Balanced parentheses using StateT

gistfile1.scala

object PragmaticParens {
  import scalaz._, Scalaz._, typelevel._

  private def f(c: Char): StateT[Option, Nat, Unit] =
    StateT(nat => (c match {
      case '(' => Some(nat.succ)
      case ')' => nat.pred
      case _   => Some(nat)
    }) map (_ -> ()))

  def balanced(s: String): Boolean =
    s.toList.traverse_[({type λ[α]=StateT[Option, Nat, α]})#λ](f).exec(Zero).exists(_.isZero)

}