Re: [Haskell-cafe] Parallel interruptible computations

On Mon, Sep 8, 2014 at 7:55 AM, Chris Wong wrote:

Hi Corentin

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Hi guys, thanks for the nice answers! I'll give you a little bit more context: I'm designing an event engine. I have instances for Applicative, Alternative, Monad, MonadPlus. It's like that:

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The Applicative instance is good if you have two events and you want both of them to fire ("and"). The Alternative instance is good if you have two events and you need only one to fire ("or"). But what if you have several events, but you need only a part of them to fire in order to construct a final result? Say you have 10 events, but

On Mon, Sep 8, 2014 at 8:23 AM, Corentin Dupont wrote: the 5

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first to fire will give you enough data to construct a result. You cannot do that with Applicative/Alternative because with Applicative, you need *all* events results, with Alternative you need *only one*.

That's why I added this primitive "ShortcutEvents" in my DSL, but I'm not convinced by it. So my questions are: 1. is ShortcutEvents expressible in term of Applicative/Alternative/Monad/MonadPlus? 2. if not is their a well known typeclass that covers this case? 3. if not is their a better way to write it? I especially don't like the list of Event, I'd prefer a more generic writing. What if I want a structure containing the events, instead of a list? What if I want event of various types (say a pair (Event a, Event b) for example)?

Note that I'm not working with streams of events (like in traditional FRP frameworks): just with single events (the "BaseEvents") that I want to combine with each other. Those "BaseEvents" will fire only once. The final result of the combination of events will trigger a callback.

There's one thing I don't quite understand: why is Event expressed as a free monad/applicative structure? Based on your description alone, it sounds like

type Event a = Maybe (BaseEvent a)

would suffice. Or am I missing something?

Hi Chris! "Event" is a small DSL that I interpret in the back end. It allows me to write nice expressions about events. Say you have functions to create text fields and buttons on the GUI: -- Create an event binded to a text field, with the first argument as a title. -- Once validated, the event returns the content of the text field. inputText :: String -> Event String -- Create an event binded to a button, with the first argument as a title. inputButton :: String -> Event () You could then express nice combinations: -- using Applicative: create a form with two fields data NameSurname = NameSurname String String form1 :: Event NameSurname form1 = NameSurname <$> onInputText "Name:" <*> onInputText "Surname:" -- using Alternative: create two buttons, first button clicked returns False, the second True form2 :: Event Boolean form2 = True <$ inputButton "click here for True" <|> False <$ inputButton "click here for False" -- using Monad: create the two buttons of form2, if "True" button is clicked, then a text field appears asking for a name. form3 :: Event String form3 = do myBool <- form2 if myBool then onInputText "Name:" else return "No name" But I am lacking a way to express the situation where I have a bunch of events, which can be cancelled as soon as a result can be calculated: ShortcutEvents :: [Event a] -> ([Maybe a] -> Maybe b) -> Event b You can think of it as a generalization of the "or" shortcut, where the evaluation is cut short if the first argument evaluates to True. With your type: type Event a = Maybe (BaseEvent a) Could you express the combinations above??