scala - 为什么 scala 编译器说这种类型用在非特化位置？

Question

我在 package 对象中有这个方法:

def extractLoop[@specialized T](x: Map[T, T]) = {
    val whatever = x.head
    val stop = whatever._1
    def iteration(
            acc: Seq[T] = Seq(whatever._1, whatever._2),
            last: T = whatever._2): Seq[T] = {
        val next = x(last)
        if (next == stop) acc
        else iteration(acc :+ next, next)
    }
    iteration()
}

但我还是不明白，为什么编译器(我有 2.9.2 版)说 type T is unused or used in non-specializable positions.?

Answer 1

我认为原因很简单，您使用的是非专用的 Map[T, T]。

一些插图:

scala> class MyMap[A,B]
defined class MyMap
scala> def extractLoop[@specialized T](x: MyMap[T, T]) = {
     |   sys.error("TODO")
     | }
<console>:8: warning: type T is unused or used in non-specializable positions.
       def extractLoop[@specialized T](x: MyMap[T, T]) = {
           ^
extractLoop: [T](x: MyMap[T,T])Nothing

但是如果你特化 MyMap 的两个类型参数，你不会有警告:

scala> class MyMap[@specialized A,@specialized B]
defined class MyMap
scala> def extractLoop[@specialized T](x: MyMap[T, T]) = {
     |   sys.error("TODO")
     | }
extractLoop: [T](x: MyMap[T,T])Nothing