Scala Lenses

Lenses.scala

/**
 * Created by epentangelo on 27/08/2015.
 */

object Lenses {
  type Lens [a, b] = (a => b, a => b => a)

  def makeLens[a, b](get: a => b, set: a => b => a) : Lens[a, b] = (get, set)

  def get[a, b] (lens: Lens[a, b]) (obj: a) : b = lens._1(obj)

  def set[a, b] (lens: Lens[a, b]) (obj: a) (v:b) : a = lens._2(obj)(v)

  def compose[a, b, c] (x: Lens[a,b]) (y: Lens[b, c]) : Lens[a, c] =
    makeLens[a, c](obj => get (y) (get(x)(obj)), obj => v => set (x) (obj) (set (y) (get (x)(obj)) (v)))
}

case class Person(firstName: String, lastName: String) {
  override def toString() = "(firstName: " + firstName.toString() + " lastName: " + lastName.toString() + ")"
}

case class Family(husband: Person, wife: Person) {
  override def toString() = "(husband: " + husband.toString() + " wife: " + wife.toString() + ")"
}

object Main {
   def main (args: Array[String]) {
     val husband = Person("Homer", "Simpson")
     val wife = Person("Marge", "Simpson")
     val family = Family(husband, wife)
     println(family)

     val wifeLens : Lenses.Lens[Person, String] =
       Lenses.makeLens[Person, String] ({ case Person(firstName, _) => firstName },
                                        { case Person(_, lastName) => (v:String) => Person(v, lastName) })

     val familyLens : Lenses.Lens[Family, Person] =
       Lenses.makeLens[Family, Person] ({ case Family (wife, _) => wife},
                                        { case Family (_, husband) => (v: Person) => Family(husband, v)})

     // println (Lenses.get[Person, String] (wifeLens) (wife))

     // println (Lenses.get[Family, Person](familyLens) (family))

     val composedLens: Lenses.Lens[Family, String] = Lenses.compose (familyLens) (wifeLens)

     println (Lenses.set (composedLens) (family) ("Margareth"))

   }
}