Scala Typeclass Example

typeclassExample.scala

//typeclass example

//Note : List already has a sum method working the same way 
//List(1,2,3).sum returns 6
//but this example is simpler

// Monoid = summable + neutral element
trait Monoid[T] {
  def combine(a: T, b: T): T
  def empty: T
}

//default instances
object Monoid {
  implicit val intMonoid: Monoid[Int] = new Monoid[Int] { 
    override def combine(a: Int, b: Int) = a +b 
    override def empty = 0
  }
  implicit val stringMonoid: Monoid[String] = new Monoid[String] { 
    override def combine(a: String, b: String) = a +b 
    override def empty = ""
  }
}

//add sumAll method to Scala List type
implicit class ListWithSumAll[T](list: List[T]){
  def combineAll()(implicit monoid: Monoid[T]): T = 
    list.foldLeft(monoid.empty)((sum, element) => monoid.combine(sum, element))
} 

List(1,2,3).combineAll //6
List("1","2","3").combineAll //123

// add your instances here
case class Color(red: Int, green: Int, blue: Int)

implicit val colorMonoid: Monoid[Color] = new Monoid[Color] {
  override def combine(c1: Color, c2: Color) = {
    val red = Math.min(c1.red + c2.red, 255)
    val green = Math.min(c1.green + c2.green, 255)
    val blue = Math.min(c1.blue + c2.blue, 255)
    Color(red, blue, green)
  }
  
  override def empty = Color(0,0,0)
}

List(Color(10,10,10), Color(200, 255, 10), Color(20, 55, 1)).combineAll // Color(62,66,32)
List[Color](Color(10,10,10)).combineAll // Color(10,10,10)
List[Color]().combineAll // Color(0,0,0)