如何避免在Scala中使用Monad变形金刚踩踏板？

Question

我有以下代码使用Readermonad进行配置,还必须处理,IO[Option[String]]我最终得到的代码是我的encode函数中的阶梯.

我怎样才能为我的函数中的monad变换器制定Reader并OptionT避免丑陋的嵌套for理解encode？

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)}

谢谢!

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.

作为脚注:liftReaderT如果您定义这样的UnapplyCo实例,我提到的漂亮语法就可用了:

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
}

我不确定Scalaz 7目前是否提供此实例是否有原因,但它可能值得研究.