Ceasar cipher challenge solution

ceasar.hs

{- Performs the Ceasar cipher encryption.

   Compile this file by running:
     ghc ceasar.hs

   You'll need to have ghc package installed.
-}
import Data.Char

main = do
  putStrLn "Enter string to encrypt:"
  plainText <- getLine
  putStrLn "Enter key:"
  keyString <- getLine
  putStrLn $ evaluateInput keyString plainText

evaluateInput keyString plainText = case reads keyString of
  [(key, _)] -> "Your encrypted string:\n" ++ show (ceasar key plainText)
  _ -> "Invalid key number."

ceasar key plainText = map (ceasarEncryptCharacter key) plainText

ceasarEncryptCharacter :: Int -> Char -> Char
ceasarEncryptCharacter key character
  | isLower character = fromCode smallA (toCode smallA character + key)
  | isAlpha character = fromCode bigA (toCode bigA character + key)
  | otherwise = character

fromCode offset code = chr (mod code 26 + offset)

toCode offset character = mod (ord character - offset) 26

smallA = ord 'a'
bigA = ord 'A'