Safe Haskell | None |
---|---|

Language | Haskell98 |

- data Event a
- data Behavior a
- type Handler a = a -> IO ()
- newEvent :: IO (Event a, Handler a)
- register :: Event a -> Handler a -> IO (IO ())
- currentValue :: MonadIO m => Behavior a -> m a
- module Control.Applicative
- never :: Event a
- filterJust :: Event (Maybe a) -> Event a
- unionWith :: (a -> a -> a) -> Event a -> Event a -> Event a
- accumE :: MonadIO m => a -> Event (a -> a) -> m (Event a)
- apply :: Behavior (a -> b) -> Event a -> Event b
- stepper :: MonadIO m => a -> Event a -> m (Behavior a)
- (<@>) :: Behavior (a -> b) -> Event a -> Event b
- (<@) :: Behavior a -> Event b -> Event a
- filterE :: (a -> Bool) -> Event 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] -> Event [a]
- concatenate :: [a -> a] -> a -> a
- accumB :: MonadIO m => a -> Event (a -> a) -> m (Behavior a)
- mapAccum :: MonadIO m => acc -> Event (acc -> (x, acc)) -> m (Event x, Behavior acc)
- data Tidings a
- tidings :: Behavior a -> Event a -> Tidings a
- facts :: Tidings a -> Behavior a
- rumors :: Tidings a -> Event a
- onChange :: Behavior a -> Handler a -> IO ()
- unsafeMapIO :: (a -> IO b) -> Event a -> Event b
- newEventsNamed :: Ord name => Handler (name, Event a, Handler a) -> IO (name -> Event a)

# Synopsis

Functional reactive programming.

# Types

At its core, Functional Reactive Programming (FRP) is about two
data types `Event`

and `Behavior`

and the various ways to combine them.

`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 occurence,

type Event a = [(Time,a)]

`Behavior a`

represents a value that varies in time. Think of it as

type Behavior a = Time -> a

# IO

Functions to connect events to the outside world.

type Handler a = a -> IO () Source

An *event handler* is a function that takes an
*event value* and performs some computation.

newEvent :: IO (Event a, Handler a) Source

Create a new event. Also returns a function that triggers an event occurrence.

register :: Event a -> Handler a -> IO (IO ()) Source

Register an event `Handler`

for an `Event`

.
All registered handlers will be called whenever the event occurs.

When registering an event handler, you will also be given an action that unregisters this handler again.

do unregisterMyHandler <- register event myHandler

FIXME: Unregistering event handlers does not work yet.

currentValue :: MonadIO m => Behavior a -> m a Source

Read the current value of a `Behavior`

.

# Core Combinators

module Control.Applicative

filterJust :: Event (Maybe a) -> Event a Source

Return all event occurrences that are `Just`

values, discard the rest.
Think of it as

filterJust es = [(time,a) | (time,Just a) <- es]

unionWith :: (a -> a -> a) -> Event a -> Event a -> Event a Source

Merge two event streams of the same type. In case of simultaneous occurrences, the event values are combined with the binary function. Think of it as

unionWith f ((timex,x):xs) ((timey,y):ys) | timex == timey = (timex,f x y) : unionWith f xs ys | timex < timey = (timex,x) : unionWith f xs ((timey,y):ys) | timex > timey = (timey,y) : unionWith f ((timex,x):xs) ys

apply :: Behavior (a -> b) -> Event a -> Event b Source

Apply a time-varying function to a stream of events. Think of it as

apply bf ex = [(time, bf time x) | (time, x) <- ex]

stepper :: MonadIO m => a -> Event a -> m (Behavior a) Source

Construct a time-varying function from an initial value and a stream of new values. Think of it as

stepper x0 ex = return $ \time -> last (x0 : [x | (timex,x) <- ex, timex < time])

Note that the smaller-than-sign in the comparision `timex < time`

means
that the value of the behavior changes "slightly after"
the event occurrences. This allows for recursive definitions.

*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. Think of it as `pure x = \time -> x`

.

(<*>) :: Behavior (a -> b) -> Behavior a -> Behavior b

Combine behaviors in applicative style.
Think of it as `bf <*> bx = \time -> bf time $ bx time`

.

# Derived Combinators

## Filtering

filterE :: (a -> Bool) -> Event a -> Event a Source

Return all event occurrences that fulfill the predicate, discard the rest.

filterApply :: Behavior (a -> Bool) -> Event a -> Event a Source

Return all event occurrences that fulfill the time-varying predicate,
discard the rest. Generalization of `filterE`

.

whenE :: Behavior Bool -> Event a -> Event a Source

Return event occurrences only when the behavior is `True`

.
Variant of `filterApply`

.

## Union

concatenate :: [a -> a] -> a -> a Source

Apply a list of functions in succession.
Useful in conjunction with `unions`

.

concatenate [f,g,h] = f . g . h

## Accumulation

Note: All accumulation functions are strict in the accumulated value!
acc -> (x,acc) is the order used by `unfoldr`

and `State`

.

accumB :: MonadIO m => a -> Event (a -> a) -> m (Behavior a) Source

The `accumB`

function is similar to a *strict* left fold, `foldl'`

.
It starts with an initial value and combines it with incoming events.
For example, think

accumB "x" [(time1,(++"y")),(time2,(++"z"))] = stepper "x" [(time1,"xy"),(time2,"xyz")]

Note that the value of the behavior changes "slightly after" the events occur. This allows for recursive definitions.

# Additional Notes

Recursion in the `IO`

monad is possible, but somewhat limited.
The main rule is that the sequence of IO actions must be known
in advance, only the values may be recursive.

Good:

mdo let e2 = apply (const <$> b) e1 -- applying a behavior is not an IO action b <- accumB $ (+1) <$ e2

Bad:

mdo b <- accumB $ (+1) <$ e2 -- actions executed here could depend ... let e2 = apply (const <$> b) e1 -- ... on this value

# Tidings

tidings :: Behavior a -> Event a -> Tidings a Source

Smart constructor. Combine facts and rumors into `Tidings`

.

# Internal

Functions reserved for special circumstances. Do not use unless you know what you're doing.

unsafeMapIO :: (a -> IO b) -> Event a -> Event b Source

:: Ord name | |

=> Handler (name, Event a, Handler a) | Initialization procedure. |

-> IO (name -> Event a) | Series of events. |

Create a series of events with delayed initialization.

For each name, the initialization handler will be called exactly once when the event is first "brought to life", e.g. when an event handler is registered to it.