FinallyTaglessOrFreemonad.scala

package io.atlassian.aws

import java.io.File

import scalaz.{Free, Functor}

sealed trait FileActionOp[A]
case class Pure[A](x: A) extends FileActionOp[A]
case class MakeDir[A](d: File, x: A) extends FileActionOp[A]
case class CreateFile[A](d: File, x: A) extends FileActionOp[A]

object X {
  implicit val functorF: Functor[FileActionOp] =  new Functor[FileActionOp] {
    override def map[A, B](fa: FileActionOp[A])(f: A => B): FileActionOp[B] =
      fa match {
        case Pure(x) => Pure(f(x))
        case MakeDir(d, x) => MakeDir(d, f(x))
        case CreateFile(d, x) => CreateFile(d, f(x))
      }
  }

  type FileAction[A] = Free[FileActionOp,A]

  val createFile: Free[FileActionOp, String] = Free.liftF(CreateFile(new File(""), "afsal"))
  val makeDir: Free[FileActionOp, String] = Free.liftF(MakeDir(new File(""), "afsal"))

  // My program is doing a lot of input and output operations. First it creates a file. Then it creates a directory.
  // Please test if myProgram does the right thing! That it first createFile and then makeDir
  val myProgram: Free[FileActionOp, Unit] =
    for {
      _ <- createFile
      _ <- makeDir
    } yield ()

  // Unwrapping the Free to a list of operations recursively. Yes it is a boiler plate. 
  // Come on, do you want to write to a file and test?
  // Or do you want to make a file operation program and test ? In which case you can test only the final result which is Unit
  // Or do you want to write this boiler plate and test if the program has all the steps that are required?
  def inspectProgram[A](f: Free[FileActionOp,A]): List[FileActionOp[None.type]] = f.fold(
    (a:A) => List(),
    (x:FileActionOp[FileAction[A]]) => x match {
      case Pure(x) => inspectProgram(x)
      case MakeDir(d,x) => MakeDir(d,None) :: inspectProgram(x)
      case CreateFile(d,x) => CreateFile(d,None) :: inspectProgram(x)
    }
  )
}

object Main {
  def main(args: Array[String]): Unit = {
    val s = X.inspectProgram[Unit](X.myProgram)

    // M also worried about the order, and it tests that as well!
    assert(s == List(CreateFile(new File(""), None), MakeDir(new File(""), None)))
  }
}