constraints\Examples\ConcClass\BadChild does not correctly implement all required methods  (Typing[4110])
  concrete-type-errors.php:18:24,46: Some methods are incompatible with those declared in type Hack\UserDocumentation\TypeConstants\Constraints\Examples\ConcClass\ParentWithoutConstraint
Read the following to see why:
  concrete-type-errors.php:21:14,16: The assigned type of this type constant is inconsistent with its parent
  concrete-type-errors.php:21:20,22: This is an int
  concrete-type-errors.php:15:20,27: It is incompatible with an array key (int/string)

***/