[英]Avoid diverging implicit expansion on recursive MTL class
我正在嘗試使用Hlist在cats mtl中創建可組合狀態類型,並定義了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,也調用此隱式定義來解析隱式arg M
,並且Selector和Replacer無法解析,因此它不會在cats.mtl隱式中查找
我嘗試定義S2 <:!<HList(和=:!=),但這根本沒有幫助。
可測試的代碼如下所示:
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的證據(如您所建議的),並在Selector&Replacer之前查找MonadState似乎對我有用。
懷疑它可能與編譯器沒有將S綁定到任何東西有關-只知道它是HList的上限。 (但這是我最好的猜測)
希望這可以幫助。
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
}
}
嘗試
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)
}
}
至少錯誤發生了變化。
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