[Haskell-cafe] Data declaration vs type classes

[Imants Cekusins]

> 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.