RollingSum

[johnynek]

Awful mutability....

    def finish[V:Semigroup](in: Iterator[V]): Iterator[(Option[V], V)] = {
      var prev: Option[V] = None
      in.map { v =>
        val stored = prev
        prev = Some(prev.map(Semigroup.plus(_, v)).getOrElse(v))
        (stored, v)
      }
    }

[ryanlecompte]

Don't feel badly about the var... Iterator's scanLeft (which returns another lazy Iterator) also uses the same var approach! :) https://github.com/scala/scala/blob/master/src/library/scala/collection/Iterator.scala#L487

[sritchie]

@johnynek where do you want to use this? What's the issue? I guess the task is to add this to the Semigroup object?