A brainfuck interpreter in Scala

bf.scala

object Test extends App {
  new Interpreter("++++++++++[>+++++++>++++++++++>+++>+<<<<-]>++.>+.+++++++..+++.>++.<<+++++++++++++++.>.+++.------.--------.>+.>.")
}

class Interpreter(str: String) {

  val prog = """[^\+\-\<\>\.\,\[\]]""".r replaceAllIn (str, "")
  val openBrackets = findBraces(0, Nil, Map.empty)
  val closeBrackets = openBrackets map { _.swap }
  exec()

  def findBraces(pos: Int, braces: List[Int], m: Map[Int, Int]): Map[Int, Int] =
    if (pos >= prog.length) m
    else prog(pos) match {
      case '[' => findBraces(pos+1, pos :: braces, m)
      case ']' => findBraces(pos+1, braces.tail, m + (braces.head -> pos))
      case _ => findBraces(pos+1, braces, m)
    }

  def exec(pos: Int = 0, tape: Tape = Tape(Nil, 0, Nil)): Unit = {
    def nextPos: Char => Int = {
      case '[' if tape.isZero => openBrackets(pos)
      case ']' if !tape.isZero => closeBrackets(pos)
      case _ => pos
    }
    if (pos < prog.length)
      exec(nextPos(prog(pos))+1, tape exec prog(pos))
  }
}

case class Tape(left: List[Int], cell: Int, right: List[Int]) {

  def headOf(xs: List[Int]) = if (xs == Nil) 0 else xs.head
  def tailOf(xs: List[Int]) = if (xs == Nil) Nil else xs.tail
  def isZero = (cell == 0)

  def exec: Char => Tape = {
    case '+' => copy(cell = (cell+1) & 255)
    case '-' => copy(cell = (cell-1) & 255)
    case '<' => Tape(tailOf(left), headOf(left), cell :: right)
    case '>' => Tape(cell :: left, headOf(right), tailOf(right))
    case '.' => print(cell.toChar); this
    case ',' => print(">"); copy(cell = io.StdIn.readInt & 255)
    case '[' | ']' => this
    case c => sys.error("Unexpected token: " + c)
  }
}