Functor => Pointed => Applicative => Monad

John Lato jwlato at
Tue Nov 30 18:37:46 CET 2010

On Tue, Nov 30, 2010 at 4:05 PM, David Menendez <dave at> wrote:

> On Tue, Nov 30, 2010 at 5:31 AM, John Lato <jwlato at> wrote:
> >> From: David Menendez <dave at>
> >>
> >> Is Pointed useful at all? The last time this discussion came up, I
> >> asked for algorithms which were generic over pointed functors (in the
> >> same way that traverse is generic over applicative functors) and no
> >> one could think of any.
> >
> > It's useful for 'singleton' on probabilistic data structures (e.g. a
> Bloom
> > filter).
> Again, I'm asking for clients, not implementations. Are there any
> useful, non-trivial functions which are parameterized by arbitrary
> Pointed functors?

I consider "singleton" itself to be a client, albeit somewhat trivial.  But
as a result of the last discussion, I'm no longer sure that "singleton" is a
proper use of "pointed" and I shouldn't have brought it up now.  I'm still
in favor of splitting Applicative for philosophical reasons though.

In any case there's the function posted by Ross Patterson, which seems to
fit all your criteria.

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