How can I restrict types in overridden method in Scala?

Question

trait Rendered
trait TemplateRendered extends Rendered
trait Media {
  def send[T <: Rendered](cooked:T)
}
case class EmailMedia() extends Media {
  override def send(cooked:TemplateRendered) {}  // compile error this line
}

I want to have a Media template with a send() method that accepts a subclass Rendered object. In the concrete classes (EmailMedia) I want to lock that down to a particular subclass of Rendered, or TemplateRendered in this case. (i.e. make the type in the class more restrictive/specific than the type in the trait)

How can I do that?

The attempt here wasn't liked by the compiler. Tried this too:

case class EmailMedia() extends Media {
  override def send[T <: TemplateRendered](cooked:T) {}
}

Have a look at [scala override trait with generic method](http://stackoverflow.com/q/17728998/1194486) question. — falconepl, Sep 21 '15 at 13:59

falconepl · Accepted Answer · 2015-09-21T14:20:04.710

How about:

trait Rendered
trait TemplateRendered extends Rendered

trait Media[T <: Rendered] {
  def send(cooked: T): Unit
}

case class EmailMedia() extends Media[TemplateRendered] {
  override def send(cooked: TemplateRendered): Unit = {}
}

Do you expect Media to provide more than one generic method? If that is the case, you might consider moving that generic parameter to a type member to keep things a bit more readable:

trait Rendered
trait TemplateRendered extends Rendered
trait BazzRendered extends Rendered

trait Media {
  type SendRend <: Rendered
  type FooRend <: Rendered

  def send(cooked: SendRend): Unit
  def foo(bar: FooRend): Unit
}

case class EmailMedia() extends Media {
  type SendRend = TemplateRendered
  type FooRend = BazzRendered

  override def send(cooked: SendRend): Unit = {}
  override def foo(bar: FooRend): Unit = {}
}

This is called F-bound polymorphism, by the way. +1 – dcastro Sep 21 '15 at 14:13 — dcastro, Sep 21 '15 at 14:13

How can I restrict types in overridden method in Scala?

1 Answers1