[Haskell-beginners] Context reducion Stack overflow

[Brent Yorgey]

On Tue, May 05, 2009 at 10:28:10AM -0300, Marco Túlio Gontijo e Silva wrote:
> 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]

I don't follow.  In order to get 
 
  instance M (IORef [a]) a

from

  instance (F f, M m (f a)) => M m a

would require

  instance M (IORef [a]) (f a)

for some f, which you don't have.

I might try rewriting M as

  class M f a where
    mm :: (a -> a) -> f a -> IO ()

and then your automatic lifting instance would be something like

  instance (F f, M f2 a) => M (f :.: f2) a

where :.: denotes functor composition.


-Brent