Reverse Polish Notation Calculator

-- Reverse Polish Notation (v1.0.0, a newbie's script :D)
import qualified Data.Char as C

rpn :: String -> String
rpn expr = let res = calc [] $ words expr
           in show $ (if length res == 1 then head else last) res
    where
        isOper = (`elem` ["+","-","*","/","^"])
        isNum = all (== True) . map (C.isNumber)

        calc l [] | length l == 1 = l
                  | otherwise     = ["There aren't sufficient operators."]
        calc l (x:xs) | isOper x  = if length l >= 2
                                       then if (x == "/") && (ultimate == 0)
                                                then ["Division by zero"]
                                                else calc (operate x) xs
                                       else ["There aren't sufficient operands."]
                      | isNum x   = calc (push x l) xs
                      | otherwise = ["There are tokens which aren't numbers or operators."]
            where
                operate "+" = push (show $ penultimate + ultimate) begin
                operate "-" = push (show $ penultimate - ultimate) begin
                operate "*" = push (show $ penultimate * ultimate) begin
                operate "/" = push (show $ penultimate / ultimate) begin
                operate "^" = push (show $ penultimate ^ (read $ show penultimate :: Int)) begin

                begin = init $ init l
                ultimate = read $ last l :: Float  -- because we cannot use the identifier 'last', so it's similar to 'penultimate'
                penultimate = read (last $ init l) :: Float

                push n = (++ [n])

-- INPUT BUG: power operator generates an error. Use the function directly and not this 'main' block.
main :: IO()
main = do
    putStr ">> "
    n <- (readLn :: IO String)
    if (/= "EXIT") (map (C.toUpper) n)
        then do
            putStrLn $ "Result: " ++ (rpn n)
            mainrpn
        else putStrLn "End."