斯卡拉，类型类和“找不到隐含价值”

Question

我面临着以下类型类的一些奇怪的问题：由于某种原因，隐式对象ContentUploader没有呼叫决心上传DemoActor方法。

import akka.actor.Actor 
import java.io.File 
import org.slf4j.LoggerFactory 

class DemoActor extends Actor { 

    import DemoActor.UploaderImpl._ 

    override def receive = { 
    case (x: DemoActor.Content) => 
     DemoActor.upload(x) 
    } 

} 

object DemoActor { 

    val LOG = LoggerFactory.getLogger("DemoActor") 

    sealed trait UploadData { 
    val data: Array[File] 
    } 

    case class Content(data: Array[File]) extends UploadData 

    case class UploadResult(url: String, contentType: String, size: Long) 

    trait S3Uploader[T <: UploadData] { 

    def uploadToS3(filez: Array[File]): Iterable[UploadResult] 

    } 

    object UploaderImpl { 

    val LOG = LoggerFactory.getLogger("Uploader") 

    private def contentType(name: String): String = { 
     "application/octet-stream" 
    } 

    private def doUpload(filez: Array[File], bucketName: String) = { 
     LOG.debug("Uploading: {} to {}", filez, bucketName) 
     filez.flatMap { 
     case f => 
      try { 
      val key = f.getName 
      val mime = contentType(f.getName) 
      Some(UploadResult("http://" + bucketName + ".s3.amazonaws.com/" + key, mime, f.length())) 
      } catch { 
      case e => 
       LOG.error("Can not upload", e) 
       None 
      } 
     } 
    } 

    implicit object ContentUploader extends S3Uploader[Content] { 

     lazy val bucketName = "resources.aws.bucketname" 

     lazy val awsSecret = "resources.aws.secret.key" 

     lazy val awsAccess = "resources.aws.access.key" 

     override def uploadToS3(filez: Array[File]) = doUpload(filez, bucketName) 

    } 

    } 

    def upload[T <: UploadData](src: T)(implicit uploader: S3Uploader[T]) = uploader.uploadToS3(src.data) 


} 

我在这里错过了什么？

UPD

如果我搬到类DemoActor的定义里面对象 DemoActor，像

import akka.actor.Actor 
import java.io.File 
import org.slf4j.LoggerFactory 

object DemoActor { 

    val LOG = LoggerFactory.getLogger("DemoActor") 

    sealed trait UploadData { 
    val data: Array[File] 
    } 

    case class Content(data: Array[File]) extends UploadData 

    case class UploadResult(url: String, contentType: String, size: Long) 

    trait S3Uploader[UploadData] { 

    def uploadToS3(filez: Array[File]): Iterable[UploadResult] 

    } 

    object UploaderImpl { 

    val LOG = LoggerFactory.getLogger("Uploader") 

    private def contentType(name: String): String = { 
     "application/octet-stream" 
    } 

    private def doUpload(filez: Array[File], bucketName: String) = { 
     LOG.debug("Uploading: {} to {}", filez, bucketName) 
     filez.flatMap { 
     case f => 
      try { 
      val key = f.getName 
      val mime = contentType(f.getName) 
      Some(UploadResult("http://" + bucketName + ".s3.amazonaws.com/" + key, mime, f.length())) 
      } catch { 
      case e => 
       LOG.error("Can not upload", e) 
       None 
      } 
     } 
    } 

    implicit object ContentUploader extends S3Uploader[DemoActor.Content] { 

     lazy val bucketName = "resources.aws.bucketname" 

     lazy val awsSecret = "resources.aws.secret.key" 

     lazy val awsAccess = "resources.aws.access.key" 

     override def uploadToS3(filez: Array[File]) = doUpload(filez, bucketName) 

    } 

    } 

    def upload[T <: UploadData](src: T)(implicit uploader: S3Uploader[T]) = uploader.uploadToS3(src.data) 

    class DemoActor extends Actor { 

    import DemoActor.UploaderImpl._ 

    override def receive = { 
     case (x: DemoActor.Content) => 
     DemoActor.upload(x) 
    } 

    } 


} 

则一切正常。有没有命名空间的问题？

Answer 1

它没有找到它，因为隐式前向引用必须明确键入才能被考虑，而这不是。

如果这是令人困惑的，也许有两种解决方法可能会说清楚。首先，你可以声明隐式的类型。除去来自物体的implicit，并声明一个val指向它：

implicit val contentUploader: S3Uploader[DemoActor.Content] = ContentUploader 

第二种方式移动的class DemoActor声明的文件的末尾，所以它的所述object DemoActor声明之后保持。

它这样工作的原因是，编译器必须在文件的其余部分完全键入之前搜索隐式，所以它当时不知道该对象ContentUploader满足搜索。