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"
digitToWord _ = "nope"

digits :: Int -> [Int]
digits num = go num []
  where
    go n lst
      | q == 0 = [r] ++ lst
      | otherwise = go q $ [r] ++ lst
      where q = div n 10
            r = mod n 10

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

wordsToString :: [String] -> String
wordsToString ws = concat $ intersperse "-" ws

say :: Int -> String
say = wordsToString . digitsToWords . digits