How to avoid stair-stepping with Monad Transformers in scala?

Question

I have the following code that uses the Reader monad for configuration and also has to deal with IO[Option[String]] and I've ended up with code that stair-steps in my encode function.

How can I formulate a monad transformer for Reader and OptionT to avoid the ugly nested for comprehensions in my encode function?

def encode(fileName: String): Reader[Config, IO[Unit]] = for {
   ffmpegWrapper <- findFfmpegWrapper
   ffmpegBin <- findFfmpeg
} yield (for {
    w <- ffmpegWrapper
    b <- ffmpegBin
    stream <- callFfmpeg(getCommand(w, b, fileName)).liftM[OptionT]
} yield stream) map (_ foreach (println)) getOrElse Unit.box {}


def getCommand(ffmpegWrapper: String, ffmpegBin: String,
             videoFile: String) = s"$ffmpegWrapper $ffmpegBin $videoFile  '-vcodec libx264 -s 1024x576' /tmp/out.mp4"

def callFfmpeg(command: String): IO[Stream[String]] = IO {
  Process(command).lines_!
}

def findFile(path:List[String]): OptionT[IO,String] = OptionT[IO,String](IO{path.find(new File(_).exists)})

def findFfmpeg:Reader[Config, OptionT[IO,String]] = Reader {c=>findFile(c.ffmpegLocations)}

def findFfmpegWrapper:Reader[Config, OptionT[IO,String]] = Reader {c=>findFile(c.ffmpegWrapperLocations)}

Thank-you!

Answer 1

If you look at the definition of Reader in the Scalaz source, you'll see this:

    type Reader[-E, +A] = ReaderT[Id, E, A]

Which tells us that the Reader monad you are using is just a specialization of a monad transformer where the monad being wrapped is the trivial Id monad. You can use ReaderT directly, but wrapping your OptionT[IO, _] monad instead of just wrapping everything in a Reader. For example, the following should do what you want:

type OptionIO[+A] = OptionT[IO, A]

def findFfmpeg: ReaderT[OptionIO, Config, String] =
  Kleisli[OptionIO, Config, String](c => findFile(c.ffmpegLocations))

def findFfmpegWrapper: ReaderT[OptionIO, Config, String] =
  Kleisli[OptionIO, Config, String](c => findFile(c.ffmpegWrapperLocations))

def encode(fileName: String): ReaderT[OptionIO, Config, Unit] = (for {
   w <- findFfmpegWrapper
   b <- findFfmpeg
   stream <- Kleisli[OptionIO, Config, Stream[String]](
     _ => callFfmpeg(getCommand(w, b, fileName)).liftM[OptionT]
   )
} yield stream).map(_ foreach println)

In principle you should be able to replace the part after stream <- with the following:

callFfmpeg(getCommand(w, b, fileName)).liftM[OptionT].liftReaderT[Config]

But for some reason the Unapply machinery that liftReaderT relies on doesn't seem to be working in this case. Writing the Kleisli part out explicitly isn't all that horrible, fortunately.

---

As a footnote: the nice liftReaderT syntax I mentioned becomes available if you define an UnapplyCo instance like this:

implicit def unapplyMFA1[TC[_[_]], F[+_], M0[F[+_], +_], A0](
  implicit TC0: TC[({ type L[x] = M0[F, x] })#L]
): UnapplyCo[TC, M0[F, A0]] {
  type M[+X] = M0[F, X]
  type A = A0
} = new UnapplyCo[TC, M0[F, A0]] {
  type M[+X] = M0[F, X]
  type A = A0
  def TC = TC0
  def leibniz = Leibniz.refl
}

I'm not sure off the top of my head whether there's a reason Scalaz 7 doesn't currently provide this instance, but it's probably worth looking into.