'deriving Tree (Eq,Ord)'

[Bulat Ziganshin]

Hello John,

Wednesday, April 5, 2006, 12:02:16 AM, you wrote:
>> 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).

> indeed, that is actually a requirement of the original proposal.

i can agree with it with one additional requirement to library writers
- provide "internal" modules that export data definitions. without
this, it's a big problem to add new functionality with optimal
performance. for example, to add Binary instance for Data.Map, the
only way is to use fromList/toList functions, what is not the
fastest method

(i still not tested whether TH provide access to structure of abstract
types :) )



-- 
Best regards,
 Bulat                            mailto:Bulat.Ziganshin at gmail.com