How concrete directives would look like

ConcreteDirectives.scala

def extract[L <: HList](f: Request => L): Directive[L] = new Directive[L] {
  def happly(inner: L ⇒ Route): Route = ctx ⇒ inner(f(ctx))(ctx)
}
def host: Directive[String :: HNil] = {
  extract { case Request(url) =>
    val Array(protocol, rest) = url.split("://")
    rest.split("/", 1).head :: HNil
  }
}
def protocol: Directive[String :: HNil] = {
  extract { case Request(url) =>
    val Array(protocol, rest) = url.split("://")
    protocol :: HNil
  }
}
val protocolAndHost: Directive[String :: String :: HNil] = protocol & host