Scala vs Haskell类型分类: "catchall" instances

以下Haskell类型的类和实例:

class Able a where
  able :: a -> Int

instance Able Int where
  able x = x

通常像这样翻译成Scala:

trait Able[A] {
  def able(a: A): Int
}

implicit object AbleInt extends Able[Int] {
  def able(a: Int) = a
}

现在，在Haskell中，我可以定义一种包罗万象的实例，从而为所有Maybe类型创建一个实例:

instance Able a => Able (Maybe a) where
  able (Just a) = able a
  able Nothing  = 0

如果存在Able，Maybe Int等的实例Maybe Bool，则此方法为Able，Int等定义了Bool的实例。

在Scala中如何做到这一点？

最佳答案

您将根据对等类型A的实例的隐式参数构造实例。例如:

implicit def AbleOption[A](implicit peer: Able[A]) = new Able[Option[A]] {
  def able(a: Option[A]) = a match {
    case Some(x) => peer.able(x)
    case None    => 0
  }
}

assert(implicitly[Able[Option[Int]]].able(None)    == 0)
assert(implicitly[Able[Option[Int]]].able(Some(3)) == 3)