Scala Tagless Final - 不编译

Question

这是人为登录过程的基本 Scala Tagless Final 模式实现。它不会编译，因为它在结尾附近显示“没有找到参数的隐式......”

但是，如果我从程序泛型类型中删除

': Monad: LoginProcessAlgebra[F]'

，特定错误就会消失，但理解开始抱怨，因为

F[_]

不再缩小到 Monad

问题： 为什么 Scala 认为 State 不是 Monad？

import cats.Monad
import cats.data.State
import cats.implicits._

import java.util.UUID
import scala.language.higherKinds

case class Credentials(uid: String, pwd: String)
case class Session(sessionId: String, credentials: Credentials)

object LoginProcessTaglessFinal {

  trait LoginProcessAlgebra[F[_]] {
    def captureCredentials(name: String, password: String): F[Credentials]
    def login(credentials: Credentials): F[Session]
  }

  type LoginProcessState = State[LoginProcessAlgebra[_], _]
  type LoginProcessStateA[A] = LoginProcessState[A]

  implicit object LoginProcessInterpreterUsingState extends LoginProcessAlgebra[LoginProcessStateA] {
    override def captureCredentials(name: String, password: String): LoginProcessStateA[Credentials] =
      State(login => (login, Credentials(name, password)))
    override def login(credentials: Credentials): LoginProcessStateA[Session] =
      State(login => (login, Session(UUID.randomUUID().toString, credentials)))
  }

  def program[F[_]: Monad: LoginProcessAlgebra[F]](userName: String, password: String)
    (implicit interpreter: LoginProcessAlgebra[F]): F[Session] = for {
      credentials <-  interpreter.captureCredentials(userName, password)
      session <-  interpreter.login(credentials)
    } yield session


  val sessionState = program("someUserName", "p455w0rd")
  //compile error here 
  //due to 'No implicits found for parameters ...'
}

Answer 1

import scala.language.higherKinds

已弃用。没有必要。

编译错误更早开始

https://scastie.scala-lang.org/DmytroMitin/cVmdwXy7RKSWUbn9V0TBGw（Scala 2）

https://scastie.scala-lang.org/DmytroMitin/cVmdwXy7RKSWUbn9V0TBGw/6（Scala 3）

你在这些行中的种类不正确

type LoginProcessState = State[LoginProcessAlgebra[_], _]
type LoginProcessStateA[A] = LoginProcessState[A]

这似乎是 Scala 3.

我换了

type LoginProcessState = State[LoginProcessAlgebra[_], _]

与

// scalacOptions += "-Ykind-projector"

type LoginProcessState = State[LoginProcessAlgebra[?], *]

（即 类型 lambda

[A] =>> State[LoginProcessAlgebra[?], A]

到 存在类型

LoginProcessAlgebra[?]

，另见 Polymorphic method works with type lambda, but not with type wildcard in Scala 3）。

我也换了

def program[F[_]: Monad: LoginProcessAlgebra[F]](userName: String, password: String)
    (implicit interpreter: LoginProcessAlgebra[F]): F[Session]

与

def program[F[_]: Monad](userName: String, password: String)
    (implicit interpreter: LoginProcessAlgebra[F]): F[Session]

（

LoginProcessAlgebra

可以是上下文绑定，而不是

LoginProcessAlgebra[F]

；你也有上下文绑定和相同的隐式再次，这是不正确的）。

现在错误是

Ambiguous given instances: both value catsStdInstancesForOption in trait OptionInstances and value catsStdInstancesForVector in trait VectorInstances match type cats.Monad[F] of an implicit parameter of method program in object LoginProcessTaglessFinal

https://scastie.scala-lang.org/DmytroMitin/cVmdwXy7RKSWUbn9V0TBGw/3

你只需要指定类型参数

LoginProcessStateA

。无法推断。

val sessionState = program[LoginProcessStateA]("someUserName", "p455w0rd")

https://scastie.scala-lang.org/DmytroMitin/cVmdwXy7RKSWUbn9V0TBGw/5

LoginProcessStateA

好像和

LoginProcessState

一样。我正在删除前者。

https://scastie.scala-lang.org/DmytroMitin/cVmdwXy7RKSWUbn9V0TBGw/13