num2text (PT)

-- For Portuguese language, only.
-- It may not work with other languages.

import Data.Maybe (fromJust)

num2ext :: Integer -> String
num2ext n | n <    0 || n >= maxlim = "ERRO"
          | n >=   0 && n <= 9      = f (n,0)
          | n >=  10 && n <  20     = f (n,1)
          | n >=  20 && n < 100     = f (n `div`  10, 2) +:+ if n `mod`  10 == 0 then "" else num2ext (n `mod`  10)
          | n == 100                = "cem"  -- caso especial
          | n >  100 && n < 1000    = f (n `div` 100, 3) +:+ if n `mod` 100 == 0 then "" else num2ext (n `mod` 100)
          | otherwise               = unwords $ map (\(x,y) -> if x `mod` 1000 == 0 then "" else num2ext x +:+ f (y, is1 x)) (reverse $ zip (reverse $ digs3 n) [0..])

    where
        (+:+) [] [] = []
        (+:+) xs [] = xs
        (+:+) [] ys = ys
        (+:+) xs ys = unwords [xs, ys]

        maxlim = (10^) . (*3) $ 5

        is1 1 = 5
        is1 _ = 4

        digs3 0 = []
        digs3 x = let d = x `div` 1000
                      m = x `mod` 1000
                  in digs3 d ++ m:[]

        countdigs = length . digits

        digits x | x >= 10   = (x `div` 10) : digits (x `mod` 10)
                 | otherwise = [x]

        fst3 (x,_,_) = x
        snd3 (_,x,_) = x
        thr3 (_,_,x) = x

        f (c,0) = thr3 . fromJust $ lookup c conv100
        f (c,1) =        fromJust $ lookup c conv10
        f (c,2) = snd3 . fromJust $ lookup c conv100
        f (c,3) = fst3 . fromJust $ lookup c conv100
        f (c,4) = snd .  fromJust $ lookup c convplus
        f (c,5) = fst .  fromJust $ lookup c convplus

        conv10 = zip [10..]
                 ["dez",
                  "onze",
                  "doze",
                  "treze",
                  "catorze",
                  "quinze",
                  "dezasseis",
                  "dezassete",
                  "dezoito",
                  "dezanove"]

        conv100 = zip [0..]
                  [("",            "",         "zero"),
                   ("cento",       "dez",      "um"),
                   ("duzentos",    "vinte",    "dois"),
                   ("trezentos",   "trinta",   "três"),
                   ("quatrocentos","quarenta", "quatro"),
                   ("quinhentos",  "cinquenta","cinco"),
                   ("seiscentos",  "sessenta", "seis"),
                   ("setecentos",  "setenta",  "sete"),
                   ("oitocentos",  "oitenta",  "oito"),
                   ("novecentos",  "noventa",  "nove")]

        convplus = zip [0..]
                   [("",                ""),
                    ("mil",             "mil"),
                    ("milhão",          "milhões"),
                    ("milhar de milhão","milhares de milhões"),
                    ("bilião",          "biliões")]