ksson

{
type expr =
  | Op of (float -> float -> float)
  | Const of float
  | NL
}

let digit = ['0'-'9']
let float = digit+ ('.' digit+)?

rule token = parse
| [ ' ' '\t' ] { token lexbuf }
| float as f { let f = float_of_string f in Const f }
| '+' { Op (+.) }
| '/' { Op (/.)  }
| '-' { Op (-.) }
| '*' { Op ( *. ) }
| '\n' { NL }
| eof { exit 0 }
| _ as c { failwith (Printf.sprintf "invalid char: %c\n" c) }

{
let main =
  let lexbuf = Lexing.from_channel stdin in
  let rec aux stack =
    match token lexbuf with
    | Const f -> aux (f::stack)
    | NL -> Format.printf "%f@." (List.hd stack); aux stack
    | Op f ->
        match stack with
        | x :: y :: r -> aux ( f y x :: r)
        | _ -> assert false
  in
  aux []
}