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

current := 0.0

loop {
  ("[ ", current, " ]").concat.println
  ().read_line.try(
    (line) -> {
      input := line.trim
      num := (input, 1).substring.trim.parse_float.try(
        (val) -> val
        () -> 0.0
      )
      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 = (current, num).div
      } else if mode.eq("=") {
        &current = num
      } else if input.eq("exit") {
        (())
      } else {
        "Expected one of +-*/= followed by a number".eprintln
      }
    }
    // CTRL+D
    () -> (())
  )
}