I'm using Scala and I want to extend a (singleton) object with a trait, which delivers a data structure and some methods, like this:
trait Tray[T] {
val tray = ListBuffer.empty[T]
def add[T] (t: T) = tray += t
def get[T]: List[T] = tray.toList
}
And then I'll would like to mix-in the trait into an object, like this:
object Test with Tray[Int]
But there are type mismatches in add and get:
Test.add(1)
// ...
How can I'll get this to work? Or what is my mistake?
The problem is that you're shadowing the trait's type parameter with the T on the add and get methods. See my answer here for more detail about the problem.
Here's the correct code:
trait Tray[T] {
val tray = ListBuffer.empty[T]
def add (t: T) = tray += t // add[T] --> add
def get: List[T] = tray.toList // get[T] --> add
}
object Test extends Tray[Int]
Note the use of extends in the object definition—see section 5.4 of the spec for an explanation of why with alone doesn't work here.
