The objection to this I recall hearing is that it would make it possible for different parts of the program to see different instances for a given type (local instances, effectively), which would have "interesting" results. I don't know if there's a way to implement the feature which avoids this, or whether also adding support for local instances is actually part of the plan. (If I'm thinking this through correctly, this is currently avoided because instances are always exported, and Main imports all modules transitively, so any conflicting instances will be visible in Main at the latest.) On Fri, Jan 7, 2011 at 2:07 PM, Marco Túlio Gontijo e Silva wrote:

Hi.

I'm planning to propose a language extension for Haskell to make it possible to control the visibility of type class instances in module export and import lists as a research project.  I'm planning to implement this in GHC.  I've already mentioned it in #ghc@irc.freenode.net and got interesting comments from tibbe and quicksilver.

I'd be glad to receive more feedback on the idea.  If you have any comments, please reply to this thread or mail me individually, at your will.

Greetings. -- marcot http://marcot.eti.br/ [Flattr=54498]

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