[Haskell-cafe] Do you recognize this type?

[Corentin Dupont]

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 :)


On Thu, Jun 19, 2014 at 5:17 PM, Corentin Dupont <corentin.dupont at gmail.com>
wrote:

> Hi guys,
> I am making a DSL for event composition (inspired from digestive-functor &
> reactive-banana) and I find myself wanting a primitive like that:
>
> -- given a list of events, evaluate the function on those that have
> already fired.
> -- returns an event yelding the result when ready.
> ListEvent  :: [Event a] -> ([a] -> Maybe b) -> Event b
>
> Does this type tell you something? Is there a better abstraction that can
> encompass it?
> My DSL is like that:
>
> -- | Composable events
> data Event a where
>    SumEvent   :: Event a -> Event a -> Event a        -- The first event
> to fire will be returned
>    AppEvent   :: Event (a -> b) -> Event a -> Event b -- Both events
> should fire, and then the result is returned
>    PureEvent  :: a -> Event a                         -- Create a fake
> event. The result is useable with no delay.
>    EmptyEvent :: Event a                              -- An event that is
> never fired.
>    BaseEvent  :: (Typeable a) => Field a -> Event a   -- Embed a base event
>
> It's easy to define instances for Applicative and Alternative.
> But then I have a use case that I cannot solve:
> In the case of votations (which use this DSL), sometime you can end
> prematuraly the voting, even if not everybody voted yet: for example if the
> majority is already reached.
> In this case no need to wait for everybody, we can end the vote.
>
> This does not seem to be possible with Applicative or Monad interfaces:
> things have to be sequenced in a monad, they cannot be evaluated in
> parralel.
> For example:
>
> do
>    v1 <- getVote1
>    v2 <- getVote2
>    v3 <- getVote3
>    evalMajority(v1, v2, v3)
>
> In this example I have to wait for all 3 votes to be completed, but in
> fact only two would suffice to achieve majority.
> That's why I added "ListEvent" to the DSL above which allow to achieve
> that but I'm wondering if there is a better/bigger abstraction to put it in.
>
> Thanks!!
> Corentin
>
>
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