Scala best way of turning a Collection into a Map-by-key? (2nd variant)

(This is a variant to this Q&A) Say I have this: ``` List( "foo", "bar", "spam" ) ``` I want to create a Map for which the key is the length of the String and the value is a Collection of all the Strings that have that length. In other words, given the about List, we'd get: ``` Map( 3 -> List(foo, bar), 4 -> List(spam) ) ``` The code I've written to do this is: ``` list.foldLeft(Map[Long, List[String]]()) { (m, s) => m(s.length) = s :: ( if ( m.contains(s.length) ) m(s.length) else Nil ) } ``` This works, but it adds a lot of ugliness to the elegant answer Daniel Spiewak provided to the original question (referenced above). Any ideas how to improve the solution for my variant? Thanks! Sean