Question

Really struggling to figure out extending the immutable Set with a class that will represent a Set of concrete type. I’m doing this to try and create a nice DSL.

I’d like to have a class Thing, and when you add ‘things’ together you get a ThingSet object, which extends Set.

class Thing(val name:String){
  def +(other: Thing):ThingSet = new ThingSet() + other
}

I just can’t figure out how to make the ThingSet object. I know I need to mix in traits like GenericSetTemplate, SetLike etc. But I just can’t make it work.

Please, can anybody give me some pointers, as I can’t find anything explicit enough to learn from. I’ve tried looking at the BitSet and HashSet implementations, but get lost.

Answer 1

Adapting from this Daily Scala post as well as the source to BitSet, which is a wonderful example here because it is not a parameterized collection, and is rather short and simple.

import scala.collection.SetLike
import scala.collection.generic.{GenericSetTemplate, GenericCompanion, CanBuildFrom}
import scala.collection.mutable.{Builder, SetBuilder}

class ThingSet(seq : Thing*) extends Set[Thing] 
                             with SetLike[Thing, ThingSet]
                             with Serializable {
    override def empty: ThingSet = new ThingSet()
    def + (elem: Thing) : ThingSet = if (seq contains elem) this 
        else new ThingSet(elem +: seq: _*)
    def - (elem: Thing) : ThingSet = if (!(seq contains elem)) this
        else new ThingSet(seq filterNot (elem ==): _*)
    def contains (elem: Thing) : Boolean = seq exists (elem ==)
    def iterator : Iterator[Thing] = seq.iterator
}

object ThingSet {
    def empty: ThingSet = new ThingSet()
    def newBuilder: Builder[Thing, ThingSet] = new SetBuilder[Thing, ThingSet](empty)
    def apply(elems: Thing*): ThingSet = (empty /: elems) (_ + _)
    def thingSetCanBuildFrom = new CanBuildFrom[ThingSet, Thing, ThingSet] {
        def apply(from: ThingSet) = newBuilder
        def apply() = newBuilder
    }
}