Question

Possible Duplicate:
scala: adding a method to List?

I struggle to formulate what I’m trying to do, but the code example should be pretty straightforward. If anyone knows a better way to phrase it, you are free to edit the title. 🙂

trait DiceThrow {
  list: List[Int] =>   // something like this??
  def yatzee = list.filter(_ == list.head).length >= 5
}

object Main extends App {
  val aThrow = List(4,4,4,4,4) with DiceThrow
  aThrow.yatzee  // => true    is what I want
}

So I want the aThrow: List[Int] to have some extra methods, like knowing whether it’s a yatzee or not. This is just one example I made up where adding some extra methods to e.g. a List could be useful.

Is this possible somehow? Or is there another approach that’s more the scala way? I believe it’s possible with implicit conversion(?)(they’re still pretty “magic” to me), but that seems unnecessarily dirty this use-case?

Answer 1

You can use enrich (pimp) my library pattern:

class DiceList(list: List[Int]) {
  def yatzee = list.filter(_ == list.head).length >= 5
}

implicit def list2DiceList(list: List[Int]) = new DiceList(list)

In scala 2.10 it could be simplified with implicit classes:

implicit class DiceList(list: List[Int]) {
  def yatzee = list.filter(_ == list.head).length >= 5
}

Then you could use it like:

object Main extends App {
  val aThrow = List(4,4,4,4,4)
  aThrow.yatzee  // => true
}