容器嵌套类型的 Scala 隐式转换

Question

考虑以下示例：

case class A()

case class B()

object Conversions {
  implicit def aToB(a: A): B = B()

  implicit def convert[U, T](seq: Seq[U])(implicit converter: U => T): Seq[T] = {
    seq.map(converter)
  }
}

object Main {
  import Conversions._

  def main(args: Array[String]): Unit = {

    val sa = Seq(A())

    def example(): Seq[B] = sa
  }
}

这个例子不会被版本 2.11.8 的 scala 编译器编译。我使用 IntelliJ Idea 进行编译，但实际上 idea 不会即时产生错误并显示隐式用于转换： Screenshot from Intellij Idea
为了解决这个问题，我使用了此处描述的方法： "Scala: Making implicit conversion A->B work for Option[A] -> Option[B]"
我的代码开始如下所示：

case class A()

case class B()

object Conversions {
  implicit def aToB(a: A): B = B()

  trait ContainerFunctor[Container[_]] {
    def map[A, B](container: Container[A], f: A => B): Container[B]
  }

  implicit object SeqFunctor extends ContainerFunctor[Seq] {
    override def map[A, B](container: Seq[A], f: (A) => B): Seq[B] = {
      Option(container).map(_.map(f)).getOrElse(Seq.empty[B])
    }
  }

  implicit def functorConvert[F[_], A, B](x: F[A])(implicit f: A => B, functor: ContainerFunctor[F]): F[B] = functor.map(x, f)
}

object Main {

  import Conversions._

  def main(args: Array[String]): Unit = {

    val sa = Seq(A())

    def example(): Seq[B] = sa
  }
}

这段代码编译得很好，可以按需运行。

我的问题是：
为什么第一种方法编译失败？
这在某种程度上与类型擦除有关吗？如果是，Functor 的使用如何帮助它？
编译器如何为这两种情况解析隐式？

Answer 1

Why the first approach fails to compile?

I've opened a bug for this issue.

这似乎是隐式搜索中的编译器怪癖。由于您提供了转换 Seq[A] => Seq[B] 的 convert 方法，因此编译器无法正确对齐类型。这是使用 Ytyper-debug:

编译的输出

|    [search #3] start `[U, T](seq: Seq[U])(implicit converter: U => T)Seq[T]` inferring type T, searching for adaptation to pt=A => T (silent: method example in Test) implicits disabled
|    [search #3] considering aToB
|    |-- { ((a: A) => Conversions.aToB(a)) } : pt=A => ? EXPRmode (silent: method example in Test) implicits disabled
|    |    |-- ((a: A) => Conversions.aToB(a)) : pt=A => ? EXPRmode (silent: method example in Test) implicits disabled
|    |    |    |-- Conversions.aToB(a) EXPRmode (silent: value $anonfun in Test) implicits disabled
|    |    |    |    |-- Conversions.aToB BYVALmode-EXPRmode-FUNmode-POLYmode (silent: value $anonfun in Test) implicits disabled
|    |    |    |    |    \-> (a: A)B
|    |    |    |    |-- a : pt=A BYVALmode-EXPRmode (silent: value $anonfun in Test) implicits disabled
|    |    |    |    |    \-> A
|    |    |    |    \-> B
|    |    |    \-> A => B
|    |    \-> A => B
|    [adapt] aToB adapted to { ((a: A) => Conversions.aToB(a)) } based on pt A => T
|    [search #3] solve tvars=?T, tvars.constr= >: B
|    solving for (T: ?T)
|    [search #3] success inferred value of type A => =?B is SearchResult({
|      ((a: A) => Conversions.aToB(a))
|    }, TreeTypeSubstituter(List(type T),List(B)))
|    solving for (A: ?A)
|    solving for (A: ?A)
|    solving for (A: ?A)
|    solving for (A: ?A)
|    [search #3] considering $conforms
|    solving for (A: ?A)
|    [adapt] $conforms adapted to [A]=> <:<[A,A] based on pt A => T
|    [search #3] solve tvars=?T, tvars.constr= >: A
|    solving for (T: ?T)
|    [search #3] success inferred value of type A => =?A is SearchResult(scala.Predef.$conforms[A], TreeTypeSubstituter(List(type T),List(A)))

似乎搜索 #3 正在尝试调整 conforms (<:<)，这需要整个隐式搜索从 A => B 到 A => A。如果我用 -Yno-predef 编译，隐式转换成功：

|    |    |-- [U, T](seq: Seq[U])(implicit converter: U => T)Seq[T] : pt=Seq[B] EXPRmode (silent: method example in Test) implicits disabled
|    |    |    [search #4] start `[U, T](seq: Seq[U])(implicit converter: U => T)Seq[T]`, searching for adaptation to pt=A => B (silent: method example in Test) implicits disabled
|    |    |    [search #4] considering aToB
|    |    |    |-- { ((a: A) => Conversions.aToB(a)) } : pt=A => B EXPRmode (silent: method example in Test) implicits disabled
|    |    |    |    |-- ((a: A) => Conversions.aToB(a)) : pt=A => B EXPRmode (silent: method example in Test) implicits disabled
|    |    |    |    |    |-- Conversions.aToB(a) : pt=B EXPRmode (silent: value $anonfun in Test) implicits disabled
|    |    |    |    |    |    |-- Conversions.aToB BYVALmode-EXPRmode-FUNmode-POLYmode (silent: value $anonfun in Test) implicits disabled
|    |    |    |    |    |    |    \-> (a: A)B
|    |    |    |    |    |    |-- a : pt=A BYVALmode-EXPRmode (silent: value $anonfun in Test) implicits disabled
|    |    |    |    |    |    |    \-> A
|    |    |    |    |    |    \-> B
|    |    |    |    |    \-> A => B
|    |    |    |    \-> A => B
|    |    |    [adapt] aToB adapted to { ((a: A) => Conversions.aToB(a)) } based on pt A => B
|    |    |    [search #4] success inferred value of type A => B is SearchResult({
|    |    |      ((a: A) => Conversions.aToB(a))
|    |    |    }, )
|    |    |    |-- [U, T](seq: Seq[U])(implicit converter: U => T)Seq[T] : pt=Seq[B] EXPRmode (silent: method example in Test) implicits disabled
|    |    |    |    \-> Seq[B]
|    |    |    [adapt] [U, T](seq: Seq[U])(implicit converter: U => T)Seq[T] adapted to [U, T](seq: Seq[U])(implicit converter: U => T)Seq[T] based on pt Seq[B]
|    |    |    \-> Seq[B]
|    |    [adapt] Seq[A] adapted to [U, T](seq: Seq[U])(implicit converter: U => T)Seq[T] based on pt Seq[B]
|    |    \-> Seq[B]
|    \-> [def example] ()Seq[B]

Is this somehow related to type erasure and if yes, how usage of Functor helps with it?

第二个示例之所以有效，是因为您现在明确说明了如何使用 Functor 类型 class 将 Seq[A] 映射到 Seq[B] 以及何时编译器看到一个 Seq[A]，它有一个隐含的将它转换为 Seq[B]:

def example(): Seq[B] = Conversions.functorConvert[Seq, A, B](sa)({
        ((a: A) => Conversions.aToB(a))
}, Conversions.SeqFunctor);

请注意，您需要从 A => B 和 Functor[Seq] 进行转换才能映射所有 A 以将它们转换为 B，这是它使用 conversions.aToB.

的作用

容器嵌套类型的 Scala 隐式转换

Scala implicit conversion of container nested types

scala

implicit

functor

implicit-conversion

scala-compiler