Filter an iterator by keeping only the lines which are unique by a transformation function

UniqueBy.scala

import scala.collection.AbstractIterator
import scala.collection.Iterator

object UniqueBy extends App {
  val lines = io.Source.fromFile("""somefile""").getLines

  uniqueBy(lines)(_.trim.headOption.getOrElse("")).foreach(println)

  def uniqueBy[T, U](it: Iterator[T])(pred: T => U) = new AbstractIterator[T] {

    var seen = Set.empty[U]
    var n = if (it.hasNext) Some(it.next()) else None

    def hasNext = n.nonEmpty
    def next() = {
      val res = n.get
      seen += pred(res)
      n = None
      while (it.hasNext && n.isEmpty) {
        val nn = it.next()
        if (!seen.contains(pred(nn))) n = Some(nn)
      }
      res
    }
  }
}