函数缺少参数实例的隐式值(仅在sbt清理之后)

Question

我用小猫做了一些实验(https://github.com/milessabin/kittens)并且在编译我的代码时遇到问题。我收到以下错误。

[error] ...danireyscalakittensKittens.scala:23: could not find implicit value for parameter instance: cats.Functor[danirey.scala.kittens.AdtDefns.Tree]
[error]     val funct = Functor[Tree]
[error]                        ^
[error] one error found
[error] (compile:compileIncremental) Compilation failed

完整的文件如下

package danirey.scala.kittens
/**
  * @author Dani
  */
import cats.Functor
import cats.syntax.AllSyntax
import cats.derived.functor._
import legacy._
import cats.derived.iterable.legacy._
import org.typelevel.discipline.scalatest.Discipline
import shapeless.cachedImplicit
object Kittens extends App {
  val ft = new FunctorExperiment()
  ft.print()
}
class FunctorExperiment extends AllSyntax {
  import AdtDefns._
  def print():Unit = {
    val funct = Functor[Tree]
    val tree: Tree[String] = Node(
        Leaf("Reto"),
        Node(
            Leaf("Sandra"),
            Leaf("Mike")
        )
    )
    println(funct.map(tree)(_.length))
  }
}

我在ScalaTest中使用了几乎相同的代码，它的编译没有任何问题。

package danirey.scala.kittens
import cats.Functor
import cats.syntax.AllSyntax
import cats.derived.functor._
import legacy._
import cats.derived.iterable.legacy._
import org.scalatest.FunSuite
import org.typelevel.discipline.scalatest.Discipline
import shapeless.cachedImplicit
/**
  * @author Dani
  */
class FunctorExperimentTest extends FunSuite with Discipline with AllSyntax {
  import AdtDefns._
  test("functors experiment") {
    val funct = Functor[Tree]
    val tree: Tree[String] = Node(
        Leaf("Reto"),
        Node(
            Leaf("Sandra"),
            Leaf("Mike")
        )
    )
    println(funct.map(tree)(_.length))
  }
}

我的build.sbt如下name:="无定形实验"

version := "1.0-SNAPSHOT"
scalaVersion := "2.11.8"
exportJars := true
libraryDependencies ++= Seq(
  "com.chuusai" % "shapeless_2.11" % "2.3.0",
  "org.typelevel" % "kittens_2.11" % "1.0.0-M2",
  "org.scalatest"   %% "scalatest"      % "3.0.0-M7" % "test"
)
scalacOptions ++= Seq(
  "-feature",
  "-language:higherKinds",
  "-language:implicitConversions",
  "-unchecked"
)

最有趣的是，它作为增量编译的一部分进行编译。

如果我注释第16、23和32行，然后执行"sbt-compile"，然后再次删除注释并执行它编译的"sbt编译/包"，我甚至可以执行该程序。但一旦我运行"sbt-clean"，它就不会再编译了。

AdtDefns对象基本上是https://github.com/milessabin/kittens/blob/master/core/src/test/scala/cats/derived/adtdefns.scala相关部分为

object AdtDefns {
  sealed trait Tree[T]
  final case class Leaf[T](t: T) extends Tree[T]
  final case class Node[T](l: Tree[T], r: Tree[T]) extends Tree[T]
}

PS：如果有人能为scala小猫创建一个标签，那就太好了

Answer 1

@DaniRey我们在项目中使用了小猫，但只使用了序列部分。我不知道有任何项目使用小猫衍生。你的用户案例是什么？