Scala what does the “def function = Type {” mean?

Question

I'm using Play 2.x and found the following syntax in action handlers eg

object Application extends Controller { 
    /**
     * Index action handler
     */
    def index = Action { implicit request => 
      Ok(Json.obj("one" -> "two"))
    }
}

Here I understand everything except the = Action which is not the type of the function, because the function return type is play.api.mvc.Result . So what does the = Action mean?

To make the understanding worse I now introduced authentication and based on examples changed my Application to:

object Application extends Controller with Secured { 
    /**
     * Index action handler
     */
    def index = IsAuthenticated { username => implicit request => 
      Ok(Json.obj("one" -> "two"))
    }
}

This works but why the Action is no longer necessary? was it necessary at all? how can I combine several of these types (whatever they mean): Action or DBAction , IsAuthenticated etc?

Answer 1

Action is not a type, it is a function. If it were a type the signature for index would look like

def index: Action = { implicit request =>

Notice the addition of the : and the location of the =

The relevant documentation states an Action is a

function that handles a request and generates a result to be sent to the client

As you noticed you are returning a play.api.mvc.Result

Answer 2

Ok now thanks to a colleague I understood what it is. That syntax that simply means returning an object, in this case an object of type Action and the action takes as constructor argument a function that takes as input a Request and returns a SimpleResult , it can be rewritten as eg

object Application extends Controller { 
    /**
     * Index action handler
     */
     def index : Action[AnyContent] = { 
         Action(implicit request => Ok(Json.obj("one" -> "two")))
     }
 }

What makes it actually confusing is that in Scala they can switch between parenthesis and curly brackets somewhat indistinctly. Therefore makes it tricky to realize it is not a function body what I was looking at but a constructor parameter to an action (which is an anonymous function)