"- Calculator -".println
"Type =<num> to set the value to that number.".println
"Type +<num>, -<num> or *<num> to change the value.".println
"Type exit to exit.".println

current := 0.0

{() -> {
  ("[ ", current, " ]").concat.println
  input := ().read_line.trim
  num := (input, 1).substring.trim.parse_float.try((
    () -> 0.0,
    val -> val
  ))
  mode := input.substring(0, 1)
  if mode.eq("+") {
    &current = (current, num).sum
  } else if mode.eq("-") {
    &current = (current, (num, -1).product).sum
  } else if mode.eq("*") {
    &current = (current, num).product
  } else if mode.eq("=") {
    &current = num
  } else if input.eq("exit") {
    (())
  }
}}.loop