Scala contravariance difficulties

I'm new with Scala so I'm starting with 开发者_开发知识库rewriting my old code in Scala. Now, I'm rewriting a Map, that contains some values and "history" of their modifications (like adding, deleting etc):

import scala.collection.immutable._
class Storage[A,+B](private var oldValues: Map[A,B]) extends Map[A,B] {
    private var addedValues = new HashMap[A,B]
    private var modifiedValues = new HashMap[A,B]
    private var deletedValues = new HashSet[A]  
}

When I overriding method "+" I can't compile it:

override def +[B1 >: B](kv: (A,B1)) = {
    deletedValues = deletedValues - kv._1
    addedValues = addedValues + kv //type mismatch; found : (A, B1) required: (A, B)
    modifiedValues = modifiedValues + kv //type mismatch; found : (A, B1) required: (A, B)
    currentValues()
}

Could someone tell me what can I do in this kind of situation?

---

The problem is the covariance of (the value type) B. Since you're using mutable state, you should probably use the mutable Map trait, which isn't covariant in type B anyway. How about extending the HashMap implementation? The following compiles, but I haven't tested it,

import collection.mutable._

class Storage[A,B](private var oldValues: Map[A,B]) extends HashMap[A,B] {
  private var addedValues: Map[A,B] = new HashMap[A,B]
  private var modifiedValues: Map[A, B] = new HashMap[A,B]
  private var deletedValues: Set[A] = new HashSet[A]  

  // Overriding this method will redefine the behavior of HashMap.put and HashMap.+= 
  override def addEntry(e: DefaultEntry[A, B]) {
    super.addEntry(e)
    // your extension code below
    val kv = (e.key, e.value)
    deletedValues -= kv._1
    addedValues += kv
    modifiedValues += kv
    // currentValues() // not defined yet
  }
}

Calls like storage += (key, value) will use your modified addEntry method. If you haven't done so already, you might want to familiarize yourself with the source-code for HashMap linked from the ScalaDoc.

---

You could make your class immutable.

import scala.collection.immutable._

class Storage[A,+B] private (
   val oldValues: Map[A,B] = Map(),
   val addedValues: Map[A, B] = Map(),
   val modifiedValues: Map[A, B] = Map(),
   val deletedValues: Set[A] = Set()) extends Map[A,B] {
  override def +[B1 >: B](kv: (A,B1)) =
    new Storage(oldValues,
            addedValues + kv,
            modifiedValues + kv,
            deletedValues - kv._1)
  ...
}