'deriving Tree (Eq,Ord)'

Robert Dockins robdockins at fastmail.fm
Tue Apr 4 13:13:32 EDT 2006

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

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