[SOLVED] Types in Scala

Types in Scala - lower bounds

on code below.

My expectation is that T must be a of type B or A, so call to lowerBound(new D) should probably not compile (?). Similar experiments with upperbound give me expected typecheck errors.

Thanks for giving the hint.

object varianceCheck {
  class A {
    override def toString = this.getClass.getCanonicalName
  }

  class B extends A
  class C extends B
  class D extends C

  def lowerBound[T >: B](param: T) = { param }

  println(lowerBound(new D))                      //> varianceCheck.D
}

Solution

With your implementation you can write:

scala>   def lowerBound[T >: B](param: T) = { param }
lowerBound: [T >: B](param: T)T

scala> lowerBound(new AnyRef {})
res0: AnyRef = $anon$1@2eef224

where AnyRef is a super type of all object/reference types (actually it is an alias for Java Object class). And this is right, T >: B expresses that the type parameter T or the abstract type T refer to a supertype of type B.

You just have a bad example with toString, cause this method has all object types, but if you change it to, let's say on someMethod, your lowerBound won't compile:

<console>:18: error: value someMethod is not a member of type parameter T
       def lowerBound[T >: B](param: T) = { param.someMethod }

If you change this to T <: B, which means that parameter of type T is a subclass of B, than everything is good, cause this param has someMethod method:

def lowerBound[T <: B](param: T) = { param.someMethod }