On Tuesday 04 April 2006 18:46, you wrote:
Benjamin Franksen wrote:
However, it would help programmers a lot, specialy library users. And it would lift some of the burden that is currently on the library writer to never forget to derive all possibly useful instances for public data types.
If you allow 'deriving' in other modules it can violate the 'privacy' of modules. For example (if these instances were not declared): deriving Data.Map.Map (Show)
Now I can look at what map nodes look like.
deriving Data.Map.Map (Data)
Now I can even mess with the internal structure, violating invariants.
Ouch! This would be bad. Hmm... not all exported data types are abstract. I guess we could allow it for data types with no hidden constructors, but then maybe it is not quite as useful as I thought... Ben