An Event a is an object representing an event of type a. You can register actions through it – see the on function – and detach them later on.

An Event has many purposes. The one in mind when writing that package was to interface over C callback-based reactive system. Consider the following Haskell wrapper function, which is based on imperative style:

  -- Create a new button and register an action to launch when the button’s
  -- state changes.
  createButton :: (ButtonState -> IO ()) -> IO Button
  createButton callback = do
    -- create the button
    button <- ...
    forkIO . forever $ do
      -- launch a thread in which we can test whether the state has changed
      when stateHasChanged $ callback newState
    pure button

We can enhance that by representing the action of registering to the event and detaching from it by immediately returning a value:

  createButton :: IO (Button,Event ButtonState)
  createButton = do
    -- create the button
    button <- ...
    -- create an Event
    (ev,t) <- newEvent
    forkIO . forever $
      -- check the new state
      when stateHasChanged $ trigger t newState
    pure (button,ev)

The Trigger can also be returned to manually invoke the Event.