Exercise from page 294 of Haskell Book - Numbers into words

Page_294.hs

module Page294 where

--Numbers into words

import Data.List (intersperse)

digitToWord :: Int -> String
digitToWord 0 = "zero"
digitToWord 1 = "one"
digitToWord 2 = "two"
digitToWord 3 = "three"
digitToWord 4 = "four"
digitToWord 5 = "five"
digitToWord 6 = "six"
digitToWord 7 = "seven"
digitToWord 8 = "eight"
digitToWord 9 = "nine"

digits :: Int -> [Int]
digits n = go n []
  where go n lst
         | (div n 10) == 0 = reverse $ lst ++ [(mod n 10)]
         | otherwise = go (div n 10) $ lst ++ [(mod n 10)]

digitsToWords :: [Int] -> [String]
digitsToWords ds = map digitToWord ds