Asynchronous Operations: Async Vs. Awaitable
Why is the async keyword needed for functions that return Awaitable? Because it is possible to have non-async functions that
return awaitables; the async keyword is merely an implementation detail. For this reason, async is not allowed in interfaces. For example:
<?hh // strict
namespace Hack\UserDocumentation\AsyncOps\AsyncVsAwaitables\Examples\Impl;
interface Car {
// It doesn't matter to the caller how this is implemented, only that it
// returns an Awaitable<void>
public function drive(): Awaitable<void>;
}
This can be implemented with an async function, like this:
<?hh // strict
namespace Hack\UserDocumentation\AsyncOps\AsyncVsAwaitables\Examples\Impl;
require_once "interface.inc.php";
class Ford implements Car {
public async function drive(): Awaitable<void> {
// ...
}
}
It can also be implemented by a non-async function, like this:
<?hh // strict
namespace Hack\UserDocumentation\AsyncOps\AsyncVsAwaitables\Examples\Impl;
require_once "interface.inc.php";
class VolkswagenDiesel implements Car {
public function drive(): Awaitable<void> {
// ...
return $this->driveNormally();
}
private async function driveNormally(): Awaitable<void> {
// ...
}
}
The use of async is strongly encouraged for all functions, except for:
- Interface method declarations
- Abstract method declarations
The async keyword should be used in most other cases, including implementations of interface or abstract methods.