printnum.hs

import Data.List

main :: IO ()
main = do
    putStrLn "Enter a number"
    number <- getLine
    putStrLn $ longForm number

longForm n =
    let ints = map makeInt n
        strings = toStrings ints 0
    in intercalate " " strings

longForm n = intercalate " " $ toStrings (map makeInt n) 0

makeInt :: Char -> Int
makeInt n = read (n:[])

toStrings :: [Int] -> Int -> [String]
toStrings [] _ = []
toStrings xs depth = leading ++ trailing ++ place
    where trailing = convertTriple (last3 xs)
          leading = toStrings (dropLast3 xs) (depth + 1)
          place
            | trailing == [] = []
            | depth == 0 = []
            | depth == 1 = ["thousand"]
            | depth == 2 = ["million"]
            | depth == 3 = ["billion"]
            | depth == 4 = ["trillion"]
            | depth == 5 = ["quadrillion"]
            | otherwise  = ["bajillion"]

last3 :: [Int] -> (Int, Int, Int)
last3 xs = (a, b, c)
    where list | length xs <  3 = take (3 - length xs) [0,0..] ++ xs
               | length xs >= 3 = drop (length xs - 3) xs
          (a:b:c:[]) = list

dropLast3 :: [a] -> [a]
dropLast3 xs = reverse . (drop 3) $ reverse xs

convertTriple :: (Int, Int, Int) -> [String]
convertTriple (h,t,o)
    | t == 1 && o /= 0 = h' ++ to'
    | otherwise        = h' ++ t' ++ o'
    where h' = if h/=0 then getFromList ones h ++ ["hundred"] else []
          t' = getFromList tens t
          o' = getFromList ones o
          to' = getFromList teens o

getFromList :: [String] -> Int -> [String]
getFromList _ 0 = []
getFromList list n = [list !! (n - 1)]

ones = ["one",
        "two",
        "three",
        "four",
        "five",
        "six",
        "seven",
        "eight",
        "nine"]

tens = ["ten",
        "twenty",
        "thirty",
        "forty",
        "fifty",
        "sixty",
        "seventy",
        "eighty",
        "ninety"]

teens = ["eleven",
        "twelve",
        "thirteen",
        "fourteen",
        "fifteen",
        "sixteen",
        "seventeen",
        "eighteen",
        "nineteen"]