Question

Could someone demystify this code which is part of the zentasks example in the Play20 framework. I’m curious how this works, granted I’m new to Scala from Java so a lot of things are tough to wrap my head around.

def IsAuthenticated(f: => String => Request[AnyContent] => Result) = 
  Security.Authenticated(username, onUnauthorized) { user =>
    Action(request => f(user)(request))
  }

Answer 1

You need to split up the signature a bit. f is a function that takes a not-yet-computed string => String and returns another function that accepts a Request[AnyContent] and returns a result.

The Security.Authenticated call accepts two parameters lists. One that has username and onUnauthorized. The second takes a function accepting the user and returning an action.

The Action.apply method accepts a function Request[AnyContent] => Result

so, the f is called in ‘curried’ fashion. That is the first function is called, and then the resulting function is immediately used f(user)(request).

Here’s the same thing desugared (at least, as best I can) and ugly:

def isAuthenticated(f: => String => Request[AnyContent] => Result) =
  Security.Authenticated(username, onUnauthorized) { user: String =>
     Action.apply { request: Request[AnyContent] =>
       val hiddenTmp: Request[AnyContent] => Result = f(user)
       hiddenTemp.apply(request)
     }
  }

You can see the compiler is doing a bit of work removing type annotations. Hopefully that helps explain how it desugars into raw scala. Essentially, the function does a lot of functional composition.