Em Seg, 2009-05-04 às 15:54 -0400, Brent Yorgey escreveu:
On Mon, May 04, 2009 at 11:59:54AM -0300, Marco Túlio Gontijo e Silva wrote:
instance (F f, M m (f a)) => M m a where mm f v = mm (m f) v
Perhaps you mean
instance (F f, M m a) => M m (f a) where ...
?
No, I really meant what I wrote. An example: If I the instances I wrote:
instance F [] where m = map
instance M (IORef a) a where mm = flip modifyIORef
I want to define:
instance M (IORef [a]) a where mm f v = mm (m f) v
This could of course be written as: mm = mm . m I'd like to get this last instance automaticly from that definition, having: f = [] m = IORef [a]
What you have written means that to have an instance M m a, you must have an instance M m (f a), for which you must have an instance M m (f (f a)), for which you must have an instance M m (f (f (f a)))...
After this explanation I think I located the problem. My problem seems to be with how IncoherentInstances work, by picking always the most generic instance. Thanks for that. Do you know if there a way to get around this, maybe using another GHC extension? Greetings. -- marcot http://marcot.iaaeee.org/