Question

What I am trying to do is to write a custom class extending Map[DateTime, T]. The extended class is to take a Map[DateTime, T] as a constructor argument and call corresponding methods of the supplied map for every method a Map is to implement.

I had no problem writing

def -(key: DateTime) = iv.-(key)
def get(key: DateTime) = iv.get(key)

but I’ve got stuck on

+[B1 :>B](kv: (A, B1)):Map[A, B1]

How am I meant to write it?

Isn’t there a simple reference Map wrapper implementation to start from?

PS: Don’t offer to declare it as type DtMap[T] = Map[DateTime, T] – I am going to have to add my own methods into the class.

UPDATE: So, the result looks like this, to consolidate it so that somebody would be able to use it as a starting point for his Map extension (may the solution have some hidden problems – please let me know):

case class DtValMap[T](iv: Map[DateTime, T]) extends Map[DateTime, T] {
  def -(key: DateTime) = new DtValMap(iv.-(key))
  def get(key: DateTime) = iv.get(key)
  def +[T1 >: T](kv: (DateTime, T1)): DtValMap[T1] = new DtValMap(iv + kv)
  def iterator: Iterator[(DateTime, T)] = iv.iterator
}

UPDATE2: I’ve noticed the typo which was causing the problem now: I had :> (wrong) instead of >:. But I intentionally don’t correct it in the text above as it would make the question make no sense.

Answer 1

If I understand correctly, there is

class YourMap[T](iv: Map[DateTime, T]) extends Map[DateTime, T] 

Can’t you just do

def +[T1 >: T](kv: (DateTime, T1)): YourMap[T1] = new YourMap(iv + kv)

I’m not too sure about your def -, If I understand correctly, it returns a Map, not a YourMap. If this is ok, why can’t you do just the same for +?

In the line of type DtMap[T], have you considered adding the method with pimp my library, rather than with class extension? Collection classes are complex, and so is extending them.