[Haskell-cafe] Composable events: Applicative and Alternative

[Corentin Dupont]

I created a DSL to manage and create events (see under). It's an instance
of Applicative and Alternative, which make them compose nicely.
The idea is that when an event is firing, I look the corresponding event in
the tree and replace it by its value using Pure.

For example onInputText will trigger the display of a form with a text
field on the screen, and when the player validates the form, the same
onInputText is searched in the tree by the engine and replaced by the text
value. This is where the difficulty begins:

--> How to make Event an instance of Eq? How to search and replace through
it?

The problem is the EventProduct primitive, that forbids to make the DSL an
instance of Eq:
EventProduct :: Event (a -> b) -> Event a -> Event b

Should I replace it by:
EventProduct :: Event a -> Event b -> Event (a,b)

But then, bye-bye Applicative/Alternative instances!
How can I make the tree of events searchable/traversable and still maintain
the Applicative instance?

Thanks!!
Corentin


On Sat, May 3, 2014 at 8:02 PM, Corentin Dupont
<corentin.dupont at gmail.com>wrote:

> Hi Cafe!!
>
> For my game Nomyx, I am using events that the player can program. For
> example, the player can register a callback that will be triggered when a
> new player arrives. He can also program some forms (with buttons,
> checkboxes, textboxes...) to appear on the Web GUI. The problem is those
> events are not composable: he has to create and handle them one by one.
>
> So I'm thinking of making those events composable by making them an
> instance of Applicative and Alternative.
> For Applicative, this makes events composable very much like in
> Applicative-Functors and Reform. I can build neat composed events such as
> (full program below):
>
> onInputMyRecord :: Event MyRecord
> onInputMyRecord = MyRecord <$> onInputText <*> onInputCheckbox
>
> For Alternative, I haven't seen any example of it on the net. The idea is
> that the first event that fires is used to build the alternative:
>
> onInputMyAlternative :: Event Bool
> onInputMyAlternative = (True <$ onInputButton) <|> (False <$ onInputButton)
>
> Here is an example program:
>
> {-# LANGUAGE GADTs #-}
>
> module ComposableEvents where
>
> import Control.Applicative
> import Data.Time
> import Data.Traversable
>
> type PlayerNumber = Int
>
> data Event a where
>    OnInputText :: PlayerNumber -> Event String           -- A textbox
> will be created for the player. When filled, this event will fire and
> return the result
>    OnInputCheckbox :: PlayerNumber -> Event Bool         -- Idem with a
> checkbox
>    OnInputButton :: PlayerNumber -> Event ()             -- Idem with a
> button
>    OnTime :: UTCTime -> Event ()                         -- time event
>    EventSum :: Event a -> Event a -> Event a             -- The first
> event to fire will be returned
>    EventProduct :: Event (a -> b) -> Event a -> Event b  -- both events
> should fire, and then the result is returned
>    Fmap :: (a -> b) -> Event a -> Event b                -- transforms
> the value returned by an event.
>    Pure :: a -> Event a                                  -- Create a fake
> event. The result is useable with no delay.
>    Empty :: Event a                                      -- An event that
> is never fired.
>
> instance Functor Event where
>    fmap = Fmap
>
> instance Applicative Event where
>    pure = Pure
>    (<*>) = EventProduct
>
> instance Alternative Event where
>    (<|>) = EventSum
>    empty = Empty
>
> onInputText = OnInputText
> onInputCheckbox = OnInputCheckbox
> onInputButton = OnInputButton
> onTime = OnTime
>
> -- A product type
> data MyRecord = MyRecord String Bool
>
> -- A sum type
> data MyAlternative = A | B
>
> -- Using the Applicative instance, we can build a product type from two
> separate event results.
> -- The event callback should be called only when all two events have fired.
> onInputMyRecord :: Event MyRecord
> onInputMyRecord = MyRecord <$> onInputText 1 <*> onInputCheckbox 1
>
> -- other possible implementation (given a monad instance)
> -- onInputMyRecord' = do
> --    s <- onInputText
> --    b <- onInputCheckbox
> --    return $ MyRecord s b
>
> -- Using the Alternative instance, we build a sum type.
> -- The event callback should be called when the first event have fired.
> onInputMyAlternative :: Event MyAlternative
> onInputMyAlternative = (const A <$> onInputButton 1) <|> (const B <$>
> onInputButton 1)
>
> allPlayers = [1 .. 10]
>
> -- Now complex events can be created, such as voting systems:
> voteEvent :: UTCTime -> Event ([Maybe Bool])
> voteEvent time = sequenceA $ map (singleVote time) allPlayers
>
> singleVote :: UTCTime -> PlayerNumber -> Event (Maybe Bool)
> singleVote timeLimit pn = (Just <$> onInputCheckbox pn) <|> (const Nothing
> <$> onTime timeLimit)
>
> vote :: UTCTime -> Event Bool
> vote timeLimit = unanimity <$> (voteEvent timeLimit)
>
> unanimity :: [Maybe Bool] -> Bool
> unanimity = all (== Just True)
>
>
> --Evaluation
> --evalEvent :: Event a -> State Game a
> --evalEvent = undefined
>
> With this DSL, I can create complex events such as time limited votes very
> neatly...
> There is much left to do for a full implem: the way to register callbacks
> on complex events, the evaluator and the event manager.
> Have you heard about a similar implementation? It seems pretty useful.
> Maybe in FRP frameworks?
>
> Thanks a lot!!
> Corentin
> PS: I copied this example also in
> https://github.com/cdupont/Nomyx-design/blob/master/ComposableEvents.hs.
>
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