找不到 Tuple2K 的仿函数实例

Question

我有一个玩具 DSL

case class Logging[A](msg: String, action: A)
case class Persist[A](msg: String, action: A)
type Effect[A] = EitherK[Logging, Persist, A]

我想和一个同样玩具的口译员配对

case class CoLogging[A](run: String => A)
case class CoPersist[A](run: String => A)
type Interp[A] = Tuple2K[CoLogging, CoPersist, A]

这是一个程序示例：

def prog(implicit L: Logs[Effect], P: Persists[Effect]): Free[Effect, Unit] =
  P.store("bar") >> L.log("foo")

这里是口译员：

def interpretEffect(implicit CL: CoLogs[IO], CP: CoPersists[IO]): Cofree[Interp, IO[Unit]] = 
  Cofree.unfold(IO.pure(())) { a: IO[Unit] => Tuple2K(CoLogging(CL.coLog(a)), CoPersist(CP.coPersist(a))) }

我已经付出了尽职调查并定义了函子和注入隐式。编译器抱怨找不到实例 cats.Functor[[A]cats.data.Tuple2K[example.CoLogging,example.CoPersist,A]]，即使我正在导入 cats.data.Tuple2K._ where the instance is implicitly defined.

我看不出我做错了什么，这一定是愚蠢的事情。你有什么主意吗？所有代码可见in this gist.

Answer 1

要在您的代码中使用猫函子，您需要尝试在您的 build.sbt 文件中添加设置。

scalacOptions += "-Ypartial-unification"

也许这会帮助你。这是 scala 编译器的限制 https://issues.scala-lang.org/browse/SI-2712

Answer 2

The compiler complains that it cannot find an instance cats.Functor[[A]cats.data.Tuple2K[example.CoLogging,example.CoPersist,A]], even though I am importing cats.data.Tuple2K._ where the instance is implicitly defined.

如果定义了 Functor[F] 和 Functor[G]，则

Functor[Tuple2K[F, G, ?]] 通过 Tuple2KInstances8#catsDataFunctorForTuple2K 定义。问题是 Functor[CoLogging] 和 Functor[CoPersist] 不是。

添加

object CoLogging {
  implicit val coLoggingFunctor: Functor[CoLogging] = new Functor[CoLogging] {
    override def map[A, B](fa: CoLogging[A])(f: A => B): CoLogging[B] = CoLogging(fa.run andThen f)
  }
}

和

object CoPersist {
  implicit val coPersistFunctor: Functor[CoPersist] = new Functor[CoPersist] {
    override def map[A, B](fa: CoPersist[A])(f: A => B): CoPersist[B] = CoPersist(fa.run andThen f)
  }
}

一切都应该编译。

关键是隐式的顺序。将对象 functors 移动到开头，一切都应该编译。