在单例对象内找不到隐式值 [英] Could not find implicit value inside singleton object

问题描述

我有此代码:

trait Context {
  implicit val e: Encoder

  trait Encoder {
    def write(): Unit = {
      println("Test")
    }
  }

}

trait AsyncEncoders {
  this: Context =>

  class AsyncEncoder extends Encoder {
  }

  implicit val e = new AsyncEncoder()
}

class ConcreteContext extends Context with AsyncEncoders {
}

当我这样使用它时(案例1 ):

When I use it like this (case 1):

object Main extends App {
  implicit val c = new ConcreteContext()

  import c._

  implicitly[Encoder].write()
}

然后编译并打印Test.

但是当我尝试在单例对象内调用相同的代码时(情况2 ):

But when I try to call the same code inside singleton object (case 2):

object TestObject {
  def apply()(implicit c: ConcreteContext): Unit = {
    import c._
    implicitly[Encoder].write()
  }
}

object Main extends App {
  implicit val c = new ConcreteContext()

  TestObject()
}

编译失败，并显示以下信息:

compilation fails with:

path/to/Main.scala:29:找不到参数e的隐式值:c.Encoder 隐式地[c.Encoder] .write()

path/to/Main.scala:29: could not find implicit value for parameter e: c.Encoder implicitly[c.Encoder].write()

如果我更改了(情况3 ):

implicit val e = new AsyncEncoder()

到

implicit val e: Encoder = new AsyncEncoder()

然后编译并按预期运行.

then it compiles and runs as expected.

但是由于某种原因，这对我来说是不可接受的.

But for some reason this is not acceptable for me.

在上述情况下为什么编译失败?

Why does compilation fail in the above case?

推荐答案

在评论中说，Scala 2.12.0中没有问题.

As it said in comments there is no issue in Scala 2.12.0.

对于Scala 2.11.8，我使用以下变通办法，假设Encoder只有一种方法:

For Scala 2.11.8 I used the following workaround assuming that Encoder will have only one method:

trait Context {
  implicit val e: Encoder

  type BaseEncoder = () => Unit

  type Encoder <: BaseEncoder
}

trait AsyncEncoders {
  this: Context =>

  type Encoder = AsyncEncoder

  class AsyncEncoder extends BaseEncoder {
    override def apply(): Unit = {
      println("Test")
    }
  }

  implicit val e = new AsyncEncoder()
}

class ConcreteContext extends Context with AsyncEncoders {
}

object TestObject {
  def apply()(implicit c: ConcreteContext): Unit = {
    import c._
    implicitly[Encoder].apply()
  }
}

object Main extends App {
  implicit val c = new ConcreteContext()

  TestObject()
}

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