Reactive.Banana.Frameworks

Synopsis

Build event networks using existing event-based frameworks and run them.

Simple use

interpretAsHandler :: (forall t. Event t a -> Event t b) -> AddHandler a -> AddHandler b

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

Building event networks with input/output

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 constrast, 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 Moment 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 :: forall t. Frameworks t => Moment t ()
      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
          let
              behavior1 = accumB ...
              ...
              event15 = union event13 event14
  
          -- output: animate some event occurences
          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.

compile :: (forall t. Frameworks t => Moment t ()) -> IO EventNetwork

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

Event networks are described in the Moment monad and use the Frameworks class constraint.

class Frameworks t

Class constraint on the type parameter t of the Moment monad.

Indicates that we can add input and output to an event network.

module Control.Event.Handler

fromAddHandler :: Frameworks t => AddHandler a -> Moment t (Event t a)

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 :: Frameworks t => a -> AddHandler a -> Moment t (Behavior t a)

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

This is essentially just an application of the stepper combinator.

fromPoll :: Frameworks t => IO a -> Moment t (Behavior t a)

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 :: Frameworks t => Event t (IO ()) -> Moment t ()

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. Fortuantely, 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

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.

Instances

Monad Future

Functor Future

Applicative Future

reactimate' :: Frameworks t => Event t (Future (IO ())) -> Moment t ()

Output. Execute the IO action whenever the event occurs.

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

initial :: Behavior t a -> Moment t a

Output, observe the initial value contained in a Behavior.

changes :: Frameworks t => Behavior t a -> Moment t (Event t (Future a))

Output, observe when a Behavior changes.

Strictly speaking, a Behavior denotes a value that varies continuously in time, so there is no well-defined event which indicates when the behavior changes.

Still, for reasons of efficiency, the library provides a way to observe changes when the behavior is a step function, for instance as created by stepper. There are no formal guarantees, but the idea is that

changes (stepper x e) = return (calm e)

Note: The values of the event will not become available until event processing is complete. It can be used only in the context of reactimate'.

imposeChanges :: Frameworks t => Behavior t a -> Event t () -> Behavior t a

Impose a different sampling event on a Behavior.

The Behavior will vary continuously 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.

newtype FrameworksMoment a

Dummy type needed to simulate impredicative polymorphism.

Constructors

FrameworksMoment

Fields

runFrameworksMoment :: forall t. Frameworks t => Moment t a

execute :: Frameworks t => Event t (FrameworksMoment a) -> Moment t (Event t a)

Dynamically add input and output to an existing event network.

Note: You can even do IO actions here, but there is no guarantee about the order in which they are executed.

liftIOLater :: Frameworks t => IO () -> Moment t ()

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.

module Control.Monad.IO.Class

Running event networks

data EventNetwork

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

actuate :: EventNetwork -> IO ()

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 ()

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.

Utilities

This section collects a few convenience functions for unusual use cases. For instance:

• The event-based framework you want to hook into is poorly designed
• You have to write your own event loop and roll a little event framework

newEvent :: Frameworks t => Moment t (Event t a, Handler a)

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.

Internal

interpretFrameworks :: (forall t. Event t a -> Event t b) -> [a] -> IO [[b]]

Interpret by using a framework internally. Only useful for testing library internals.

showNetwork :: EventNetwork -> IO String

A multiline description of the current Latches and Pulses in the EventNetwork.

Incidentally, evaluation the returned string to normal form will also force the EventNetwork to some kind of normal form. This may be useful for benchmarking purposes.