Contexts And Capabilities: Context Constants
Note: Context and capabilities are enabled by default since HHVM 4.93.
Note: Context constant constraints are available since HHVM 4.108.
Classes and interfaces may define context constants:
class WithConstant {
const ctx C = [io];
public function has_io()[self::C]: void {
echo "I have IO!";
}
}
They may be abstract,
interface IWithConstant {
abstract const ctx C;
}
may have one or more bounds,
abstract class WithConstant {
// Subclasses must require *at least* [io]
abstract const ctx CAnotherOne as [io];
// Subclasses must require *at most* [defaults]
abstract const ctx COne super [defaults];
// Subclasses must require *at most* [defaults] and *at least* [io, rand]
abstract const ctx CMany super [defaults] as [io, rand];
}
and may have defaults, though only when abstract
interface IWithConstant {
abstract const ctx C = [defaults];
abstract const ctx CWithBound super [defaults] = [io];
}
When inheriting a class containing a context constant with a default, the first non-abstract class that doesn’t define an implementation of that constant gets the default synthesized for it.
One may define a member function whose context depends on the this type or the exact value of context constant.
class ClassWithConstant {
const ctx C = [io];
}
abstract class AnotherClassWithConstant {
abstract const ctx C;
abstract public function usesC()[this::C, ClassWithConstant::C]: void;
}
One may define a function whose context depends on the dynamic context constant of one or more passed in arguments.
function uses_const_ctx(SomeClassWithConstant $t)[$t::C]: void {
$t->usesC();
}
One may reference the dependent context constant of a argument in later arguments as well as in the return type.
function uses_const_ctx_more(
SomeClassWithConstant $t,
(function()[$t::C]: void) $f
)[$t::C]: (function()[$t::C]: void) {
$f();
$t->usesC();
return $f;
}