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我想采用这种模式：

 def accept[T](a: RList[T]) = true
 def accept[T, V](a: RList[T], b: RList[V])(implicit ev: a.S =:= b.S) = true
 def accept[T, V, Q](a: RList[T], b: RList[V], c: RList[Q])(implicit ev: a.S =:= b.S, ev2: b.S =:= c.S) = true

但让它接受 a KList，而不是手动覆盖所有的arities。基本上我想说，“取任意数量的RList具有相同S成员类型的 s”

RList是一个特征，它包含一个 type S。（有关 RList 的更多背景信息以及我这样做的原因，请参阅：Constrain function based on origin (Path Dependent type?Type Generation?)）

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1 回答 1

2

It looks like what you are doing is just trying to get the compiler to check that the types agree, since your methods ALWAYS return true.

Can you perhaps, instead of making methods which accept an arbitrary number of these, accept a "List of RLists" which is guaranteed to have S all matching?

Here is how such a list might be constructed:

package rl {

// A simplified version of your RList:
trait RList[T] {
  type S
  def data: List[T]
}

// A list of RLists which have identical S
sealed trait RListList 

// RListNil is an empty list
trait RListNil extends RListList {
  def ::[H <: RList[_]](h: H) = rl.::[h.S,H,RListNil](h, this)
}
// there is exactly one RListNil
case object RListNil extends RListNil

// List can be a cons cell of lists sharing the same S
final case class ::[S, H <: RList[_], T <: RListList](head: H, tail: T) extends RListList {

  // We only allow you to cons another to this if we can find evidence that the S matches
  def ::[H2 <: RList[_]](h: H2)(implicit ev: =:=[h.S,S]) = rl.::[S,H2,::[S,H,T]](h, this)
}

Now, if we try to construct a RListList that doesn't have all the S types agreeing, the compiler will catch us:

object RListTest {

  val list1 = new RList[Int] { type S = String; def data = List(1,2,3,4) }
  val list2 = new RList[String] { type S = String; def data = List("1","2","3","4") }
  val list3 = new RList[Double] { type S = Float; def data = List(1.1,2.2,3.3,4.4) }

  val listOfLists1 = list1 :: RListNil // fine
  val listOfLists2 = list2 :: listOfLists1 // still fine, since list1 and list2 have the same S
  val listOfLists3 = list3 :: listOfLists2 // compiler error: Cannot prove that java.lang.String =:= Float

}

This is using dependent method types, which means you need to use scala 2.10 or you need to compile with the -Ydependent-method-types switch in 2.9.x

于 2012-08-21T06:09:53.767 回答