http://blog.higher-order.com/blog/2012/09/13/what-purity-is-and-isnt/

what-purity-is-and-isnt.scala

sealed trait Sigma
case class Lit(s: String) extends Sigma
case class Concat(s: Sigma, t: Sigma) extends Sigma
case class Ext extends Sigma

def eval1(sig: Sigma): Sigma = sig match {
  case Concat(s, t) => {
    val Lit(e1) = eval1(s)
    val Lit(e2) = eval1(s)
    Lit(e1 + e2)
  }
  case Ext => Lit(readLine)
  case x => x
}

def eval2(sig: Sigma, stdin: String): Sigma = sig match {
  case Concat(s, t) => {
    val Lit(e1) = eval2(s)
    val Lit(e2) = eval2(s)
    Lit(e1 + e2)
  }
  case Ext => Lit(stdin)
  case x => x
}