----------------------------------------------------------------------------------------- -- | -- Module : FRP.Yampa.Event -- Copyright : (c) Antony Courtney and Henrik Nilsson, Yale University, 2003 -- License : BSD-style (see the LICENSE file in the distribution) -- -- Maintainer : nilsson@cs.yale.edu -- Stability : provisional -- Portability : portable -- -- Definition of Yampa Event type. -- -- Note on naming conventions used in this module. -- -- Names here might have to be rethought. It's really a bit messy. -- In general, the aim has been short and convenient names (like 'tag', -- 'attach', 'lMerge') and thus we have tried to stay away from suffixing/ -- prefixing conventions. E.g. 'Event' as a common suffix would be very -- verbose. -- -- However, part of the names come from a desire to stay close to similar -- functions for the Maybe type. e.g. 'event', 'fromEvent', 'isEvent'. -- In many cases, this use of 'Event' can could understood to refer to the -- constructor 'Event', not to the type name 'Event'. Thus this use of -- event should not be seen as a suffixing-with-type-name convention. But -- that is obviously not easy to see, and, more over, interpreting 'Event' -- as the name of the type might make equally good or better sense. E.g. -- 'fromEvent' can also be seen as a function taking an event signal, -- which is a partial function on time, to a normal signal. The latter is -- then undefined when the source event function is undefined. -- -- In other cases, it has been necessary to somehow stay out of the way of -- names used by the prelude or other commonly imported modules/modules -- which could be expected to be used heavily in Yampa code. In those cases -- a suffix 'E' have been added. Examples are 'filterE' (exists in Prelude) -- and 'joinE' (exists in Monad). Maybe the suffix isn't necessary in the -- last case. -- -- Some functions (actually only one currently, 'mapFilterE') have got an 'E' -- suffix just because they're closely related (by name or semantics) to one -- which already has an 'E' suffix. Another candidate would be 'splitE' to -- complement 'joinE'. But events carrying pairs could obviously have other -- sources than a 'joinE', so currently it is called 'split'. -- -- 2003-05-19: Actually, have now changed to 'splitE' to avoid a clash -- with the method 'split' in the class RandomGen. -- -- 2003-05-19: What about 'gate'? Stands out compared to e.g. 'filterE'. -- -- Currently the 'E' suffix is considered an exception. Maybe we should use -- completely different names to avoid the 'E' suffix. If the functions -- are not used that often, 'Event' might be approriate. Alternatively the -- suffix 'E' should be adopted globaly (except if the name already contains -- 'event' in some form?). -- -- Arguably, having both a type 'Event' and a constructor 'Event' is confusing -- since there are more than one constructor. But the name 'Event' for the -- constructor is quite apt. It's really the type name that is wrong. But -- no one has found a better name, and changing it would be a really major -- undertaking. Yes, the constructor 'Event' is not exported, but we still -- need to talk conceptually about them. On the other hand, if we consider -- Event-signals as partial functions on time, maybe it isn't so confusing: -- they just don't have a value between events, so 'NoEvent' does not really -- exist conceptually. -- -- ToDo: -- - Either: reveal NoEvent and Event -- or: introcuce 'event = Event', call what's now 'event' 'fromEvent', -- and call what's now called 'fromEvent' something else, like -- 'unsafeFromEvent'??? Better, dump it! After all, using current -- names, 'fromEvent = event undefined'! ----------------------------------------------------------------------------------------- module FRP.Yampa.Event where import FRP.Yampa.Diagnostics import FRP.Yampa.Forceable infixl 8 `tag`, `attach`, `gate` infixl 7 `joinE` infixl 6 `lMerge`, `rMerge`, `merge` ------------------------------------------------------------------------------ -- The Event type ------------------------------------------------------------------------------ -- The type Event represents a single possible event occurrence. -- It is isomorphic to Maybe, but its constructors are not exposed outside -- the AFRP implementation. -- There could possibly be further constructors, but note that the NeverEvent- -- idea does not work, at least not in the current AFRP implementation. -- Also note that it unfortunately is possible to partially break the -- abstractions through judicious use of e.g. snap and switching. data Event a = NoEvent | Event a deriving (Show) -- Make the NoEvent constructor available. Useful e.g. for initialization, -- ((-->) & friends), and it's easily available anyway (e.g. mergeEvents []). noEvent :: Event a noEvent = NoEvent -- Suppress any event in the first component of a pair. noEventFst :: (Event a, b) -> (Event c, b) noEventFst (_, b) = (NoEvent, b) -- Suppress any event in the second component of a pair. noEventSnd :: (a, Event b) -> (a, Event c) noEventSnd (a, _) = (a, NoEvent) ------------------------------------------------------------------------------ -- Eq instance ------------------------------------------------------------------------------ -- Right now, we could derive this instance. But that could possibly change. instance Eq a => Eq (Event a) where NoEvent == NoEvent = True (Event x) == (Event y) = x == y _ == _ = False ------------------------------------------------------------------------------ -- Ord instance ------------------------------------------------------------------------------ instance Ord a => Ord (Event a) where compare NoEvent NoEvent = EQ compare NoEvent (Event _) = LT compare (Event _) NoEvent = GT compare (Event x) (Event y) = compare x y ------------------------------------------------------------------------------ -- Functor instance ------------------------------------------------------------------------------ instance Functor Event where fmap _ NoEvent = NoEvent fmap f (Event a) = Event (f a) ------------------------------------------------------------------------------ -- Forceable instance ------------------------------------------------------------------------------ instance Forceable a => Forceable (Event a) where force ea@NoEvent = ea force ea@(Event a) = force a `seq` ea ------------------------------------------------------------------------------ -- Internal utilities for event construction ------------------------------------------------------------------------------ -- These utilities are to be considered strictly internal to AFRP for the -- time being. maybeToEvent :: Maybe a -> Event a maybeToEvent Nothing = NoEvent maybeToEvent (Just a) = Event a ------------------------------------------------------------------------------ -- Utility functions similar to those available for Maybe ------------------------------------------------------------------------------ -- An event-based version of the maybe function. event :: a -> (b -> a) -> Event b -> a event a _ NoEvent = a event _ f (Event b) = f b fromEvent :: Event a -> a fromEvent (Event a) = a fromEvent NoEvent = usrErr "AFRP" "fromEvent" "Not an event." isEvent :: Event a -> Bool isEvent NoEvent = False isEvent (Event _) = True isNoEvent :: Event a -> Bool isNoEvent = not . isEvent ------------------------------------------------------------------------------ -- Event tagging ------------------------------------------------------------------------------ -- Tags an (occurring) event with a value ("replacing" the old value). tag :: Event a -> b -> Event b e `tag` b = fmap (const b) e tagWith :: b -> Event a -> Event b tagWith = flip tag -- Attaches an extra value to the value of an occurring event. attach :: Event a -> b -> Event (a, b) e `attach` b = fmap (\a -> (a, b)) e ------------------------------------------------------------------------------ -- Event merging (disjunction) and joining (conjunction) ------------------------------------------------------------------------------ -- !!! I think this is too complicated. rMerge can be obtained simply by -- !!! swapping the arguments. So the only time it is possibly of any -- !!! interest is for partial app. "merge" is inherently dangerous. -- !!! But this is NOT obvious from its type: it's type is just like -- !!! the others. This is the only example of such a def. -- !!! Finally: mergeEvents is left-biased, but this is not reflected in -- !!! its name. -- Left-biased event merge. lMerge :: Event a -> Event a -> Event a le `lMerge` re = event re Event le -- Right-biased event merge. rMerge :: Event a -> Event a -> Event a le `rMerge` re = event le Event re -- Unbiased event merge: simultaneous occurrence is an error. merge :: Event a -> Event a -> Event a merge = mergeBy (usrErr "AFRP" "merge" "Simultaneous event occurrence.") -- Event merge paramterezied on the conflict resolution function. mergeBy :: (a -> a -> a) -> Event a -> Event a -> Event a mergeBy _ NoEvent NoEvent = NoEvent mergeBy _ le@(Event _) NoEvent = le mergeBy _ NoEvent re@(Event _) = re mergeBy resolve (Event l) (Event r) = Event (resolve l r) -- A generic event merge utility: mapMerge :: (a -> c) -> (b -> c) -> (a -> b -> c) -> Event a -> Event b -> Event c mapMerge _ _ _ NoEvent NoEvent = NoEvent mapMerge lf _ _ (Event l) NoEvent = Event (lf l) mapMerge _ rf _ NoEvent (Event r) = Event (rf r) mapMerge _ _ lrf (Event l) (Event r) = Event (lrf l r) -- Merging of a list of events; foremost event has priority. mergeEvents :: [Event a] -> Event a mergeEvents = foldr lMerge NoEvent -- Collects simultaneous event occurrences; no event if none. catEvents :: [Event a] -> Event [a] catEvents eas = case [ a | Event a <- eas ] of [] -> NoEvent as -> Event as -- Join (conjucntion) of two events. joinE :: Event a -> Event b -> Event (a,b) joinE NoEvent _ = NoEvent joinE _ NoEvent = NoEvent joinE (Event l) (Event r) = Event (l,r) -- Split event carrying pairs into two events. splitE :: Event (a,b) -> (Event a, Event b) splitE NoEvent = (NoEvent, NoEvent) splitE (Event (a,b)) = (Event a, Event b) ------------------------------------------------------------------------------ -- Event filtering ------------------------------------------------------------------------------ -- Filter out events that don't satisfy some predicate. filterE :: (a -> Bool) -> Event a -> Event a filterE p e@(Event a) = if (p a) then e else NoEvent filterE _ NoEvent = NoEvent -- Combined event mapping and filtering. mapFilterE :: (a -> Maybe b) -> Event a -> Event b mapFilterE _ NoEvent = NoEvent mapFilterE f (Event a) = case f a of Nothing -> NoEvent Just b -> Event b -- Enable/disable event occurences based on an external condition. gate :: Event a -> Bool -> Event a _ `gate` False = NoEvent e `gate` True = e