Scala Functional Programming Patterns

Pipes and filters – the Scala version

Here is the Scala version. The for comprehension is similar in spirit to the preceding Unix shell pipe line:

object InvertedIdx extends App {
  val m = List("Carr" -> "And So To Murder",
          "Carr" -> "The Arabian Nights Murder",
          "Carr" -> "The Mad Hatter Mystery",
          "Christie" -> "The Murder Of Roger Ackroyd",
          "Christie" -> "The Sittaford Mystery",
          "Carr" -> "The Plague Court Murders") // 1
  val ignoreWordsSet = Set("To", "The", "And", "So", "Of") // 2
  val invertedIdx = (for { // 3
    (k, v) <- m // 4
    w <- v.split("\\W") // 5
    if !ignoreWordsSet.contains(w) // 6
  }
    yield(w -> k)).groupBy(_._1).map { case (k,v) => (k,v.map(_._2))} // 7
  println(invertedIdx)
}

The code is simple. We just write the code for comprehension, ...