Re: [Haskell-cafe] Data declaration vs type classes

it's very hard to pass them around, manipulate them and compute with them without using non-standard and awkward techniques.

well here is one simple use case when class is very convenient: class ConvertByteString a where toByteString::a -> ByteString fromByteString::ByteString -> a no problems defining instances of this class, passing and calling them whatsoever.