避免在递归 MTL 类上发散隐式扩展

Question

我正在尝试使用 Hlist 在猫 mtl 中创建一个可组合的状态类型，并定义了一个 MonadState，如下所示

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，调用此隐式定义是为了解析隐式参数M，即使S2不是 HList，并且选择器和替换器无法解析，因此它不会查找 cats.mtl 隐式

我尝试定义 S2 <：！<HList(和>

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(如您所建议的那样(，并在选择器和替换器之前查找MonadState似乎已经为我完成了这个技巧。

怀疑这可能与编译器未将 S 绑定到任何东西有关 - 只是知道它是 HList 的上限。 (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
}
}

Answer 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)
}
}

至少错误更改。