applicative_example

OptionGolf.scala

import scalaz._

/**
 * A simple example of how applicative functors can shorten code
 * adapted from chapter 8? of Real World Haskell
 */


object OptionGolf {

  import Scalaz._

  val moviel = Map("title" -> "South Park", 
		  "user" -> "Terrence", 
		  "rating" -> "3")

  val incompleteMovie = Map("name" -> "Jaws")

  case class MovieReview(revTitle: String, revUser: String, revReview: String)

  /*
   * the ugly way
   */
  def rev1(movie: Map[String,String]): Option[MovieReview] = 
    movie get "title" match {
      case None => None
      case Some(title) => movie get "user" match {
	case None => None
	case Some(user) => movie get "rating" match {
	  case None => None
	  case Some(rating) => Some(MovieReview(title,user,rating))
	}
      }
    }
    
  /*
   * using monads: slightly better
   */
  def rev2(movie: Map[String,String]): Option[MovieReview] = 
    for (title <- movie get "title";
	 user <- movie get "user";
	 rating <- movie get "rating")
    yield MovieReview (title,user,rating)

  /*
   * but with applicative functors you can express it all in one line
   */
  def rev3(movie: Map[String,String]): Option[MovieReview] = 
    movie.get("title") <*> (movie.get("user") <*> (movie.get("rating") ∘ MovieReview.curried))

  def main(args: Array[String]) {
    println(rev3(moviel))
    println(rev3(incompleteMovie))
  }

}