foo.hs

lookupBS :: Csv.NamedRecord -> BS.ByteString -> Csv.Parser BS.ByteString
lookupBS = Csv.lookup

lenientDecodeUtf8 :: BS.ByteString -> T.Text
lenientDecodeUtf8 = T.decodeUtf8With T.lenientDecode

(.:?) :: Csv.FromField a => Csv.NamedRecord -> BS.ByteString -> Csv.Parser a
rec .:? field = case Csv.runParser (rec .: field) of
    Left err -> fail ("Failed to parse field " <> name <> ": " <> err)
    Right r -> pure r
  where
    name = T.unpack (lenientDecodeUtf8 field)

(.:!) :: Csv.FromField a => Csv.NamedRecord -> BS.ByteString -> Csv.Parser a
rec .:! field = case Csv.runParser (lookupBS rec field) of
    Left err -> fail ("Failed to parse field " <> name <> ": " <> err)
    Right r | BS.null r
            -> fail ("Missing field " <> name)
            | otherwise
            -> rec .:? field
  where
    name = T.unpack (lenientDecodeUtf8 field)