[英]Unable to recover typeclass instances for types with kind F[A1, A2] with syntax extensions
我一直在使用Scala中的類型類模式來更好地理解它是如何工作的,因為我熟悉Scalaz並且想要弄清楚它是如何工作的“引擎蓋下”。
(您可以使用https://ammonite.io/ REPL運行以下代碼)
import $plugin.$ivy.`org.spire-math::kind-projector:0.9.3`
sealed trait Maybe[+A] // Option
final case class Just[+A](value: A) extends Maybe[A]
final case object Null extends Maybe[Nothing]
sealed trait Direction[+E, +A] // Either
final case class Up[+E, +A](value: A) extends Direction[E, A]
final case class Down[+E, +A](value: E) extends Direction[E, A]
trait Functor[F[_]] {
def map[A, B](fa: F[A])(f: A => B): F[B]
}
object FunctorSyntax {
implicit final class FunctorExtensions[F[_], A](private val self: F[A]) extends AnyVal {
def map[B](f: A => B)(implicit instance: Functor[F]): F[B] = {
instance.map(self)(f)
}
}
}
object FunctorInstances {
implicit val maybeFunctorInstance: Functor[Maybe] = new Functor[Maybe] {
def map[A, B](fa: Maybe[A])(f: A => B): Maybe[B] = fa match {
case Just(a) => Just(f(a))
case n@Null => n
}
}
implicit def directionFunctorInstance[E]: Functor[Direction[E, ?]] = new Functor[Direction[E, ?]] {
def map[A, B](fa: Direction[E, A])(f: A => B): Direction[E, B] = fa match {
case Up(a) => Up(f(a))
case Down(e) => Down(e)
}
}
}
所以,我已經寫了一些類似物Option
( Maybe
),並Either
( Direction
),該Functor
定義,有些情況下Functor
,有的一些語法擴展,所以我可以叫.map
上有效的函子。
以下代碼有效:
import FunctorInstances._
import FunctorSyntax._
val x: Maybe[Int] = Just(5)
println(x.map(_ + 1)) // prints "Just(6)"
正如所料。 但以下不是:
val y: Direction[String, Int] = Up(5)
println(y.map(_ + 1)) // errors out
拋出錯誤help.sc:48: value map is not a member of ammonite.$file.help.Direction[String,Int]
簡單地說,我不希望發生此錯誤,並且.map
可以在任意Direction[E, ?]
.map
Direction[E, ?]
。
我認為Scala無法看到Direction[String, Int]
可以被解構為F = Direction[String, ?]
FunctorExtensions
F = Direction[String, ?]
和A = String
,從而阻止FunctorExtensions
類將自己包裹在val y: Direction[String, Int]
周圍val y: Direction[String, Int]
。 不幸的是,我不知道如何解決這個問題。
注意:實例本身仍然可以implicitly
恢復
val instance = implicitly[Functor[Direction[String, ?]]]
println(instance.map(y)(_ + 1)) // prints "Up(6)"
如果您使用的是Scala 2.11或Scala 2.12,則可能缺少-Ypartial-unification
scalac選項。
將scalacOptions += "-Ypartial-unification"
到您的構建中,或者您可以使用默認情況下啟用它的Scala 2.13。
請參閱https://github.com/typelevel/cats/blob/v1.6.1/README.md#getting-started
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