module Calculator where

calculator env = 
   template
      num := []
      haveDot := False
      stack := []
   in let
      digit n = do
         num := n:num
         
      dot = do
         num := '.' : num
         haveDot := True
         
      enter = do
         if num /= [] then
            if not haveDot then
               dot
            digit '0'
            stack   := read (reverse num) : stack
            num     := []
            haveDot := False
         
      op f = do
         enter
         case stack of
            a:b:rest ->
               stack := f b a : rest
               redraw
            _ ->
               err
      
      clear = do
         num     := []
         haveDot := False
         stack   := []
         redraw
         
      redraw = do
         env.putStr "\n# "
         forall n <- reverse stack do
            env.putStr (show n ++ " ")
         env.putStr (reverse num)
      
      err = do
         env.putChar '\BEL'
         redraw
      
      input c = action
         env.putChar c
         case c of 
            ' '             -> enter
            'c'             -> clear
            '+'             -> op (+)
            '-'             -> op (-)
            '*'             -> op (*)
            '/'             -> op (/)
            '\n'            -> redraw
            'q'             -> env.quit
            '.'             -> dot
            c  | isDigit c  -> digit c
            _               -> err

      init = action
         env.putStr "\nWelcome to O'Calc v0.00001\n"
         redraw
         
   in struct ..Prog

struct Prog =
   init   :: Action
   input  :: Char -> Action
   
run prog env = do
   p <- prog env
   p.init
   env.setReader p.input

main = run calculator