On Apr 4, 2006, at 12:46 PM, Twan van Laarhoven 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.
The obvious solution is to allow out-of-defining-module deriving only for data types where all constructors are exported. Presumably, if someone explicitly hid (at least some of) the constructors of a data type, they did it for a reason and we can hope that they carefully considered the external interface of that type (including instances). Rob Dockins Speak softly and drive a Sherman tank. Laugh hard; it's a long way to the bank. -- TMBG