Martin Odersky's Phone Coder example

Coder.scala

package demo
  
class Coder(words: List[String]) {

  private val mnemonics = Map(
      '2' -> "ABC", '3' -> "DEF", '4' -> "GHI", '5' -> "JKL", 
      '6' -> "MNO", '7' -> "PQRS", '8' -> "TUV", '9' -> "WXYZ")
      
  /** Invert the mnemonics map to give a map from chars 'A' ... 'Z' to '2' ... '9' */
  private val charCode: Map[Char, Char] = 
    for ((digit, str) <- mnemonics; letter <- str) yield letter -> digit
          
  /** Maps a word to the digit string it can represent, e.g. “Java” -> “5282” */
  private def wordCode(word: String): String = word.toUpperCase map charCode
    
  /** A map from digit strings to the words that represent them, 
    * e,g. “5282” -> List(“Java”, “Kata”, “Lava”, ...) 
    * Note: A missing number should map to the empty set, e.g. "1111" -> List()
    */
  private val wordsForNum: Map[String, Seq[String]] = (words groupBy wordCode) withDefaultValue List()
  
  /** Return all ways to encode a number as a list of words */
  def encode(number: String): Set[List[String]] = 
    if (number.isEmpty) Set(List())
    else {
      for {
        split <- 1 to number.length
        word <- wordsForNum(number take split)
        rest <- encode(number drop split)
      } yield word :: rest
    }.toSet

    
  /** Maps a number to a list of all word phrases that can represent it */
  def translate(number: String): Set[String] = encode(number) map (_ mkString " ")
}

object Coder extends App {
  val dict = io.Source.fromFile("/usr/share/dict/words")
     .getLines.filter(_.length > 1).filter(_.matches("[a-zA-Z]+")).toList  
  val coder = new Coder("Scala" :: "rocks" :: dict)
  println(coder.translate("7225276257"))
}