A recursive function which verifies the balancing of parentheses in a string, which we represent as a List[Char]. Recfun 2013, Week 1.

Balance.scala

package recfun
import common._
import scala.annotation.tailrec
 
object Balance {
  def balance(chars: List[Char]): Boolean = {
    @tailrec
    def loop(chars: List[Char], open: Int): Boolean = {
      chars match {
        case Nil => open == 0
        case '(' :: xs => loop(xs, open + 1)
        case ')' :: xs => (open > 0) && loop(xs, open - 1)
        case any => loop(chars.tail, open)
      }
    }
    loop(chars, 0)
  }
}