<br><br><div class="gmail_quote">On Thu, Jun 14, 2012 at 12:15 PM, Corentin Dupont <span dir="ltr"><<a href="mailto:corentin.dupont@gmail.com" target="_blank">corentin.dupont@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
Hi folks,<br>I'm trying to make a simple event driven engine. It simply consists of two functions:<br>- "addEvent", where you pass the event name with a callback, <br>- "triggerEvent" where you pass the event name with the data. <br>
the data shall be passed to the callback of the corresponding event.<br><br>I have trouble making it correctly typed.<br>Here is my try:<br><i><br>type Player = Int --dummy types for the example<br>type Rule = Int<br>data EventEnum = NewPlayer | NewRule deriving Eq<br>
data Data = P Player | R Rule<br>data Handler = H (Data -> IO ())<br><br>addEvent :: EventEnum -> Handler -> [(EventEnum, Handler)] -> [(EventEnum, Handler)]<br>addEvent e h es = (e,h):es<br><br>triggerEvent :: EventEnum -> Data -> [(EventEnum, Handler)] -> IO ()<br>
triggerEvent e d es = do<br> let r = lookup e es<br> case r of <br> Nothing -> return ()<br> Just (H h) -> h d</i><br><br>The trouble is that I want the user to be only able to add an event that is compatible with its handler:<br>
For example the event NewPlayer should have a handler of type Player -> IO (). The data passed when triggering this event should be only of type Player.<br>How can I do that? It sound like dependant typing...<br><br></blockquote>
<div><br></div><div>Haven't tried it, and I don't know if it actually does what you want in the big picture. But you can do "dynamic" dependent typing with dummy (free) type variables.</div><div><br></div>
<div><i>type Player = Int --dummy types for the example<br>type Rule = Int<br>data Event d = New deriving Eq -- not necessary for this example, but you might want to enumerate other events.<br><br></i></div><div><i>class Handled data where -- Corresponds to your "Data" type<br>
<br></i></div><div><i>data Handler d = H (d -> IO ())</i></div><div><i><br></i></div><div><i>instance Handled Player</i></div><div><i>instance Handled Rule</i></div><div><i><br></i></div><div><i>addEvent :: (Handled d) => Event d -> Handler d -> [(Event d, Handler d)] -> [(Event d, Handler)]</i></div>
<div><i>triggerEvent :: (Handled d) => Event d -> d -> [(Event d, Handler d)] -> IO ()</i></div><div><i><br></i></div><div>Basically, this means that Events are "keyed" into separate spaces by the Handled types. (New :: Event Player) has a different type as (New :: Event Rule).</div>
<div><br></div><div>You might want to look into ScopedTypeVariables.</div></div>