更高种类的隐式参数解析 [英] Implicit parameter resolution for higher kinded types

问题描述

考虑以下代码:

object foo {

    trait Bar[Q[_]]

    implicit object OptionBar extends Bar[Option]

    def test[T, C[_]](c: C[T])(implicit bar: Bar[C]) = ()

    def main(args: Array[String]) {
      test(Some(42): Option[Int])  //???
    }
}

这可行，但是我需要输入Some(42)作为Option [Int]，否则隐式对象OptionBar将无法解析(因为预计会使用Bar [Some]).有没有一种方法可以避免进行显式输入，以便即使我使用某些"或无"输入测试，也可以在测试中获得隐式的OptionBar对象?

This works, but I need to type the Some(42) as Option[Int], else the implicit object OptionBar won't be resolved (because a Bar[Some] is expected instead). Is there a way to avoid the explicit typing, so that I get the implicit OptionBar object in test even if I feed test with a Some or None?

[澄清]

• 我在这里以Option为例，如果我为抽象类等使用Bar，它也应该工作.
• 当其他不相关的条形图在范围内(例如，implicit object listBar extends Bar[list]
)时，该解决方案也应该起作用

• I used Option here just as example, it should also work if I have a Bar for an abstract class etc.
• The solution should also work when other, unrelated Bars are in scope, say implicit object listBar extends Bar[list]

[更新]

似乎使Bar的参数成为协变量可以达到目的:

It seems that making Bar's parameter contravariant does the trick:

object foo {

  trait Bar[-Q[_]] //<---------------

  implicit object OptionBar extends Bar[Option]
  implicit object ListBar extends Bar[List]

  def test[T, C[_]](c: C[T])(implicit bar: Bar[C]) = ()

  def main(args:Array[String]) {
    test(Some(42))
  }
}

但是，这当然是对Bar可能性的严重限制，因此我仍然希望有一个更好的答案.

But of course this is a severe limitation of the possibilities in Bar, so I still hope for a better answer.

推荐答案

它并非在所有情况下都有效，但是如上所述，您可以尝试以下方法:

It's not going to work in all cases, but as stated, you can try this:

object foo {
  trait Bar[Q[_]]

  implicit object OptionBar extends Bar[Option]

  def test[T, C[_], D](c: D)(implicit bar: Bar[C], ev: D <:< C[T]) = ()

  def main(args: Array[String]) {
    test(Some(42)) //???
  }
}

有趣的是，尽管它表达了相同的内容，但这并不能推断:

Interestingly, this doesn't infer, although it expresses the same thing:

def test[T, C[_], D <: C[T]](c: D)(implicit bar: Bar[C]) = ()

要了解有关<:<的更多信息，请参见:

To learn more about <:<, see:

• 做什么< ;:<，<％<和=:=在Scala 2.8中的平均值，它们在哪里记录?
• <:< Scala中的运算符

• What do <:<, <%<, and =:= mean in Scala 2.8, and where are they documented?
• <:< operator in scala

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