Free Monad in Scala with Cats

free.scala

object ImageApp extends App {

  import cats._
  import cats.free._
  import cats.data._

  case class Image(bytes: Array[Byte])

  sealed trait ImageOp[A] // Image Algebra

  case class Open[A](url: String) extends ImageOp[Image]

  case class Grayscale[A]() extends ImageOp[Unit]

  case class Resize[A](ratio: Double) extends ImageOp[Unit]

  type ImageOpF[A] = Free[ImageOp, A]

  object ImageOps {

    def open(url: String): ImageOpF[Image] = Free.liftF[ImageOp, Image](Open(url))

    def grayscale: ImageOpF[Unit] = Free.liftF[ImageOp, Unit](Grayscale())

    def resize(ratio: Double): ImageOpF[Unit] = Free.liftF[ImageOp, Unit](Resize(ratio))


  }

  val imageOpCompiler = new (ImageOp ~> Id) {

    def apply[A](imageOp: ImageOp[A]): Id[A] = {
      imageOp match {
        case Grayscale() =>
          println("Converting image to grayscale")
          ()

        case Open(url) =>
          println(s"Opening ${url}")
          Image(null)

        case Resize(ratio) =>
          println(s"Resizing image by ${ratio} percent")
          ()
      }
    }

  }

  import ImageOps._
  val program: ImageOpF[Unit] = for {
    _ <- open("http://tagueule")
    _ <- grayscale
    u <- resize(0.5)
  } yield u


  val compiled = program.compile(imageOpCompiler).run
  println(compiled)
}