Can't access generic function type in a inner PartialFunction

Question

I am currently writing a generic function to execute Dispatch async requests, but I can't access generic type in Dispatch handler:

private def execQuery[MessageType](query : Req, errorMsg : String)
{
    Http(query OK as.String).either
        .onSuccess
    {
        case Left(error) => println(errorMsg)
        case Right(json) => println( new MessageType(json) ) // error here
    }
}

I have an error on new MessageType : "Cannot resolve symbol MessageType" in "new MessageType(json)". Can you help me ?

Thank you in advance

Victor

EDIT : I have found an other interesting way here http://www.brentsowers.com/2011/11/writing-generic-functions-that-take.html. You have to use the manifest feature :

class DynamicTestClass() {
    def output() {
        println("Hello from a dynamically sent class")
    }
}

def testFunc[T : Manifest]() : T = {
    manifest[T].erasure.newInstance().asInstanceOf[T]
}
val dynamicTestClassInstance = testFunc[DynamicTestClass]()
dynamicTestClassInstance.output()

It seems to work !

Answer 1

You can't do

def mustMakeA[A](b:String):A = new A(b)

in scala.

A few practical reason. How is scalac supposed to know if A has public constructors and its argument?

However you can use a smarter design, for example, a type class that "knows" how to construct the parameter:

class Message(val value:String)

trait Constructor[A] {
   def cons(b:String) 
}

implicit class MessageHasConstructor(m:Message) extends Constructor[Message] {
   def cons(b:String) = new Message(b)
}

Et voilà, now we rewrite our mustMakeA as:

def mustMakeA[A:Constructor](b:String):A = implicitly[Constructor[A]].cons(b)

mustMakeA[Message]("Example") // would give us a `new Message("Example")`

Btw, I didn't test the code, so it might require some tweaking.