public enum Either<L, R> {
  case Left(L)
  case Right(R)
 }

func mayThrow() throws -> Int {
  return 100
}

func measureNano<T>(_ block: ()-> T) -> (Int,T) {
   return (0, block())
}

func rescue<T>(_ block: @autoclosure () throws -> T) -> Either<Error,T> {
 do { 
    return .Right(try block()) 
 }
 catch{
    return .Left(error) 
  }
}

func main() {
  measureNano  { // this line will raise a compiler error 
    rescue(try mayThrow()) 
  }
}