scala - 避免递归 MTL 类上的隐式扩展发散

Question

我正在尝试使用 Hlist 在 cats mtl 中创建可组合状态类型，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]

然而，即使 S2 不是 HList，并且 Selector 和 Replacer 无法解析并且它不会在cats.mtl 隐式

我尝试定义 S2 <:!< HList (和 =:!=)，但这根本没有帮助。

可测试的代码如下所示:

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]]
  }

Answer 1

添加查找 S2 不是 HList 的证据(如您所建议的)并在选择器和替换器之前查找 MonadState 似乎对我来说已经成功了。

怀疑这可能与编译器没有将 S 绑定(bind)到任何东西有关 - 只是知道它的上限是 HList 。(但这是我最好的猜测)

希望这有帮助。

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
  }
}