Four

module Main
where

import Data.List (unwords)
import Data.Char (toUpper)
import Control.Exception (IOException, try)

minNum, maxNum :: Int
minNum = 0
maxNum = 99

minText, maxText :: String
minText = show minNum
maxText = show maxNum

parsingFailedText :: String
parsingFailedText = unwords ["Not a number between", minText, "and", maxText]

getNumber :: IO Int
getNumber = do result <- try (readLn :: IO Int) :: IO (Either IOException Int)
               case result of
                    Left ex   -> fail
                    Right val -> if val >= minNum && val <= maxNum
                                    then return val
                                    else fail
            where fail = do putStrLn parsingFailedText
                            getNumber

capitalize :: String -> String
capitalize (x:xs) = toUpper x : xs

numToStr :: Int -> String
numToStr num
    | num < 0   = error "Error: Number too small"
    | num < 10  = digits !! digit
    | num < 20  = teens  !! digit
    | num < 100 = tens !! (num `div` 10) ++ "-" ++ if digit /= 0
                                                      then digits !! digit
                                                      else ""
    | otherwise = error "Error: Number too big"
    where digit  = num `mod` 10
          digits = ["zero", "one", "two", "three", "four",
                    "five", "six", "seven", "eight", "nine"]
          teens  = ["ten", "eleven", "twelve", "thirteen", "fourteen",
                    "fifteen", "sixteen", "seventeen", "eighteen", "nineteen"]
          tens   = ["zero", "ten", "twenty", "thirty", "forty",
                    "fifty", "sixty", "seventy", "eighty", "ninety"]

compute :: Int -> IO ()
compute num = do
    let str    = capitalize . numToStr $ num
    let newNum = length str
    if num /= 4
       then do putStrLn $ str ++ " is " ++ show newNum
               compute newNum
       else putStrLn "Four is the magic number"

main :: IO ()
main = do putStrLn $ "Input a number between " ++ minText ++ " and " ++ maxText
          num <- getNumber
          compute num