[英]Avoid diverging implicit expansion on recursive MTL class
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,即使S2
不是HList,也调用此隐式定义来解析隐式arg M
,并且Selector和Replacer无法解析,因此它不会在cats.mtl隐式中查找
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]]
}
添加查找S2不是HList的证据(如您所建议的),并在Selector&Replacer之前查找MonadState似乎对我有用。
怀疑它可能与编译器没有将S绑定到任何东西有关-只知道它是HList的上限。 (但这是我最好的猜测)
希望这可以帮助。
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
}
}
尝试
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。
声明:本站的技术帖子网页,遵循CC BY-SA 4.0协议,如果您需要转载,请注明本站网址或者原文地址。任何问题请咨询:yoyou2525@163.com.