StringMapArg.scala

object Example extends MistFn {
  
  // an arguments that extracts Map[String, String]
  val mapArg = allArgs.andThen(params => {
    val r = params.map({
      case (k, v: String) => Right((k,v))
      case (k, any) => Left(Failed.InvalidValue(s"$k key contains invalid value: $any"))
    }).toSeq

    if (r.exists(_.isLeft)) {
      val all = r.collect({case Left(f) => f})
      Failed.ComplexFailure(all)
    } else {
      val value = r.collect({case Right(v) => v}).toMap
      Extracted(value)
    }

  })

  override def handle = {
    withArgs(mapArg).onSparkContext((map: Map[String, String], sc: SparkContext) => {
      ...
    }).asHandle
  }
}