Bored on the plane without sudoku I thought of writing my own board.

sudokuBoard.scala

class SudokuBoard {
  val size = 9
  private val sudokuRange = 0 until size
  val board = sudokuRange.map { _ =>
    sudokuRange.map(_ => 'E').toArray
  }.toArray

  def stringify = board.map(_.mkString("[", ", ", "]")).mkString("\n")

  def assign(row: Int, column: Int, value: Char) =
    if (isValidMove(row, column, value)) board(row)(column) = value

  def isValidMove(row: Int, column: Int, value: Char): Boolean = board(row)(column) match {
    case 'E' => allowedInRow(value, row) && allowedInColumn(value, column) && allowedInSubGrid(row, column, value)
    case _ => throw new IllegalStateException("Cannot move to occupied cell")
  }

  private def allowedInRow(value: Char, row: Int) = !board(row).contains(value)
  private def allowedInColumn(value: Char, column: Int) = sudokuRange.forall( board(_)(column) != value)
  private def allowedInSubGrid(row: Int, column: Int, value: Char) = {
    val subGridXRange = subGridRangeOf(row)
    val subGridYRange = subGridRangeOf(column)
     subGridXRange.forall(row => subGridYRange.forall(board(row)(_) != value))
  }

  def subGridRangeOf(idx: Int) = idx match {
    case  x if x < 3  => 0 to 2
    case  x if x < 6  => 3 until 5
    case  x if x < 9  => 6 until 9
  }
}