Event.hs

-----------------------------------------------------------------------------------------
-- |
-- 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