simple math fun with haskell - custom number base systems

base.hs

import Data.Char (digitToInt)

numberToBaseString :: Int -> Int -> String
numberToBaseString n base
    | n < base  = show n
    | otherwise = numberToBaseString ((n - (n `mod` base)) `div` base) base ++ show (n `mod` base)

baseStringToValue :: String -> Int -> Int
baseStringToValue n base
    | n == []   = 0
    | otherwise = base * (baseStringToValue (init n) base) + digitToInt (last n)