Telephone words in Haskell

gistfile1.hs

import Data.Char

telephoneWords :: String -> [String]

telephoneWords [] = [[]]

telephoneWords (digit : digits) =
    [letter : rest
        | letter <- digitToLetters digit, rest <- telephoneWords digits]

digitToLetters :: Char -> String
digitToLetters c
    | n < 0 || n > 9 = error "Illegal telephone number"
    | n < 2 = [c]
    | n < 7 = [chr (((n - 2) * 3) + baseA + i) | i <- [0..2]]
    | n == 7 = "PQRS"
    | n == 8 = "TUV"
    | n == 9 = "WXYZ"
    where
        baseA = ord 'A'
        n = (ord c) - (ord '0')