免费的 Monad 解释器

Question

我正在关注 Chris Myers 关于 Free Monads 的演讲，但是当我实现解释器时出现编译错误

package object example {
  import cats._
  import cats.data._
  import cats.free._
  import cats.implicits._

  case class Property(Id: Int, post: String)

  type Script[A] = Free[AppAction, A]

  sealed trait AppAction[A] {
    def lift: Script[A] = Free.liftF(this)
  }

  case class FetchById(propertyId: Int) extends AppAction[Property]

  case class Save(property: Property) extends AppAction[Unit]

  case class Log(message: String) extends AppAction[Unit]

  def fetchById(id: Int): Script[Property] = FetchById(id).lift
  def save(property: Property): Script[Unit] = Save(property).lift

  def log(message: String): Script[Unit] = Log(message).lift


  def updatePost(propertyId: Int, post: String): Script[Property] = {
    for {
      property <- fetchById(propertyId)
      p1 = property.copy(post = post)
      _ <- log("Updating")
      _ <- save(p1)
    } yield p1
  }

  object TestInterp extends (AppAction ~> Id) {
    def apply[A](fa: AppAction[A]): Id[A] = fa match {
      case FetchById(_) => Property(1, "2000")
      case Save(_) => ()
      case Log(msg) => println(msg)
    }

    def run[A](script: Script[A]) = script.foldMap(this)
  }

}

我导入了猫，特别是 cats.Id 但我在测试解释器中得到了 "Expression of type Property does not conform to cats.Id"。我什至在 Id monad 中明确包装了 fetchById 的结果，但它不起作用。

Answer 1

在for-表达式中

p1 <- property.copy(post = post)

不是单子绑定步骤。就像定义辅助变量 p1 的简单赋值一样。这在这里似乎有效：

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

case class Property(id: Int, post: String)


sealed trait AppAction[A] {
  def lift: Script[A] = Free.liftF(this)
}
case class FetchById(propertyId: Int) extends AppAction[Property]
case class Save(property: Property) extends AppAction[Unit]
case class Log(message: String) extends AppAction[Unit]

type Script[A] = Free[AppAction, A]

def fetchById(id: Int): Script[Property] = FetchById(id).lift
def save(property: Property): Script[Unit] = Save(property).lift
def log(message: String): Script[Unit] = Log(message).lift

def updatePost(propertyId: Int, post: String): Script[Unit] = {
  for {
    property <- fetchById(propertyId)
    p1 = property.copy(post = post)
    _ <- log("Updating")
    _ <- save(p1)
  } yield()
}

object TestInterp extends (AppAction ~> Id) {
  def apply[A](fa: AppAction[A]): Id[A] = fa match {
    case FetchById(_) => Property(1, "2000")
    case Save(_) => ()
    case Log(msg) => println(msg)
  }

  def run[A](script: Script[A]) = script.foldMap(this)
}

免费的 Monad 解释器

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