case object of generic trait

scala, traits

Solution

You could use `Nothing` as in:

case object ImplicitMessage extends Message[Nothing]

`Nothing` is a special type which is a subtype of all possible types and has no instances.

If you experience variance problems because of `Message[T]` you can use the following trick:

object ImplicitMessage extends Message[Nothing] {
  def apply[T]: Message[T] = this.asInstanceOf[Message[T]]
}

scala> ImplicitMessage[String]
res1: Message[String] = ImplicitMessage$@4ddf95b5
scala> ImplicitMessage[Long]
res2: Message[Long] = ImplicitMessage$@4ddf95b5

Problem

In Scala: I would like to define a type Message[T] (it needs to have this signature), which can be a message holding some data of type T, or an implicit message. I have ``` trait Message[T] case object ImplicitMessage extends Message <- obviously doesn't compile case class DataMessage[T](d: T) extends Message[T] ``` How should I define ImplicitMessage? I could make it a case class, but that is obviously not that nice, as it only want one instance of it. UPDATE: I know I could simply remove [T] from Message, but I can't (requirement).

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