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Question

问题是为什么下面的代码不能用于类型推断(下面是一个REPL会话来演示),它可以修复吗？更具体地说,这与编译器用于推断返回类型的CanBuildFrom的使用有何不同？

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(2.8.1)中有这个会话:

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 = $anon$1@155bd22

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 

Answer 1

我可以看到为什么你希望它工作,但显然,类型推理器没有使用返回类型来推断T.我也期待它.

至于歧义,CanBuildFrom通过不在同一"级别"中定义所有内容来避免模棱两可.例如,这解决了歧义问题:

trait LowPriorityImplicits {
  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)}
}

object UrlLike extends LowPriorityImplicits {
  implicit object str extends UrlLike[String]{def url(s: String) = s}
}

但是,它不会使类型推断以您想要的方式工作:

scala> res0.url : URL
<console>:16: error: type mismatch;
 found   : String
 required: java.net.URL
       res0.url : URL
            ^

这表明它显然在T不考虑返回类型的情况下进行推理.

Answer 2

> trait UrlLike[T] {

trait UrlLike[+T] {