Functional solution to the Python indentation problem check

IndentationCheck.scala

import scala.io.Source

object IndentationCheck {
  
  def isValid(filename: String): Boolean = isValid(Source.fromFile(filename).getLines.toSeq)
  
  def isValid(lines: Seq[String]): Boolean = {
    val initialAccum: Option[Set[Int]] = Some(Set(0))
    lines.map(indentationOf).foldLeft(initialAccum) { (accum, i) =>
      accum match {
        case Some(history) if i >= history.max => Some(history + i)
        case Some(history) if i < history.max && history.contains(i) => Some(history.filter(_ <= i))
        case _ => None
      }
    }.isDefined
  }
  
  private def indentationOf(line: String) = line.takeWhile(_.isWhitespace).size
}