On Fri, Jun 20, 2014 at 4:55 PM, Chris Warburton <chriswarbo@googlemail.com> wrote:

Corentin Dupont <corentin.dupont@gmail.com> writes:

> Nobody on that?
> What I really want is: a computation that can be interrupted if enough
> result is already there.
> I would be surprise if that concept doesn't exist already somewhere :)

I'd be surprised if this can be done with Applicative without some
hackery.

Very roughly, we can think of Functors as one-shot actions. For example,
the IO instance of Functor lets us perform exactly 1 side-effect; all we
can do with the result is map pure functions over it.

Applicatives are a static chain of actions. For example, the IO instance
of Applicative lets us chain together any number of side-effecting
actions, but those actions can't alter the chain itself.

Monads are a dynamic chain of actions. They're like Applicatives, but
each action can choose which action to perform next based on the
previous results.

wow, I *love* this summary! Thanks for that!

>From this perspective, the "shortcut" you're looking for requires the
ability to choose the next action (count another vote or finish
prematurely) based on the previous results (the running total). That's
what Monads are good for, but Applicatives can't do it by themselves.

Indeed I could make my DSL a monad instance with some changes, that would

be easy to do. But even not a Monad cannot do the trick: imagine if in a country, you were asking

the people to vote one by one, only to interrupt them when the result is known. In fact they have to vote altogether,

in whatever order, and we count the votes as they arrive.

So I think what I need is a sort of Applicative++: not a chain of actions but a set of actions, all independent from each other (so no monad) and with no particular order.

There's probably an unsafe* way to do this, which will in fact be safe
due to your voting invariant ("if there aren't enough uncounted votes
to change the outcome, the outcome cannot change"). Maybe someone who's
performed similar optimisations can help.

Good luck!

Chris