Scala Code demystify

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 thi

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.