An immutable MultiMap for Scala

The Scala collections library (in version 2.7.7) has a MultiMap trait for mutable collections, but none for immutable ones. I hacked something up to use while waiting for an official version. I’m finding this to work well, but I don’t have much experience with collections design, so it’s likely to have some flaws. Also, this is a class and not a trait, so you can’t use it with any map you like. And from a concurrency perspective, maybe it’s sometimes better to use backing collections other than the HashSet and the HashMap.

 
import scala.collection.immutable._
 
/**
A multimap for immutable member sets (the Scala libraries 
only have one for mutable sets). 
*/
class MultiMap[A, B](val myMap: Map[A, Set[B]]) {
 
	def this() = this(new HashMap[A, Set[B]])
 
	def +(kv: Tuple2[A, B]): MultiMap[A, B] = {
	  val set = if (myMap.contains(kv._1)) {
		  myMap(kv._1) + kv._2
	  } else {
		  new HashSet() + kv._2	     	   
	  }
 
	  new MultiMap[A, B](myMap + ((kv._1, set)))
	}
 
	def -(kv: Tuple2[A, B]): MultiMap[A, B] = {
	  if (!myMap.contains(kv._1)) {
	    throw new Exception("No such key")
	  }
	  val set = myMap(kv._1) - kv._2
	  if (set.isEmpty) {
	    new MultiMap[A, B](myMap - kv._1)
	  } else {
		  new MultiMap[A, B](myMap + ((kv._1, set)))
	  }
	}
 
	def entryExists(kv: Tuple2[A, B]): Boolean = {
	  if (!myMap.contains(kv._1)) {
	    false
	  } else {
	    myMap(kv._1).contains(kv._2)
	  }
	}
 
    def keys = myMap.keys
 
     def values: Iterator[Set[B]] = myMap.values
 
    def getOrElse(key: A, elval: Collection[B]): Collection[B] = {      
      myMap.getOrElse(key, elval)
    }
 
    def apply(key: A) = myMap(key)
 
 
 
}

Usage:

 
   var theMultiMap = new MultiMap[String, Int]()
 
   theMultiMap += (("george", 1))
   theMultiMap += (("george", 3))
   theMultiMap += (("bob", 2))
   theMultiMap -= (("george", 1))

Monomorphic