Safe Haskell | None |
---|---|
Language | Haskell98 |
- data Event a
- data Behavior a
- interpret :: (Event a -> Event b) -> [Maybe a] -> IO [Maybe b]
- module Control.Applicative
- module Data.Monoid
- never :: Event a
- unionWith :: (a -> a -> a) -> Event a -> Event a -> Event a
- filterE :: (a -> Bool) -> Event a -> Event a
- apply :: Behavior (a -> b) -> Event a -> Event b
- data Moment a
- class Monad m => MonadMoment m where
- liftMoment :: Moment a -> m a
- accumE :: MonadMoment m => a -> Event (a -> a) -> m (Event a)
- stepper :: MonadMoment m => a -> Event a -> m (Behavior a)
- valueB :: MonadMoment m => Behavior a -> m a
- valueBLater :: MonadMoment m => Behavior a -> m a
- observeE :: Event (Moment a) -> Event a
- switchE :: Event (Event a) -> Event a
- switchB :: Behavior a -> Event (Behavior a) -> Behavior a
- (<@>) :: Behavior (a -> b) -> Event a -> Event b
- (<@) :: Behavior b -> Event a -> Event b
- filterJust :: Event (Maybe a) -> Event a
- filterApply :: Behavior (a -> Bool) -> Event a -> Event a
- whenE :: Behavior Bool -> Event a -> Event a
- split :: Event (Either a b) -> (Event a, Event b)
- unions :: [Event (a -> a)] -> Event (a -> a)
- accumB :: MonadMoment m => a -> Event (a -> a) -> m (Behavior a)
- mapAccum :: MonadMoment m => acc -> Event (acc -> (x, acc)) -> m (Event x, Behavior acc)
Synopsis
The main types and combinators of Functional Reactive Programming (FRP).
At its core, FRP is about two data types Event
and Behavior
and the various ways to combine them.
There is also a third type Moment
,
which is necessary for the higher-order combinators.
Core Combinators
Event and Behavior
Event a
represents a stream of events as they occur in time.
Semantically, you can think of Event a
as an infinite list of values
that are tagged with their corresponding time of occurrence,
type Event a = [(Time,a)]
Each pair is called an event occurrence. Note that within a single event stream, no two event occurrences may happen at the same time.
Behavior a
represents a value that varies in time.
Semantically, you can think of it as a function
type Behavior a = Time -> a
interpret :: (Event a -> Event b) -> [Maybe a] -> IO [Maybe b] Source
Interpret an event processing function. Useful for testing.
First-order
module Control.Applicative
module Data.Monoid
unionWith :: (a -> a -> a) -> Event a -> Event a -> Event a Source
Merge two event streams of the same type. The function argument specifies how event values are to be combined in case of a simultaneous occurrence. The semantics are
unionWith f ((timex,x):xs) ((timey,y):ys) | timex < timey = (timex,x) : unionWith f xs ((timey,y):ys) | timex > timey = (timey,y) : unionWith f ((timex,x):xs) ys | timex == timey = (timex,f x y) : unionWith f xs ys
filterE :: (a -> Bool) -> Event a -> Event a Source
Allow all events that fulfill the predicate, discard the rest. Semantically,
filterE p es = [(time,a) | (time,a) <- es, p a]
apply :: Behavior (a -> b) -> Event a -> Event b Source
Apply a time-varying function to a stream of events. Semantically,
apply bf ex = [(time, bf time x) | (time, x) <- ex]
This function is generally used in its infix variant <@>
.
Further combinators that Haddock can't document properly.
instance Applicative Behavior
Behavior
is an applicative functor. In particular, we have the following functions.
pure :: a -> Behavior a
The constant time-varying value. Semantically, pure x = \time -> x
.
(<*>) :: Behavior (a -> b) -> Behavior a -> Behavior b
Combine behaviors in applicative style.
The semantics are: bf <*> bx = \time -> bf time $ bx time
.
Moment and accumulation
The Moment
monad denotes a pure computation that happens
at one particular moment in time. Semantically, it as a reader monad
type Moment a = Time -> a
When run, the argument tells the time at which this computation happens.
Note that in this context, time really means to logical time.
Of course, every calculation on a computer takes some
amount of wall-clock time to complete.
Instead, what is meant here is the time as it relates to
Event
s and Behavior
s.
We use the fiction that every calculation within the Moment
monad takes zero logical time to perform.
class Monad m => MonadMoment m where Source
An instance of the MonadMoment
class denotes a computation
that happens at one particular moment in time.
Unlike the Moment
monad, it need not be pure anymore.
liftMoment :: Moment a -> m a Source
accumE :: MonadMoment m => a -> Event (a -> a) -> m (Event a) Source
The accumE
function accumulates a stream of event values,
similar to a strict left scan, scanl'
.
It starts with an initial value and emits a new value
whenever an event occurrence happens.
The new value is calculated by applying the function in the event
to the old value.
Example:
accumE "x" [(time1,(++"y")),(time2,(++"z"))] = trimE [(time1,"xy"),(time2,"xyz")] where trimE e start = [(time,x) | (time,x) <- e, start <= time]
Note: It makes sense to list the accumE
function as a primitive
combinator, but keep in mind that it can actually be expressed
in terms of stepper
and apply
by using recursion:
accumE a e1 = mdo let e2 = (\a f -> f a) <$> b <@> e1 b <- stepper a e2 return e2
stepper :: MonadMoment m => a -> Event a -> m (Behavior a) Source
Construct a time-varying function from an initial value and a stream of new values. The result will be a step function. Semantically,
stepper x0 ex = \time1 -> \time2 -> last (x0 : [x | (timex,x) <- ex, time1 <= timex, timex < time2])
Here is an illustration of the result Behavior at a particular time:
Note: The smaller-than-sign in the comparison timex < time2
means
that at time time2 == timex
, the value of the Behavior will
still be the previous value.
In the illustration, this is indicated by the dots at the end
of each step.
This allows for recursive definitions.
Higher-order
valueB :: MonadMoment m => Behavior a -> m a Source
Obtain the value of the Behavior
at a given moment in time.
Semantically, it corresponds to
valueB b = \time -> b time
NOTE: The value is immediately available for pattern matching.
Unfortunately, this means that valueB
is unsuitable for use
with value recursion in the Moment
monad.
If you need recursion, please use valueBLater
instead.
valueBLater :: MonadMoment m => Behavior a -> m a Source
Obtain the value of the Behavior
at a given moment in time.
Semantically, it corresponds to
valueBLater b = \time -> b time
NOTE: To allow for more recursion, the value is returned lazily
and not available for pattern matching immediately.
It can be used safely with most combinators like stepper
.
If that doesn't work for you, please use valueB
instead.
observeE :: Event (Moment a) -> Event a Source
Observe a value at those moments in time where event occurrences happen. Semantically,
observeE e = [(time, m time) | (time, m) <- e]
switchE :: Event (Event a) -> Event a Source
Dynamically switch between Event
.
Semantically,
switchE ee = concat [trim t1 t2 e | (t1,t2,e) <- intervals ee] where intervals e = [(time1, time2, x) | ((time1,x),(time2,_)) <- zip e (tail e)] trim time1 time2 e = [x | (timex,x) <- e, time1 < timex, timex <= time2]
switchB :: Behavior a -> Event (Behavior a) -> Behavior a Source
Dynamically switch between Behavior
.
Semantically,
switchB b0 eb = \time -> last (b0 : [b | (time2,b) <- eb, time2 < time]) time
Derived Combinators
Infix operators
(<@) :: Behavior b -> Event a -> Event b infixl 4 Source
Tag all event occurrences with a time-varying value. Similar to <*
.
infixl 4 <@
Filtering
filterJust :: Event (Maybe a) -> Event a Source
filterApply :: Behavior (a -> Bool) -> Event a -> Event a Source
Allow all events that fulfill the time-varying predicate, discard the rest.
Generalization of filterE
.
whenE :: Behavior Bool -> Event a -> Event a Source
Allow events only when the behavior is True
.
Variant of filterApply
.
Accumulation
Note: All accumulation functions are strict in the accumulated value!
Note: The order of arguments is acc -> (x,acc)
which is also the convention used by unfoldr
and State
.
unions :: [Event (a -> a)] -> Event (a -> a) Source
Merge event streams whose values are functions. In case of simultaneous occurrences, the functions at the beginning of the list are applied after the functions at the end.
unions [] = never unions xs = foldr1 (unionWith (.)) xs
Very useful in conjunction with accumulation functions like accumB
and accumE
.
accumB :: MonadMoment m => a -> Event (a -> a) -> m (Behavior a) Source
The accumB
function accumulates event occurrences into a Behavior
.
The value is accumulated using accumE
and converted
into a time-varying value using stepper
.
Example:
accumB "x" [(time1,(++"y")),(time2,(++"z"))] = stepper "x" [(time1,"xy"),(time2,"xyz")]
Note: As with stepper
, the value of the behavior changes "slightly after"
the events occur. This allows for recursive definitions.