this is how I would have refactored

getopt_stew.scala

  import scalaz.ValidationNel
  import scalaz.syntax.traverse._
  import scalaz.syntax.validation._
  import scalaz.std.list._

  sealed trait GetOptError
  case object UnknownArgument extends GetOptError
  case object EmptyElement extends GetOptError

  private def parseArguments(i: Int, element: String): List[ValidationNel[GetOptError, (Char, Option[String])]] = 
    if(i == element.length) Nil else {
      val c = element.charAt(i)
      options_map.get(c) match {
        // unknown argument, save that and keep looking for more errors
        case None => UnknownArgument.failNel :: parseArguments(i+1, element)

        // when we hit an opt with arguments, we grab the rest of the element as the argument
        case Some(true) => List((c, Some(element.substring(i+1))).successNel)

        // an opt without arguments, we recurse on the subsequent member of the string
        case Some(false) => (c, None).successNel :: parseArguments(i+1, element)
      }
    }

  private def validateElement(element: String): ValidationNel[GetOptError, String] =
    if( element == "--" )
      UnknownArgument.failNel
    else if( element.length == 0 )
      EmptyElement.failNel
    else 
      element.successNel

  private def parseElement(element:String):ValidationNel[GetOptError, Map[Char, Option[String]]] =
    for {
      valid <- validateElement(element)
      args <- parseArguments(0, valid).sequenceU
    } yield args.toMap
}