context bound of scalar.Coyoneda.liftTF

Question

Finished watching thought-provoking video "Composable application architecture with reasonably priced monads" from Rúnar Bjarnason, I started to write the examples provided in the video in Scalaz. I am having a little surprise when implementing the writer based interpreter for the Interact App. Here is the code,

sealed trait Interact[A]

case class Ask(prompt: String) extends Interact[String]

case class Tell(msg: String) extends Interact[Unit]

type W[A] = scalaz.Writer[Vector[String], A]

object ConsoleW extends (Interact ~> W) {
  def apply[A](i: Interact[A]) = i match {
    case Ask(prompt) =>
      println(prompt)
      Writer(Vector(prompt), readLine)
    case Tell(msg) =>
      println(msg)
      Writer(Vector.empty, ())
  }
}

When I tried to lift ConsoleW into an interpreter for the interact free monad, the compiler hinted me lack of context bound of Functor to W. I am quite surprised since Writer itself is a monad, the functor context bound should be given for free. Ok so I had to explicitly write a functor,

implicit val functor = new Functor[W] {
  def map[A, B](fa: Writer[Vector[String], A])(f: A => B) =
    fa.map(f)
}

which is pretty dumb... since I basically did nothing other than calling the map method of Writer itself. Now I can lift ConsoleW into an interpreter. However when I tried to foldMap the interact program with ConsoleW, the compiler again hinted the lack of Monad context bound for W! Ok, this is totally unexpected, let me spoon-feed you, Scalac...

val prg: Free[({type f[x] = Coyoneda[Interact, x]})#f, Unit] = ???

import scalaz.std.vector._

implicit val monad = new scalaz.Monad[W] {
  def point[A](a: => A) = Writer[Vector[String], A](Vector.empty, a)
  def bind[A, B](fa: Writer[Vector[String], A])(f: A => Writer[Vector[String], B]) = 
    fa.flatMap(f)
}

prg.foldMap(Coyoneda.liftTF(ConsoleW))

Now everything compiles, and the Writer Monad can record all the prompts into vector when the program runs. But this is unsatifying, I would like to create an interpreter with Writer as the underlying Monad without having to explicitly providing the proof that it is really a Functor and Monad. Any idea how this could work out?

Can you say more about your Scalaz version, any imports, and any additional context? This should work just fine without the additional instances, and in a quick off-hand REPL session with Scalaz 7.1.0 on Scala 2.11.5 everything looks fine to me. — Travis Brown
@Travis yes I just did import scalaz._; import Scalaz._; it did work out. But if I just import scalaz.syntax.monad._; compile failed. What am I missing in imports? — Sheng

Sheng Sheng · Accepted Answer · 2015-01-26T14:27:57

Just need to import scalaz.std.vector._, and problem solved. I was impressed by how Id.Id could just work out as is, and forgot import of writer monad should be more complicated.

context bound of scalar.Coyoneda.liftTF

1 Answers