Scala abstract method param question, List of T

Question

I have a helper method:

def controlStructure[T <: SomeObject](exceptions: Class[_]*)(body: => T) = { 
    try { 
        val tempObject = body 
        tempObject.callSomeMethod 
        Some(tempObject) 
    } catch { 
        case e if (exceptions.contains(e.getClass)) => None 
    } 
} 

called with:

controlStructure[MySomeObject](classOf[Exception]) { getMySomeObjectSomehow } 

the main point of which is to call the 'callSomeMethod' on the entity passed in (for example loaded from ORM), it incidentally wraps things up in exception handling too.

I would now like to add a new method which does the same thing but for a collection (java.util.List) of T.

I am unsure of the syntax, and structures to work with a collection of T in the method signature, and abstract type param definitions.

Thanks for your help.

Answer 1

There's no pass by name vararg in Scala. You have to pass a function if you want this. See this ticket of an enhancement request to this effect.

Answer 2

With a scala list, you are wanting something like this (I think):

  def controlStructure[T <: SomeObject](exceptions: Class[_]*)(body: => List[T]) = {
    try {
      val tempObject = body
      tempObject.foreach { _.callSomeMethod() }
      Some(tempObject)
    }
    catch {
      case e if (exceptions.contains(e.getClass)) => None
    } 
  }

I haven't worked with Java lists in scala, so I'm guessing you could do it with java.util.List like this:

  def controlStructure[T <: SomeObject](exceptions: Class[_]*)(body: => java.util.List[T]) = {
    import scala.collection.JavaConversions._
    try {
      val tempObject = body
      tempObject foreach { _.callSomeMethod() }
      Some(tempObject)
    }
    catch {
      case e if (exceptions.contains(e.getClass)) => None
    } 
  }

Answer 3

Thanks for your help Mitch. It turns out the answer is in this case to specify the return type of the method, as java.util.List[T], as for once Scala is not using its magic type inference to sort everything out. I am not sure why.

Perhaps someone can answer that ?