alt.scala

optional[String, Int](S.param("optional"), Helpers.asInt(_))

forOptional.scala

def optional[A,B](v: Box[A], func: A => Box[B]): Box[Box[B]] = 
  v match {
    case Full(v) => func(v) match {
      case Full(v) => Full(Full(v))
      case Empty => Empty
      case Failure(msg, ex, chain) => Failure(msg, ex, chain)
      case ParamFailure(msg, ex, chain, param) => ParamFailure(msg, ex, chain, param)
    }
    case Empty => Full(Empty) 
    case Failure(msg, ex, chain) => Failure(msg, ex, chain)
    case ParamFailure(msg, ex, chain, param) => ParamFailure(msg, ex, chain, param)
  }

/* In lift you would typically do: */
for {
  requiredInt <- S.param("required") ?~ "required is missing" ~> 400 // 1
  requiredInt <- Helpers.asInt(requiredInt) ?~ "required must be an integer" ~> 400 // 2 
  optionalInt <- optional(S.param("optional"), (x:String)=>Helpers.asInt(x)) ?~ 
    "optional must be an integer" ~> 400 // 3
} yield (requiredInt, optionalInt)