[Haskell-beginners] Context reducion Stack overflow

[Brent Yorgey]

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 ...

?

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)))...

What I have written above expresses what I presume you meant: we can
automatically derive an instance of (M m) for some structured type (f
a) as long as we have a base instance M m a and can lift functions (a
-> a) to (f a -> f a).
 
-Brent