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Date: Fri, 12 Oct 2012 10:58:56 +0200
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Subject: Closures and type hinting
From: christian.kaps@mohiva.com (Christian Kaps)

At the moment it isn't possible to restrict/define the arguments for a 
closure which is defined as parameter for a method. Please look at this 
small example.

interface Broker {

     public function scan(Request $request, Closure $onFound);
}

An implementation of the interface could be as follows:

class BrokerImpl implements Broker {

     /**
      * @var Service
      */
     private $service = null;

     public function scan(Request $request, Closure $onFound) {

         if ($request->contains('some value')) {
             $onFound($this->service);
         }
     }
}

The problem is that I can pass every closure to the scan method.

$broker = new BrokerImpl();
$broker->scan($request, function() { /* do something */ });

Sometimes I would like to restrict the closure passed to this method. 
So that only closures of a certain type which has an argument "Service 
$service" could be passed to this method.

$broker = new BrokerImpl();
$broker->scan($request, function(Service $service) { /* do something */ 
});

Would it not be possible to extend the Closure class and then define an 
abstract method signature for the __invoke method.

class OnFoundClosure extends Closure {

     public abstract function __invoke(Service $service);
}

And then you can define the interface as follows:
interface Broker {

     public function scan(Request $request, OnFoundClosure $onFound);
}

Now if you pass a closure to the scan method which doesn't follow the 
signature of the __invoke method, the engine should throw an error.

What do you think?