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scala - 找到第二个匹配的隐式

问题描述

考虑以下设置：

trait Foo[A]
object Foo extends Priority2

trait Priority0 {
   implicit def foo1: Foo[Int] = new Foo[Int] {}
}
trait Priority1 extends Priority0 {
   implicit def foo2: Foo[Boolean] = new Foo[Boolean] {}
}
trait Priority2 extends Priority1 {
   implicit def foo3: Foo[Double] = new Foo[Double] {}
}

现在，在 REPL 中（已加载上述代码），我可以执行以下操作：

scala> def implicitlyFoo[A](implicit foo: Foo[A]) = foo
implicitlyFoo: [A](implicit foo: Foo[A])Foo[A]

scala> implicitlyFoo
res1: Foo[Double] = Priority2$$anon$3@79703b86

有没有一种方法可以用一些类型级别的魔法进行编码，我想用 跳过实例A =:= Double，但仍然让类型推断找出是什么A？

我不想影子foo3。这是一个 MVCE：在我的真实情况下，foo3是def带有其他隐含参数的 a（并且可能在派生 other 时发挥间接作用Foo）。

我尝试过=:!=无形但无济于事：

scala> import shapeless._
import shapeless._

scala> def implicitlyFoo2[A](implicit foo: Foo[A], ev: A =:!= Double) = foo
implicitlyFoo2: [A](implicit foo: Foo[A], implicit ev: A =:!= Double)Foo[A]

scala> implicitlyFoo2
<console>:16: error: ambiguous implicit values:
 both method neqAmbig1 in package shapeless of type [A]=> A =:!= A
 and method neqAmbig2 in package shapeless of type [A]=> A =:!= A
 match expected type Double =:!= Double
       implicitlyFoo2
       ^

标签： scalatypesimplicitshapeless

解决方案

Dirty hack 是将宏上下文向下转换为其实现并使用编译器内部。

  import scala.language.experimental.macros
  import scala.reflect.macros.whitebox

  trait Foo[A] {
    def say: String
  }

  trait Priority0 {
    implicit def foo1: Foo[Int] = new Foo[Int] {
      override def say: String = "int"
    }
  }

  trait Priority1 extends Priority0 {
    implicit def foo2: Foo[Boolean] = new Foo[Boolean] {
      override def say: String = "bool"
    }
  }

  trait Priority2 extends Priority1 {
    implicit def foo3: Foo[Double] = new Foo[Double] {
      override def say: String = "double"
    }
  }

  object Foo extends Priority2


  def materializeSecondFoo[A]: Foo[A] = macro impl

  def impl(c: whitebox.Context): c.Tree = {
    import c.universe._

    val context = c.asInstanceOf[reflect.macros.runtime.Context]
    val global: context.universe.type = context.universe
    val analyzer: global.analyzer.type = global.analyzer

    var infos = List[analyzer.ImplicitInfo]()

    new analyzer.ImplicitSearch(
      tree = EmptyTree.asInstanceOf[global.Tree],
      pt = typeOf[Foo[_]].asInstanceOf[global.Type],
      isView = false,
      context0 = global.typer.context.makeImplicit(reportAmbiguousErrors = false),
      pos0 = c.enclosingPosition.asInstanceOf[global.Position]
    ) {
      override def searchImplicit(
                                   implicitInfoss: List[List[analyzer.ImplicitInfo]],
                                   isLocalToCallsite: Boolean
                                 ): analyzer.SearchResult = {
        val implicitInfos = implicitInfoss.flatten
        if (implicitInfos.nonEmpty) {
          infos = implicitInfos
        }
        super.searchImplicit(implicitInfoss, isLocalToCallsite)
      }
    }.bestImplicit

    val secondBest = infos.tail.head

    global.gen.mkAttributedRef(secondBest.pre, secondBest.sym).asInstanceOf[Tree]
  }

  materializeSecondFoo.say // bool

在 2.12.8 中测试。受激励shapeless.Cached。

在 2.13.0materializeSecondFoo.say中应该替换为

val m = materializeSecondFoo
m.say

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