http://codepad.org/onebEvej

toEnglish.hs

import Control.Applicative
import Control.Monad
import Data.Char
import Data.List
import System.IO

num2wstr :: Integer -> Maybe [String]
num2wstr 0         = Just []
num2wstr n | n < 0 = (:) <$> Just "minus" <*> (num2wstr $ abs n)

num2wstr n | n < 100 = let
  	concat  = (++) <$> tens <*> ("" : map ('-' :) simple)
		simple  = ["one", "two", "three", "four", "five", "six", "seven", "eight", "nine"]
		simplex = ["ten", "eleven", "twelve", "thirteen", "fourteen", "fifteen", "sixteen", "seventeen", "eighteen", "nineteen"]
		tens    = ["twenty", "thirty", "forty", "fifty", "sixty", "seventy", "eighty", "ninety"]
	in Just [(simple ++ simplex ++ concat) !! (fromIntegral $ n - 1)]

num2wstr n = foldM dot [] $ transpose [numbers, tail numbers]
	where
		dot' n xn xs = (\x y -> x ++ [xs] ++ y)
			<$> num2wstr (n `div` xn)
			<*> num2wstr (n `mod` xn)
		
		dot a@(_:_) _ = Just a
		dot a ([(xn,xs),(yn,_)])
			| n < xn * yn `div` xn = dot' n xn xs
			| otherwise            = Just a
		dot a ([(xn,xs)])
			| n < xn * xn          = dot' n xn xs
			| otherwise            = Nothing
		
		numbers = [
			(10 ^    2, "hundred"),
			(10 ^    3, "thousand"),
			(10 ^    6, "million"),
			(10 ^    9, "billion"),
			(10 ^   12, "trillion"),
			(10 ^   15, "quadrillion"),
			(10 ^   18, "quintillion"),
			(10 ^   21, "sextillion"),
			(10 ^   24, "septillion"),
			(10 ^   27, "octillion"),
			(10 ^   30, "nonillion"),
			(10 ^   33, "decillion"),
			(10 ^   36, "undecillion"),
			(10 ^   39, "duodecillion"),
			(10 ^   42, "tredecillion"),
			(10 ^   45, "quattuordecillion"),
			(10 ^   48, "quindecillion"),
			(10 ^   51, "sexdecillion"),
			(10 ^   54, "septendecillion"),
			(10 ^   57, "octodecillion"),
			(10 ^   60, "novemdecillion"),
			(10 ^   63, "vigintillion"),
			(10 ^   66, "unvigintillion"),
			(10 ^   69, "duovigintillion"),
			(10 ^   72, "tresvigintillion"),
			(10 ^   75, "quattuorvigintillion"),
			(10 ^   78, "quinquavigintillion"),
			(10 ^   81, "sesvigintillion"),
			(10 ^   84, "septemvigintillion"),
			(10 ^   87, "octovigintillion"),
			(10 ^   90, "novemvigintillion"),
			(10 ^   93, "trigintillion"),
			(10 ^   96, "untrigintillion"),
			(10 ^   99, "duotrigintillion"),
			(10 ^  102, "trestrigintillion"),
			(10 ^  105, "quattuortrigintillion"),
			(10 ^  108, "quinquatrigintillion"),
			(10 ^  111, "sestrigintillion"),
			(10 ^  114, "septentrigitillion"),
			(10 ^  117, "octotrigintillion"),
			(10 ^  120, "noventrigintillion"),
			(10 ^  123, "quadragintillion"),
			(10 ^  153, "quinquagintillion"),
			(10 ^  183, "sexagintillion"),
			(10 ^  213, "septuagintillion"),
			(10 ^  243, "octogintillion"),
			(10 ^  273, "nonagintillion"),
			(10 ^  303, "centillion"),
			(10 ^  306, "uncentillion"),
			(10 ^  309, "duocentillion"),
			(10 ^  312, "trescentillion"),
			(10 ^  333, "decicentillion"),
			(10 ^  336, "undecicentillion"),
			(10 ^  363, "viginticentillion"),
			(10 ^  366, "unviginticentillion"),
			(10 ^  393, "trigintacentillion"),
			(10 ^  423, "quadragintacentillion"),
			(10 ^  453, "quinquagintacentillion"),
			(10 ^  483, "sexagintacentillion"),
			(10 ^  513, "septuagintacentillion"),
			(10 ^  543, "octogintacentillion"),
			(10 ^  573, "nonagintacentillion"),
			(10 ^  603, "ducentillion"),
			(10 ^  903, "trecentillion"),
			(10 ^ 1203, "quadringentillion"),
			(10 ^ 1503, "quingentillion"),
			(10 ^ 1803, "sescentillion"),
			(10 ^ 2103, "septingentillion"),
			(10 ^ 2403, "octingentillion"),
			(10 ^ 2703, "nongentillion")]

num2wstr' :: Integer -> String
num2wstr' 0 = "Zero."
num2wstr' x =
	case num2wstr x of
		Just xs -> toUpper' . (++ ".") . intercalate " " $ xs
		Nothing -> "Number too large."
	where
		toUpper' (x:xs) = toUpper x : xs

main :: IO()
main = forever $ do
	putStr "> "
	hFlush stdout
	x <- getLine
	case reads x :: [(Integer, String)] of
		[(x, y)] | null $ y \\ " " -> putStrLn $ num2wstr' x
		otherwise                  -> putStrLn "Not a valid number."