UPC validation in Haskell

upc.hs

module UPC (is_upc) where

import Data.Char (digitToInt)

-- Is this string a valid Universal Product Code?
is_upc :: String -> Bool
is_upc u = ( length u == 12 ) && ( upc_checksum u `mod` 10 == 0 )

-- Calculate the UPC checksum using the algorithm from http://xrl.us/omxh8
upc_checksum :: String -> Int
upc_checksum upc = sum evens + 3*sum odds
    where digits = zip [1..12] $ map digitToInt upc
          odds   = [ x | (n,x) <- digits,       odd n   ]
          evens  = [ x | (n,x) <- digits, not ( odd n ) ]