Caesar's chiper -- Haskell exercise from §5.5 of "Programming in Haskell"

caesar.hs

import Data.Char

char2Int :: Char -> Int
char2Int l = ord l - ord 'a'

int2Char :: Int -> Char 
int2Char i = chr (ord 'a' + i)

charAdvance :: Int -> Char -> Char
charAdvance i c | isLower c = int2Char (((char2Int c) + i) `mod` 26)
		| otherwise = c

caesarEncode :: String -> String
caesarEncode s = map (charAdvance 3) s

caesarDecode :: String -> String
caesarDecode s = map (charAdvance (-3)) s