HttpType.scala

  /** A kleisli with a [[Request]] input and a [[Response]] output.  This type
    * is useful for writing middleware that are polymorphic over the return
    * type F.
    *
    * @tparam F the effect type in which the [[Response]] is returned
    * @tparam G the effect type of the [[Request]] and [[Response]] bodies
    */
  type Http[F[_], G[_]] = Kleisli[F, Request[G], Response[G]]

  /** A kleisli with a [[Request]] input and a [[Response]] output, such
    * that the response effect is the same as the request and response bodies'.
    * An HTTP app is total on its inputs.  An HTTP app may be run by a server,
    * and a client can be converted to or from an HTTP app.
    *
    * @tparam F the effect type in which the [[Response]] is returned, and also
    * of the [[Request]] and [[Response]] bodies.
    */
  type HttpApp[F[_]] = Http[F, F]

  /** A kleisli with a [[Request]] input and a [[Response]] output, such
    * that the response effect is an optional inside the effect of the
    * request and response bodies.  HTTP routes can conveniently be
    * constructed from a partial function and combined as a
    * `SemigroupK`.
    *
    * @tparam F the effect type of the [[Request]] and [[Response]] bodies,
    * and the base monad of the `OptionT` in which the response is returned.
    */
  type HttpRoutes[F[_]] = Http[OptionT[F, *], F]