通过 scalaz 中的 Kleisli 箭头编写函数时，无法在 Coyoneda 上找到 Free Monads 的 Bind 实例

Question

提前感谢您的帮助

我尝试通过 Kleisli 箭头组合 2 个函数。这些函数接受 String 并生成 FreeC。 kleisli 箭头的创建没有问题，但编译器抱怨找不到。为了简单起见，我将删除一些代码：

import scalaz._
import Scalaz._
import Free.FreeC
import Free._
import Kleisli._

trait AppCompose {

  def lift[F[_], G[_], A](fa: F[A])(implicit I: Inject[F, G]): FreeC[G, A] =
    Free.liftFC(I.inj(fa))

}

object BigBrother {

  sealed trait Sensor[A]
  case class Log(log: String) extends Sensor[Unit]
  case class Filter(log: String) extends Sensor[String]
  case class Secure(log: String) extends Sensor[String]

}

import BigBrother.Sensor

class BigBrother[F[_]](implicit I: Inject[Sensor,F]) extends AppCompose {
  import BigBrother._

  type FreeString[A] = FreeC[F,String]

  def log(log: String) = lift(Log(log))
  def filter(log: String) = lift(Filter(log))
  def secure(log: String) = lift(Secure(log))

  def filterAndSecure(phrase: String) = for {
    f <- filter(phrase)
    s <- secure(f)
  } yield s

  // kleisli composition attempt - alternative to filterAndSecure
  val fk = kleisli[FreeString, String, String](filter _)
  val sk = kleisli[FreeString, String, String](secure _)
  val fAndS = fk >=> sk // this is where we have a compilation issue

}

出于某种原因，我得到的是这个编译错误：

could not find implicit value for parameter b: scalaz.Bind[FreeString]
[error]   val fAndS = sk >=> fk

感觉应该解决隐式问题，因为 FreeC 在实现 Bind 特性的 monad 实例中，我正在通过导入导入所有 Free 隐式实例 Free._

我在这里错过了什么？

提前致谢！

Answer 1

感谢特拉维斯的帮助。错误的类型声明实际上是罪魁祸首之一。在 scalaz 社区通过 google 组提供的一些帮助下，这里的一些修补就是答案：

class BigBrother[F[_]](implicit I: Inject[Sensor,F]) extends AppCompose {
  import BigBrother._

  def log(log: String) = lift(Log(log))
  def filter(log: String) = lift(Filter(log))
  def secure(log: String) = lift(Secure(log))

  def filterAndSecure(phrase: String) = for {
    f <- filter(phrase)
    s <- secure(f)
  } yield s

  type CoyoF[A] = Coyoneda[F, A]
  type FreeCoF[A] = Free[CoyoF,A]

  implicit val MonadFreeSensor = Free.freeMonad[FreeCoF]

  // kleisli composition attempt - alternative to filterAndSecure
  val fk = kleisli[FreeCoF, String, String](filter _)
  val sk = kleisli[FreeCoF, String, String](secure _)
  val fAndS = fk >=> sk

}

key 是正确的类型声明并为 FreeCoF 提供类型 class monad 实例 implicit val MonadFreeSensor = Free.freeMonad[FreeCoF]

通过 scalaz 中的 Kleisli 箭头编写函数时，无法在 Coyoneda 上找到 Free Monads 的 Bind 实例

Cannot find Bind instances for Free Monads over Coyoneda when composing functions via Kleisli arrows in scalaz

functional-programming

scala

scalaz

kleisli

free-monad