I hate to nitpick, but could you please:

 1. Call this just `endOfLine`. I would expect any name ending with a `'` to have something to do with strictness.
 2. Define it directly as `(word8 nl *> return ()) <|> (string "\r\n" *> return ()) <|> (A.word8 cr >> return ())` (i.e. just inline attoparsec's definition).
 3. Add a comment, similar to the one here: https://github.com/bos/attoparsec/blob/master/Data/Attoparsec/ByteString/Internal.hs#L400