FRP.Sodium

Sodium Reactive Programming (FRP) system.

See the examples directory for test cases and examples.

The p type parameter determines the partition that your FRP is running on. A thread is automatically created for each partition used in the system based on the unique concrete p type, which must be an instance of Typeable. FRP processing runs on this thread, but synchronously will block the calling thread while it waits for FRP processing to complete.

In most cases you would just use one concrete partition type for everything, defined like this:

 {-# LANGUAGE DeriveDataTypeable, EmptyDataDecls #-}
 import Data.Typeable

 data M deriving Typeable

Later, if you want your code to be more parallel, you can add more partitions: The cross and crossE functions are used to move events and behaviours between partitions. The separation thus created allows your FRP logic to be partitioned so that the different partitions can run in parallel, with more relaxed guarantees of consistency between partitions.

Some functions are pure, and others need to run under the Reactive monad via synchronously or asynchronously. An Event p (Reactive p a) can be flattened to an Event p a using the execute primitive.

In addition to the explicit functions in the language, note that you can use

• Functor on Event and Behaviour
• Applicative on behaviour, e.g. let bsum = (+) <$> ba <*> bb
• Applicative pure is used to give a constant Behaviour.
• Recursive do (via DoRec) to make state loops with the rec keyword.

Here's an example of recursive do to write state-keeping loops. Note that all holds are delayed, so attachWith will capture the old value of the state s.

 {-# LANGUAGE DoRec #-}
 -- | Accumulate on input event, outputting the new state each time.
 accumE :: Typeable p => (a -> s -> s) -> s -> Event p a -> Reactive p (Event p s) 
 accumE f z ea = do
     rec
         let es = attachWith f ea s
         s <- hold z es
     return es

Running FRP code

data Reactive p a

Instances

Monad (Reactive p)

Functor (Reactive p)

MonadFix (Reactive p)

Applicative (Reactive p)

synchronously :: Typeable p => Reactive p a -> IO a

Execute the specified Reactive within a new transaction, blocking the caller until all resulting processing is complete and all callbacks have been called.

State changes to hold values occur after processing of the transaction is complete.

asynchronously :: Typeable p => Reactive p () -> IO ()

Execute the specified Reactive within a new transaction, firing it off without waiting for it to complete. It will be queued for executing on the FRP thread for the selected partition.

State changes to hold values occur after processing of the transaction is complete.

newEvent :: Typeable p => IO (Event p a, a -> Reactive p ())

Returns an event, and a push action for pushing a value into the event.

listenIO :: Event p a -> (a -> IO ()) -> Reactive p (IO ())

Listen for firings of this event. The returned IO () is an IO action that unregisters the listener. This is the observer pattern.

listenValueIO :: Behaviour p a -> (a -> IO ()) -> Reactive p (IO ())

Listen to the value of this behaviour with a guaranteed initial callback giving the current value, followed by callbacks for any updates.

FRP core language

data Event p a

A stream of events. The individual firings of events are called 'event occurrences'.

Instances

Functor (Event p)

data Behaviour p a

A time-varying value, British spelling.

Instances

Functor (Behaviour p)

Typeable p => Applicative (Behaviour p)

type Behavior p a = Behaviour p a

A time-varying value, American spelling.

never :: Event p a

An event that never fires.

merge :: Typeable p => Event p a -> Event p a -> Event p a

Merge two streams of events of the same type.

In the case where two event occurrences are simultaneous (i.e. both within the same transaction), both will be delivered in the same transaction.

The order is not defined, because simultaneous events should be considered to be order-agnostic.

mergeWith :: Typeable p => (a -> a -> a) -> Event p a -> Event p a -> Event p a

Merge two streams of events of the same type, combining simultaneous event occurrences.

In the case where multiple event occurrences are simultaneous (i.e. all within the same transaction), they are combined using the supplied function. The output event is guaranteed not to have more than one event occurrence per transaction.

The combine function should be commutative, because simultaneous events should be considered to be order-agnostic.

justE :: Typeable p => Event p (Maybe a) -> Event p a

Unwrap Just values, and discard event occurrences with Nothing values.

hold :: a -> Event p a -> Reactive p (Behaviour p a)

Create a behaviour with the specified initial value, that gets updated by the values coming through the event. The 'current value' of the behaviour is notionally the value as it was 'at the start of the transaction'. That is, state updates caused by event firings get processed at the end of the transaction.

valueEvent :: Typeable p => Behaviour p a -> Event p a

An event that fires once for the current value of the behaviour, and then for all changes that occur after that.

attachWith :: Typeable p => (a -> b -> c) -> Event p a -> Behaviour p b -> Event p c

Sample the behaviour at the time of the event firing. Note that the 'current value' of the behaviour that's sampled is the value as at the start of the transaction before any state changes of the current transaction are applied through holds.

switchE :: Typeable p => Behaviour p (Event p a) -> Event p a

Unwrap an event inside a behaviour to give a time-varying event implementation.

switch :: Typeable p => Behaviour p (Behaviour p a) -> Reactive p (Behaviour p a)

Unwrap a behaviour inside another behaviour to give a time-varying behaviour implementation.

execute :: Typeable p => Event p (Reactive p a) -> Event p a

Execute the specified Reactive action inside an event.

sample :: Behaviour p a -> Reactive p a

Obtain the current value of a behaviour.

Derived FRP functions

filterE :: Typeable p => (a -> Bool) -> Event p a -> Event p a

Only keep event occurrences for which the predicate is true.

attach :: Typeable p => Event p a -> Behaviour p b -> Event p (a, b)

Variant of attachWith defined as attachWith (,)

tag :: Typeable p => Event p a -> Behaviour p b -> Event p b

Variant of attachWith that throws away the event's value and captures the behaviour's.

gate :: Typeable p => Event p a -> Behaviour p Bool -> Event p a

Let event occurrences through only when the behaviour's value is True. Note that the behaviour's value is as it was at the start of the transaction, that is, no state changes from the current transaction are taken into account.

collectE :: Typeable p => (a -> s -> (b, s)) -> s -> Event p a -> Reactive p (Event p b)

Transform an event with a generalized state loop (a mealy machine). The function is passed the input and the old state and returns the new state and output value.

collect :: Typeable p => (a -> s -> (b, s)) -> s -> Behaviour p a -> Reactive p (Behaviour p b)

Transform a behaviour with a generalized state loop (a mealy machine). The function is passed the input and the old state and returns the new state and output value.

accumE :: Typeable p => (a -> s -> s) -> s -> Event p a -> Reactive p (Event p s)

Accumulate on input event, outputting the new state each time.

accum :: Typeable p => (a -> s -> s) -> s -> Event p a -> Reactive p (Behaviour p s)

Accumulate on input event, holding state.

countE :: Typeable p => Event p a -> Reactive p (Event p Int)

Count event occurrences, starting with 1 for the first occurrence.

count :: Typeable p => Event p a -> Reactive p (Behaviour p Int)

Count event occurrences, giving a behaviour that starts with 0 before the first occurrence.

once :: Typeable p => Event p a -> Reactive p (Event p a)

Throw away all event occurrences except for the first one.

Partitions

crossE :: (Typeable p, Typeable q) => Event p a -> Reactive p (Event q a)

Cross the specified event over to a different partition.

cross :: (Typeable p, Typeable q) => Behaviour p a -> Reactive p (Behaviour q a)

Cross the specified behaviour over to a different partition.