Code from the Functional Programming in Scala session.

Mnemonics.scala

import org.scalatest.FunSuite

class MnemonicsDemoSolution extends FunSuite {

  class Coder(val words: List[String]) {
    val mnem = Map(
      '2' -> "ABC", '3' -> "DEF", '4' -> "GHI", '5' -> "JKL",
      '6' -> "MNO", '7' -> "PQRS", '8' -> "TUV", '9' -> "WXYZ")
    /** Invert the mnem map to give a map from chars 'A'...'Z' to '2'...'9' */
    val charCode: Map[Char, Char] =
      for ((digit, word) <- mnem; letter <- word)
      yield (letter -> digit)
    //for ((digit, word) <-mnem; letter <- word)) yield letter -> digit
    /** Maps a word to the digit string it can represent e.g. "Java" -> "5282" */
    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", ...) */
    val wordsForNum: Map[String, Seq[String]] = words.groupBy(word => wordCode(word)) 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 {
          splitPoint <- (1 to number.length)
          word <- wordsForNum(number take splitPoint)
          rest <- encode(number drop splitPoint)
        } yield word :: rest
    }.toSet

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


  test("mnemonics") {
    val dict: List[String] = "rocks" :: io.Source.fromFile("/usr/share/dict/words")
      .getLines.filter(_.length > 2).filter(_.matches("[a-zA-Z]+")).toList


    val coder = new Coder(dict)
    val mnemonics = coder.translate("7225276257")
    println(mnemonics)
    assert(mnemonics.contains("scala rocks") == true)
  }
}