covariance question

you added a type parameter and named it Double, which shadowed the actual primitive Double

def doh[String](s:String):java.lang.String = s

tools like intellij idea prevent this type of error. this is what they (can) do (if configured to do it):
 


you can immediately spot the problem



Am 06.02.2012 22:44, schrieb mc:

1e1ac6a4-334c-4bbb-93b3-f1fff3348a89 [at] p21g2000yqm [dot] googlegroups [dot] com" type="cite">

Never mind, I found the problem:

  class StrangeIntQueue extends SimpleQueue[Double] {

    def enqueue(item: Double) = { //<<< REMOVED [Double]
      println(item.toString())
      super.enqueue(item)
    }
  }

I'm not sure what exactly was I breaking, but that was the problem.
Thanks!

On Feb 6, 4:36 pm, mc videm [dot]  [dot]  [dot]  [at] gmail [dot] com (<videm [dot]  [dot]  [dot]  [at] gmail [dot] com>) wrote:

Hello,

I created this test superclass (allowing covariant types to be
passed):

  class SimpleQueue[+A](val list: List[A]) {

    def this() = this(List[A]())
    def enqueue[B >: A](item: B): SimpleQueue[B] = new SimpleQueue[B]
(list :+ item)
  }

This is the concrete implementation which is supposed to use Double as
element type:

  class StrangeIntQueue extends SimpleQueue[Double] {

    def enqueue[Double](item: Double): SimpleQueue[Double] = {
      println(item.toString())
      super.enqueue(item) //<<< THIS DOES NOT COMPILE
    }
  }

I'm getting the following compilation error:
"type mismatch; found : TypeParameterization.this.SimpleQueue[Any]
required: TypeParameterization.this.SimpleQueue[Double]"

I understand the error message - Scala compiler is down casting Double
into the lowest common type.
However I'm not sure why is this happening? Method 'enqueue' is
parametrized by type Double, so B is Double and A is Double.
So there should be no need to down cast? Or is it that the compiler
does not know the actual type yet and is down casting based on
'enqueue' declaration?

I would appreciate an explanation of this problem, and maybe some info
on how to make it work?
Thanks!