Question

我一直在使用Scala中的类型类模式，但是当我使用的类型是通用的时，我无法弄清楚如何实现隐式伴随对象。

例如，假设我已经为类型类定义了一个特征，它提供了将事物放入Box es的函数。

case class Box[A](value: A)

trait Boxer[A] {
  def box(instance: A): Box[A]
  def unbox(box: Box[A]): A
}

implicit object IntBoxer extends Boxer[Int] {
  def box(instance: Int) = Box(instance)
  def unbox(box: Box[Int]) = box.value
}

def box[A : Boxer](value: A) = implicitly[Boxer[A]].box(value)
def unbox[A : Boxer](box: Box[A]) = implicitly[Boxer[A]].unbox(box)

这符合预期，允许我为各种类型提供Boxer的实现。但是，当我希望采用的类型是通用的时候，我有没有的想法我会怎么做。假设我希望能够在任何Boxer上使用Seq[A]。 Scala中的object不能包含类型参数，所以我不知道该去哪里：

// Will not compile - object cannot have type arguments
implicit object SeqBoxer[A] extends Boxer[Seq[A]] { ... }

// Will not compile - 'A' is unrecognized
implicit object SeqBoxer extends Boxer[Seq[A]] { ... }

// Compiles but fails on execution, as this doesn't implement an implicit
// conversion for _specific_ instances of Seq
implicit object SeqBoxer extends Boxer[Seq[_]] {
  def box(instance: Seq[_]) = Box(instance)
  def unbox(box: Box[Seq[_]]) = box.value
}

// Will not compile - doesn't technically implement Boxer[Seq[_]]
implicit object SeqBoxer extends Boxer[Seq[_]] {
  def box[A](instance: Seq[A]) = Box(instance)
  def unbox[A](box: Box[Seq[A]]) = box.value
}

// Compiles, but won't resolve with 'implicitly[Boxer[Seq[Foo]]]'
// I had high hopes for this one, too :(
implicit def seqBoxer[A]() = new Boxer[Seq[A]] {
  def box(instance: Seq[A]) = Box(instance)
  def unbox(box: Box[Seq[A]]) = box.value
}

有没有办法支持泛型类型的隐式转换，而不必为每个内部类型隐含一个单独的对象？

Scala Typeclasses与泛型

0 个答案: