我正在阅读Odersky和Spoon的article。 所以这里有一个代码清单,从这篇文章中有一个特殊的方面:
import scala.collection.mutable.{Builder, MapBuilder}
import scala.collection.generic.CanBuildFrom
object PrefixMap extends {
def empty[T] = new PrefixMap[T]
def apply[T](kvs: (String, T)*): PrefixMap[T] = {
val m: PrefixMap[T] = empty
for (kv <- kvs) m += kv
m
}
def newBuilder[T]: Builder[(String, T), PrefixMap[T]] =
new MapBuilder[String, T, PrefixMap[T]](empty)
implicit def canBuildFrom[T]
: CanBuildFrom[PrefixMap[_], (String, T), PrefixMap[T]] =
new CanBuildFrom[PrefixMap[_], (String, T), PrefixMap[T]] {
def apply(from: PrefixMap[_]) = newBuilder[T]
def apply() = newBuilder[T]
}
}
为什么他们将伴侣对象声明为object PrefixMap extends {...}
而不仅仅是object PrefixMap {...}
?
你能暗示在这种情况下延伸的目的吗?