The Conundrum

gistfile1.scala

/* Here's the first way, the "view" version: When your code calls request.pathInfo, the compiler says, 
"HttpServletRequest doesn't have pathInfo. But it can implicitly convert to RichRequest.  
I'm going to call RichRequest.servletRequestToRichRequest, and then call pathInfo on the result of that." */
class RichRequest(req: HttpServletRequest) extends Request { 
  def pathInfo = req.getPathInfo 
}


object RichRequest { 
  implicit def servletRequestToRichRequest(req: HttpServletRequest) = new RichRequest(request) 
}



/*  Here's the other way, using type classes: */
trait Request[A] { 
  def pathInfo(implicit a: A): String 
}

object Request { 
  implicit val servletRequset = new Request[HttpServletRequest] { 
    def pathInfo(req: HttpServletRequest) = req.getPathInfo 
  } 
}

/* A *Request* is a *class of types* that knows how to perform certain operations over other types.  
(If you want to impress the ladies at the bar, call it "ad hoc polymorphism.") Now ScalatraServlet is 
amended to look like this: */
trait ScalatraServlet {
  def request: HttpServletRequest // blech
  val betterRequest: Request[HttpServletRequest] // Hooray
}

// then we use betterRequest.pathInfo