Monthly Downloads: 8
Programming language: Haskell
License: BSD 3-clause "New" or "Revised" License
Tags: FRP     Reactive    
Latest version: v0.4.1

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FRP (functional reactive programming) framework inspired by RX, reactive-banana and Iteratee.

Main concepts are:

  • Event a : an event of type a. Either Next a or End
  • EventStream a : a stream of events of type a
  • EventSink a : function that processes events of type a
  • Source a : typeclass for things that can be converted into EventStream a
  • Property a : a property that changes as a function of time. has "current value"
  • PropertySink a: a function that processes property change events of type a
  • Observable a : common interface for EventStream and Property; you can assign side-effects to both using the same operators ==> and >>=!
  • Stream Combinators : transform and combile EventStreams with mapE, filterE, scanE, mergeE and combineLatestE etc
  • Property Combinators : transform and combine Properties with map, filterP, combineWithP etc

EventStream and Property

EventStream is a stream of events. It can be thought of as a list of (t, x) where t is time and x is a value. A Property is a value as a function of time. It may or may not have an initial value to start with but after the first value it will always have a current value that is can supply its "sinks" with.

For example, this is how you'd construct a stream that will pop out numbers 1, 2, 3 and 1 with a second's interval. The >>=! symbol is used to assign a side-effect; the values from the stream are printed to the console.

  sequentiallyE (seconds 1) [1, 2, 3, 1] >>=! print

A Property can be constructed from an EventStream using fromEventSource or fromEventSourceWithStartValue. A Property can be converted to an EventStream using changesP. It's noticeable that all of these methods are in the IO monad, meaning that Property and EventStream are not interchangeable without side-effects.

A Property differs from an EventStream in that it has the concept of "current value" that it will have after it has got it's first value. When you assign a side-effect to a Property, your "PropertySink" function will be called with the current value immediately, if there's a current value.

You should use EventStreams for discrete events (like mouse clicks) and Properties for values as function-of-time (like mouse position).


Property implements some of the standard typeclasses.

  • Functor : fmap = mapP
  • Applicative : based on combineWithP. You can apply a pure function to Property using this interface
  • Num, based on Applicative. (Yes, you can do a + Property a!)

So, you can use the Num interface to make calculations on Properties and constant numbers as in

  (xs, pushX) <- newPushProperty
  (ys, pushY) <- newPushProperty
  let sum = 100 + xs * 10 + ys
  sum ==> print
  pushX 1
  pushY 5
  pushX 2

So first I create two "pushable properties", and create a composite property "sum". I assign the "print" side-effect and then push some values to the properties. The output should obviously contain numbers 115 and 125.

EventStream only has an instance for Functor. All functions on EventStream are in the IO Monad, because most of them need mutable state to guarantee consistency (see below). The bright side of this is that you get a nice monadic syntax for applying transformations and side-effects, as in

  sequentiallyE (seconds 1) [1, 2, 3, 1] 
      >>= filterE (<3) 
      >>= mapE (("x=" ++) . show) 
      >>=! print

The custom infix operators introduced in bacon are

  • ==> : assign side-effect of type a -> IO ()
  • >>=! : a convenience-monadic version of the above, see usabe above. So >>=! f is equivalent to >>= (==> f)


For EventStream, there are several implementations included. In the Reactive.Bacon.EventStream.Timed module, there are

  • laterE that emits the given event after the given delay.
  • periodicallyE that repeats the given element with given interval
  • sequentiallyE that emits a list of elements with a given delay between
  • timedE that accepts a list of event-delay pairs
  • delayE that adds delay to given stream
  • throttleE that throttles the given stream

The Reactive.Bacon.PushStream module in turn allows you to create a "pushable" stream using the newPushStream function. You'll get a stream as well as a function for pushing new events into the stream.

For Property, there's newPushProperty plus the possibility to convert any EventStream in to a Property using fromEventStream or fromEventStreamWithStartValue.

Differences to RX:

  • EventStreams are strictly consistent with regard to time and sinks
  • Naming is more like Haskell/FP and less like SQL
  • No OnError event
  • The thing that is Observer in RX is roughly EventSink in bacon
  • Sink returns a handle result for each event. This means it can signal whether or not it wants more events.

One of the main motivators for reactive-bacon has been the weirdness of RX with regard to "hot" and "cold" observables. In reactive-bacon, there are no such things. The EventStream is always consistent with respect to time, so there will be no WTFs from that direction.

Differences to reactive-banana:

  • EventStream and Property have quite similar meanings as Event and Discrete in reactive-banana
  • No separate "build event network phase"
  • More combinators included

Another motivator for reactive-bacon was that reactive-banana seemed a bit inconvenient (event network building and stuff) and limited (not so many combinators) compared to RxJs of which I'm still quite fond of.

More examples?

More examples available!

See also tests.

EventStream and Property behavior in detail

The state of an EventStream can be defined as (t, os) where t is time and os the list of current sinks. This state should define the behavior of the stream in the sense that

  1. When an event is emitted, the same event is emitted to all sinks
  2. After an event has been emitted, it will never be emitted again, even if a new sink is registered.
  3. When a new sink is registered, it will get exactly the same events as the other sink, after registration. This means that the stream cannot emit any "initial" events to the new sink, unless it emits them to all of its sinks.
  4. A stream must never emit eny other events after End (not even another End)

The rules are deliberately redundant, explaining the constraints from different perspectives. The contract between an EventStream and its Sink is as follows:

  1. For each incoming event, the sink function Event a -> IO HandleResult a is called.
  2. The function returns a HandleResult which is either NoMore or More
  3. In case of NoMore the source must never call the consume function for this Sink again.

A Property behaves similarly to an EventStream except that

  1. On a call to addListener it will deliver its current value (if any) to the provided sink function.
  2. This means that if the Property has previously emitted the value x to its sinks and that is the latest value emitted, it will deliver this value to any registered Sink.
  3. Property may or may not have a current value to start with.

Implementing your own EventStream

As you'll soon enough notice, there are no bindings to GUI libraries available. So prepare to roll your own bindings if you need any.

The Reactive.Bacon.EventStream.IO module allows you to create an EventStream from a "stoppable" or "non-stoppable" process using the functions fromStoppableProcess and fromNonStoppableProcess.

You can also create an EventStream by using the EventStream constructor, in which case you'll have to manage Sinks, which are the functions registered as "observers" to your stream. So, you'll have to implement the "subscribe" function there. Then you'll also have to implement the "dispose" function that is the return value of "subscribe". The "subscribe" function will be called for each new sink added to your stream. The sink may later unsubscribe either by calling "dispose" or by returning NoMore as the result of any event. So, make sure you respect both ways of unsubscribing.

In case your "subscribe" function actually has any side-effects (even mutable state), you should use wrap to ensure consistency between sinks. For instance, if your stream starts any background process on-subscribe, you should definitely use wrap which will also ensure that a single background process is started regardless of the number of sinks.

On the other hand, if the subscription is stateless (like subscribing to mouse clicks), there's no need for wrapping.

EventStream is not a Monad

Well, that's it. It's nearly a monad, because there's a function named flatMapE that has a signature almost equivalent to >>= but there's a catch: It's not usually possible to create a new EventStream without resorting to IO, so the purely monadic signature would not make sense. Sorry dudes.

Building and using

It's simple: git clone, cabal install and it's there. You may now try it in GHCI, as in the examples above.

I've published this on HackageDB too, so you'll get you some tasty bacon by issuing the following:

cabal install reactive-bacon


  • Not "fully documented". Examples and tests should do though?
  • 27 test cases passing
  • Not tried out in "real life" yet

Design considerations

The concepts of "hot observable" vs "cold observable" have caused us trouble with RX. Cold observables (practically lists) may be used, for instance, to add a start value to a hot observable, in a case that we are actually modeling a property, such as location or a textfield value. This is problemsome because a cold observable always spits its value to any new sink. A new sink might actually expect to get the latest position instead of a fixed one.

I'm currently proposing a new abstraction Property for modeling a property that is actually a value as a function of time. A Property will always remember its latest value, so that new listeners will get the most up-to-date value to start with.

In reactive-banana terms Observable and Property roughly correspond to Event and Discrete.


  • Combinators for EventStream/Property combo, like combineWithProperty :: EventStream a -> Property b -> (a -> b -> c) -> EventStream c
  • Documentation for Hackage
  • mapReduceE, just for the sake of it
  • Try it out in the RUMP project