问题是为什么下面的代码不能使用类型推断(下面是要演示的REPL会话),它可以被修复吗?更具体地说,这和编译器用来推断返回类型的CanBuildFrom的用法有什么不同?含糊不清的暗示
鉴于此代码:
object S {
import java.net._
trait UrlLike[T] {
def url(s: String): T
}
object UrlLike {
implicit object str extends UrlLike[String]{def url(s: String) = s}
implicit object url extends UrlLike[URL]{def url(s: String) = new URL(s)}
implicit object uri extends UrlLike[URI]{def url(s: String) = new URI(s)}
}
trait UrlSupport {
val _url: String
def url[T : UrlLike]: T = implicitly[UrlLike[T]].url(_url)
}
}
我在REPL这届(2.8.1):
scala> :load c:\temp\UrlTest.scala
Loading c:\temp\UrlTest.scala...
defined module S
scala> import java.net._
import java.net._
scala> import S._
import S._
scala> new UrlSupport{val _url = "http://example.com"}
res0: java.lang.Object with S.UrlSupport = [email protected]
scala> res0.url : String
<console>:14: error: ambiguous implicit values:
both object uri in object UrlLike of type object S.UrlLike.uri
and object url in object UrlLike of type object S.UrlLike.url
match expected type S.UrlLike[T]
res0.url : String
^
scala> res0.url : URL
<console>:14: error: ambiguous implicit values:
both object uri in object UrlLike of type object S.UrlLike.uri
and object url in object UrlLike of type object S.UrlLike.url
match expected type S.UrlLike[T]
res0.url : URL
^
scala> res0.url[String]
res3: String = http://example.com
scala> res0.url[URL]
res4: java.net.URL = http://example.com
编译器是否看不到由于类型归属而导致String/URL/URI的结果? – IttayD 2011-03-22 14:45:56