避免递归 MTL 类的发散隐式扩展答案

【问题标题】：Avoid diverging implicit expansion on recursive MTL class避免递归 MTL 类的发散隐式扩展
【发布时间】：2018-06-15 23:14:11
【问题描述】：

我正在尝试使用 Hlist 在猫 mtl 中创建可组合的状态类型，并定义了 MonadState 如下

implicit def hlistStateMonad[M[_], S <: HList, S2]
(implicit S:Selector[S, S2], R:Replacer[S, S2, S2], M:MonadState[M, S]):MonadState[M, S2]

但是，即使S2 不是 HList，并且 Selector 和 Replacer 无法解析，它也不会在cats.mtl 中查找隐式 arg

我尝试定义 S2 <: hlist>

可测试代码如下：

object Test {
  implicit def hlistStateMonad[M[_], S <: HList, S2](implicit S:Selector[S, S2], R:Replacer[S, S2, S2], M:MonadState[M, S]):MonadState[M, S2] =
    new MonadState[M, S2] {

      val monad: Monad[M] = M.monad

      def inspect[A](f: S2 => A):M[A] =
        M.inspect(s => f(S(s)))

      def modify(f: S2 => S2):M[Unit] =
        M.modify(s => R(s, f(S(s))).asInstanceOf[(S2, S)]._2)

      def get:M[S2] =
        M.inspect(S.apply)

      def set(s2: S2): M[Unit] =
        M.modify(s => R(s, s2).asInstanceOf[(S2, S)]._2)
    }
}

  test("Monad state resolution with HList") {

    type M[V] = State[Int :: String :: HNil, V]

    import cats.mtl.instances.all._
    import Test._

    val m = implicitly[MonadState[M, Int]]
  }

【问题讨论】：

标签： scala shapeless scala-cats

【解决方案1】：

添加查找 S2 不是 HList 的证据（如您所建议的那样）并在 Selector & Replacer 似乎为我解决问题之前查找 MonadState。

怀疑它可能与编译器没有将 S 绑定到任何东西有关——只知道它的上限是 HList。（但这是我最好的猜测）

希望这会有所帮助。

import cats.Monad
import cats.data.State
import cats.mtl.MonadState
import org.scalatest.{FlatSpec, Matchers}
import shapeless._
import shapeless.::
import shapeless.ops.hlist.{Replacer, Selector}
import cats.mtl.instances.all._

class Test extends FlatSpec with Matchers {

  implicit def hlistStateMonad[M[_], S <: HList, S2](
      implicit
      nEv: S2 <:!< HList,
      M: MonadState[M, S],
      S: Selector[S, S2],
      R: Replacer[S, S2, S2]): MonadState[M, S2] =
    new MonadState[M, S2] {

      val monad: Monad[M] = M.monad

      def inspect[A](f: S2 => A): M[A] =
        M.inspect(s => f(S(s)))

      def modify(f: S2 => S2): M[Unit] =
        M.modify(s => R(s, f(S(s))).asInstanceOf[(S2, S)]._2)

      def get: M[S2] =
        M.inspect(S.apply)

      def set(s2: S2): M[Unit] =
        M.modify(s => R(s, s2).asInstanceOf[(S2, S)]._2)
    }

  "Monad state resolution with HList" should "compile and maybe even run?" in {

    type M[V] = State[Int :: String :: HNil, V]

    val m = implicitly[MonadState[M, Int]]

    val res = for {
      a <- m.set(5)
      b <- m.get
    } yield b + 1

    res.run(1 :: "test" :: HNil).value._2 shouldBe 6
  }
}

【讨论】：

【解决方案2】：

试试

import cats.Monad
import cats.data.State
import cats.mtl.MonadState
import shapeless.{::, HList, HNil, Lazy}
import shapeless.ops.hlist.{Replacer, Selector}

import scala.language.higherKinds

object Test {
  implicit def hlistStateMonad[M[_], S <: HList, S2](implicit S:Selector[S, S2], R:Replacer[S, S2, S2], M:Lazy[MonadState[M, S]]):MonadState[M, S2] =
new MonadState[M, S2] {

    val monad: Monad[M] = M.value.monad

    def inspect[A](f: S2 => A):M[A] =
    M.value.inspect(s => f(S(s)))

    def modify(f: S2 => S2):M[Unit] =
    M.value.modify(s => R(s, f(S(s))).asInstanceOf[(S2, S)]._2)

    def get:M[S2] =
    M.value.inspect(S.apply)

    def set(s2: S2): M[Unit] =
    M.value.modify(s => R(s, s2).asInstanceOf[(S2, S)]._2)
  }
}

至少错误发生了变化。

【讨论】：