PHP RFC: Callable interfaces

• Version: 0.1
• Date: 2016-04-06
• Author: Ben Scholzen, mail@dasprids.de
• Author: Marco Pivetta, ocramius@gmail.com
• Status: Retired
• First Published at: http://wiki.php.net/rfc/callable-interfaces

Currently, when you type-hint against “callable” in your method signatures, you can never be certain whether the function will accept the parameters you are giving it, or if the return type is what you'd expect. You can manually validate the return type, and catch exceptions about the invalid parameters, but that is not ideal.

This RFC tries to solve this problem by allowing callables to follow user-specified interfaces. This RFC is inspired by https://wiki.php.net/rfc/typesafe-callable

PHP already has a way to define objects that act as functions. That mechanism is the __invoke magic method, which is widely used in libraries and frameworks. In addition to that, Closure already implements __invoke.

__invoke already works quite well: with this proposal, generic *callable* arrays, functions and objects will be usable as if they implemented a matching interface:

interface RegisterUser {
    public function __invoke(Username $username) : UserRegistration;
}

We can now implicitly implement this interface by just defining any *callable* that matches this interface:

As a function:

function register (Username $username) : UserRegistration {
    // ... domain logic here ...
 
    return new UserRegistration($userId);
}

As a closure:

$register = function (Username $username) : UserRegistration {
    // ... domain logic here ...
 
    return new UserRegistration($userId);
};

As a static callable array:

class Register {
    public static function register(Username $username) : UserRegistration {
        // ... domain logic here ...
 
        return new UserRegistration($userId);
    }
}
 
$register = [Register::class, 'register'];

As an instance callable array:

class Register {
    public function register(Username $username) : UserRegistration {
        // ... domain logic here ...
 
        return new UserRegistration($userId);
    }
}
 
$register = [new Register(), 'register'];

We are now able to consume any of these callables wherever the interface is required in a type-hint:

function runRegistration(Username $username, RegisterUser $handler) {
    var_dump($handler($username));
}
 
runRegistration(new Username('DASPRiD'), $register);

In order for this to work, any implicitly defined callable should be cast to a *Closure* at call-time.

In pseudo-code, this would look like following, under the hood:

function passAParameterToAPhpFunction(callable $callable, $expectedParameterInterface) {
    if (! $expectedParameterInterface->isCallableInterface()) {
        passParameter($callable);
 
        return;
    }
 
    if (! $expectedParameterInterface->matches($callable)) {
         throw new TypeError('Expected X, got Y');
    }
 
    if (! is_object($callable)) {
        $callable = wrapInCompatibleAnonymousClass($callable);
    }
 
    passParameter($callable);
}

How will instanceof behave, when asked for a type-check against callable?

interface RegisterUser {
    public function __invoke(Username $username) : UserRegistration;
}
 
interface DeleteUserRegistration {
    public function __invoke(Username $username) : UserRegistration;
}
 
$register = function (Username $username) : UserRegistration {
    return new UserRegistration(...);
};
 
var_dump($register instanceof DeleteUserRegistration); // true? false? possibly want to keep current semantics here.

This RFC has been retired. Reason for that is that PHP currently (Version 7.0~7.1) allows applying function semantics to objects via the __invoke magic methods. Allowing the opposite would mix the domain of functions and objects in ways that are very hard to disentangle, and it would needlessly complicate the language semantics.

While it is unfortunate that migration to type-safe callables (https://wiki.php.net/rfc/typesafe-callable) would require some interface rewrites, that is indeed the correct solution, as it keeps the uni-directionality between object and function semantics.

This RFC expects no BC breaks.

7.1

This RFC requires a 2/3 majority to pass.

Patch will be available before voting commences.

After the project is implemented, this section should contain

1. the version(s) it was merged to
2. a link to the git commit(s)
3. a link to the PHP manual entry for the feature