More info at http://lehrblogger.com/2009/04/09/programming-a-to-z-assignment-9-evolvocabulary/

Evolvocabulary.scala

def readAllFiles(fileNames: List[String]): Map[String, Map[String, Int]] = {
  val wordsInFiles: List[(String, String)] = for {
    filename <- fileNames
    line <- filename.toFile.readLines()
    word <- line.split("\\W+|\\d+")
  } yield (word.toLowerCase.trim, filename)

  val emptyMap: Map[String, Map[String, Int]] = Map.empty.withDefaultValue(Map.empty.withDefaultValue())

  wordsInFiles.foldLeft(emptyMap) { case (map, (word, filename)) => 
      map.update(word, map.apply(word).update(filename, map.apply(word).apply(filename) + 1))
    //map.update(word, map.apply(word).update(filename, map(word)(filename) + 1))
    //map.update(word, map.apply(word)(filename) = map(word)(filename) + 1)
    //map.update(word, map(word)(filename) = map(word)(filename) + 1)
    //map.update(word, map(word)(filename) += 1)
    //map(word) = (map(word)(filename) += 1)
  }
}