On Mon, Jun 6, 2011 at 9:19 AM, Brent Yorgey
On Sun, Jun 05, 2011 at 12:51:47PM -0700, KC wrote:
If new intermediate classes crop up then there would be no point in fixing
class (Applicative m) => Monad m where
since it would have to be changed if new intermediate classes are found.
There actually is at least one intermediate class that I know of,
class Applicative m => Branching m where branch :: m Bool -> m a -> m a -> m a
subject to the laws
branch (m *> pure True) t f == m *> t branch (m *> pure False) t f == m *> f
or something like that. The idea is that Applicative computations have a fixed structure which is independent of intermediate results; Monad computations correspond to (potentially) infinitely branching trees, since intermediate results (which could be of an infinite-sized type) can be used to compute the next action; but Branching computations correspond to *finitely* branching trees, since future computation can depend on intermediate results, but only one binary choice at a time.
However, I doubt this qualifies as "useful" no matter how you define it, although I would not be sad to be proven wrong. In any case, I think it is ethically indefensible to procrastinate in doing something good just in case you might miss an opportunity to do something perfect later.
-Brent
I take it you would prefer the following signature class (Applicative m) => Monad m where -- -- Regards, KC