Question

I’d like to build a Map like this:

  def one = "one"
  def two = "two"
  def three = Some("three")

  Map[String, String]("one" -> one, "two" -> two, "three" -> three)

This won’t compile because the method three returns an Option instead of a String.
I can make this work like this:

Map[String, String]("one" -> one, "two" -> two) ++ three.map(t => Map("three" -> t)).getOrElse(Map.empty[String, String])

Now it will only add the Option to the list when it’s Some.

But there must be a more elegant way. (lift-json for example knows how to filter out Options when constructing JValue’s).

Any suggestions?
(P.S. I’ve simplified the problem here)

Answer 1

In order to provide a nice interface to your clients, you could extend one of the Map‘s to perform the unpacking:

class MyMap[A, B](private val tuples: (A, Option[B])*)
    extends collection.DefaultMap[A, B] {

  private val container =
    new collection.mutable.HashMap[A, B]()

  container ++= tuples collect {case (k, Some(v)) => (k, v)}

  def iterator = container.iterator
  def get(id: A) = container.get(id)
  override def size = container.size
}

Combine this with an implicit that turns pairs (A, B) into pairs (A, Option[B]):

implicit def pairToPairWithSomeValue[A, B](t: (A, B)): (A, Option[B]) =
  (t._1, Some(t._2))

And use it as:

def one = "one"
def two = "two"
def three = Some("three")
def four = None


val mm = new MyMap("one" -> one, "two" -> two, "three" -> three,
                   "four" -> four)

mm foreach println
  /* (two,two), (one,one), (three,three) */