reactive-banana- Library for functional reactive programming (FRP).

Safe HaskellNone





Connect to the outside world by building EventNetworks and running them.

Simple use

interpretAsHandler :: (Event a -> Moment (Event b)) -> AddHandler a -> AddHandler b Source

Simple way to write a single event handler with functional reactive programming.


After having read all about Events and Behaviors, you want to hook them up to an existing event-based framework, like wxHaskell or Gtk2Hs. How do you do that?

The module presented here allows you to

  • obtain input events from external sources and to
  • perform output in reaction to events.

In contrast, the functions from Reactive.Banana.Combinators allow you to express the output events in terms of the input events. This expression is called an event graph.

An event network is an event graph together with inputs and outputs. To build an event network, describe the inputs, outputs and event graph in the MomentIO monad and use the compile function to obtain an event network from that.

To activate an event network, use the actuate function. The network will register its input event handlers and start producing output.

A typical setup looks like this:

main = do
  -- initialize your GUI framework
  window <- newWindow

  -- describe the event network
  let networkDescription :: MomentIO ()
      networkDescription = do
          -- input: obtain  Event  from functions that register event handlers
          emouse    <- fromAddHandler $ registerMouseEvent window
          ekeyboard <- fromAddHandler $ registerKeyEvent window
          -- input: obtain  Behavior  from changes
          btext     <- fromChanges    "" $ registerTextChange editBox
          -- input: obtain  Behavior  from mutable data by polling
          bdie      <- fromPoll       $ randomRIO (1,6)

          -- express event graph
          behavior1 <- accumB ...
              event15 = union event13 event14

          -- output: animate some event occurrences
          reactimate $ fmap print event15
          reactimate $ fmap drawCircle eventCircle

  -- compile network description into a network
  network <- compile networkDescription
  -- register handlers and start producing outputs
  actuate network

In short,

  • Use fromAddHandler to obtain input events. The library uses this to register event handlers with your event-based framework.
  • Use reactimate to animate output events.
  • Use compile to put everything together in an EventNetworks and use actuate to start handling events.

Building event networks with input/output

Core functions

compile :: MomentIO () -> IO EventNetwork Source

Compile the description of an event network into an EventNetwork that you can actuate, pause and so on.

data MomentIO a Source

The MomentIO monad is used to add inputs and outputs to an event network.

fromAddHandler :: AddHandler a -> MomentIO (Event a) Source

Input, obtain an Event from an AddHandler.

When the event network is actuated, this will register a callback function such that an event will occur whenever the callback function is called.

fromChanges :: a -> AddHandler a -> MomentIO (Behavior a) Source

Input, obtain a Behavior from an AddHandler that notifies changes.

This is essentially just an application of the stepper combinator.

fromPoll :: IO a -> MomentIO (Behavior a) Source

Input, obtain a Behavior by frequently polling mutable data, like the current time.

The resulting Behavior will be updated on whenever the event network processes an input event.

This function is occasionally useful, but the recommended way to obtain Behaviors is by using fromChanges.

Ideally, the argument IO action just polls a mutable variable, it should not perform expensive computations. Neither should its side effects affect the event network significantly.

reactimate :: Event (IO ()) -> MomentIO () Source

Output. Execute the IO action whenever the event occurs.

Note: If two events occur very close to each other, there is no guarantee that the reactimates for one event will have finished before the ones for the next event start executing. This does not affect the values of events and behaviors, it only means that the reactimate for different events may interleave. Fortunately, this is a very rare occurrence, and only happens if

  • you call an event handler from inside reactimate,
  • or you use concurrency.

In these cases, the reactimates follow the control flow of your event-based framework.

Note: An event network essentially behaves like a single, huge callback function. The IO action are not run in a separate thread. The callback function will throw an exception if one of your IO actions does so as well. Your event-based framework will have to handle this situation.

data Future a Source

The Future monad is just a helper type for the changes function.

A value of type Future a is only available in the context of a reactimate but not during event processing.

reactimate' :: Event (Future (IO ())) -> MomentIO () Source

Output. Execute the IO action whenever the event occurs.

This version of reactimate can deal with values obtained from the changes function.

changes :: Behavior a -> MomentIO (Event (Future a)) Source

Output, return an Event that is adapted to the changes of a Behavior.

Remember that semantically, a Behavior is a function Behavior a = Time -> a. This means that a Behavior does not have a notion of "changes" associated with it. For instance, the following Behaviors are equal:

stepper 0 []
= stepper 0 [(time1, 0), (time2, 0)]
= stepper 0 $ zip [time1,time2..] (repeat 0)

In principle, to perform IO actions with the value of a Behavior, one has to sample it using an Event and the apply function.

However, in practice, Behaviors are usually step functions. For reasons of efficiency, the library provides a way to obtain an Event that mostly coincides with the steps of a Behavior, so that sampling is only done at a few select points in time. The idea is that

changes =<< stepper x e  =  return e

Please use changes only in a ways that do not distinguish between the different expressions for the same Behavior above.

Note that the value of the event is actually the new value, i.e. that value slightly after this point in time. (See the documentation of stepper). This is more convenient. However, the value will not become available until after event processing is complete; this is indicated by the type Future. It can be used only in the context of reactimate'.

Note: If you need a variant of the changes function that does not have the additional Future type, then the following code snippet may be useful:

plainChanges :: Behavior a -> MomentIO (Event a)
plainChanges b = do
    (e, handle) <- newEvent
    eb <- changes b
    reactimate' $ (fmap handle) <$> eb
    return e

However, this approach is not recommended, because the result Event will occur slightly later than the event returned by changes. In fact, there is no guarantee whatsoever about what slightly means in this context. Still, it is useful in some cases.

imposeChanges :: Behavior a -> Event () -> Behavior a Source

Impose a different sampling event on a Behavior.

The Behavior will have the same values as before, but the event returned by the changes function will now happen simultaneously with the imposed event.

Note: This function is useful only in very specific circumstances.

execute :: Event (MomentIO a) -> MomentIO (Event a) Source

Dynamically add input and output to an existing event network.

Note: You can even do IO actions here, which is useful if you want to register additional event handlers dynamically. However, there is no guarantee about the order in which the actions are executed. If the result Event of this function is garbage collected, it may also happen that the actions are not executed at all. If you want a reliable way to turn events into IO actions use the reactimate and reactimate' functions.

liftIOLater :: IO () -> MomentIO () Source

Lift an IO action into the Moment monad, but defer its execution until compilation time. This can be useful for recursive definitions using MonadFix.

liftIO :: Frameworks t => IO a -> Moment t a

Lift an IO action into the Moment monad.

Utility functions

This section collects a few convience functions built from the core functions.

interpretFrameworks :: (Event a -> MomentIO (Event b)) -> [Maybe a] -> IO [Maybe b] Source

Interpret an event processing function by building an EventNetwork and running it. Useful for testing, but uses MomentIO. See interpret for a plain variant.

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

Build an Event together with an IO action that can fire occurrences of this event. Variant of newAddHandler.

This function is mainly useful for passing callback functions inside a reactimate.

mapEventIO :: (a -> IO b) -> Event a -> MomentIO (Event b) Source

Build a new Event that contains the result of an IO computation. The input and result events will not be simultaneous anymore, the latter will occur later than the former.

Please use the fmap for Event if your computation is pure.


mapEventIO f e1 = do
    (e2, handler) <- newEvent
    reactimate $ (\a -> f a >>= handler) <$> e1
    return e2

newBehavior :: a -> MomentIO (Behavior a, Handler a) Source

Build a Behavior together with an IO action that can update this behavior with new values.


newBehavior a = do
    (e, fire) <- newEvent
    b         <- stepper a e
    return (b, fire)

Running event networks

data EventNetwork Source

Data type that represents a compiled event network. It may be paused or already running.

actuate :: EventNetwork -> IO () Source

Actuate an event network. The inputs will register their event handlers, so that the networks starts to produce outputs in response to input events.

pause :: EventNetwork -> IO () Source

Pause an event network. Immediately stop producing output. (In a future version, it will also unregister all event handlers for inputs.) Hence, the network stops responding to input events, but it's state will be preserved.

You can resume the network with actuate.

Note: You can stop a network even while it is processing events, i.e. you can use pause as an argument to reactimate. The network will not stop immediately though, only after the current event has been processed completely.