Scala使用无形结合较高的kinded副产品而不是自然转化

trait NaturalTransformation[F[_],G[_]] extends Serializable { self =>
  def or[H[_]](h: H ~> G): Coproduct[F,H,?] ~> G = ???
}

所以你可以用它来做(使用kind-projector作为lambda类型？)

val combine: Coproduct[Command,Query,?] ~> M = commandHandler.or(queryExecutor)

编辑：这是一个完整的例子,也定义了union(使用Id代替M进行类型检查)：

import cats._
import cats.data._

trait Ack
sealed trait Domain[Y]
sealed trait Command[Y] extends Domain[Y]
sealed trait Query[Y] extends Domain[Y]

case class Add(value:String) extends Command[Ack]
case class Remove(value:String) extends Command[Ack]
case class Exists(value:String) extends Query[Boolean]
case object List extends Query[List[String]]

def commandHandler:Command ~> Id = ???
def queryExecutor:Query ~> Id = ???

def union: Domain ~> Coproduct[Command,?] = new (Domain ~> Coproduct[Command,?]) {
  def apply[A](fa: Domain[A]): Coproduct[Command,A] = fa match {
    case command: Command[A] => Coproduct.left(command)
    case query: Query[A] => Coproduct.right(query)
  }
}

def result: Domain ~> Id = commandHandler.or(queryExecutor).compose(union)

或者如果你想避免中间Coproduct：

def unionDirect[M[_]](cmd: Command ~> M,qry: Query ~> M): Domain ~> M =
  new (Domain ~> M) {
    def apply[A](fa: Domain[A]): M[A] = fa match {
      case command: Command[A] => cmd(command)
      case query: Query[A] => qry(query)
    }
}

def resultDirect: Domain ~> Id = unionDirect(commandHandler,queryExecutor)