Events

Pux listens for DOM events and reifies them using the application's event type. For example, a simple counter needs events for incrementing and decrementing in response to clicking buttons:

data Event = Increment | Decrement

When raw DOM events are needed, they can be wrapped by the application event type:

data Event = Increment DOMEvent | Decrement DOMEvent

DOMEvent from Pux.DOM.Events is a type alias for purescript-dom's event type.

Folding over the past with foldp

Whenever an event occurs a new state is produced by folding it with the current state. This function is called foldp and is analogous to foldl, but instead of folding from left to right it folds from past to present. Continuing the counter example, the previous count is combined with the current event to produce a new count:

foldp :: ∀ fx. Event -> State -> EffModel State Event fx
foldp Increment n = { state: n + 1, effects: [] }
foldp Decrement n = { state: n - 1, effects: [] }

The EffModel

foldp produces a new state along with effects. These are wrapped in a record that Pux calls an EffModel.

type EffModel st ev fx =
  { state   :: st
  , effects :: Array (Aff (CoreEffects fx) (Maybe ev))
  }

Effectful computations

Effectful computations returned by foldp are asynchronous and handled by the Aff monad from purescript-aff. These computations may return new events which are again handled by foldp.

For example, to log to the console when an event occurs we can use purescript-aff's log:

foldp :: Event -> State -> EffModel State Event (console :: CONSOLE)
foldp Increment count =
  { state: count + 1
  , effects: [ log "increment" *> pure Nothing ]
  }

Returning Nothing after logging denotes that the computation does not generate a new event.

Some effectful computations return a value and new event. For example, an HTTP request to remotely load todos might have an event RequestTodos. When this event occurs foldp returns an effect – the AJAX request – which returns a new event ReceiveTodos:

foldp :: Event -> State -> EffModel State Event (ajax :: AJAX)
foldp (RequestTodos) st =
  { state: st { status = "Fetching todos..." }
  , effects: [ do
      res <- attempt $ get "http://jsonplaceholder.typicode.com/users/1/todos"
      let todos = either (Left <<< show) (decodeJson res.response :: Either String Todos)
      pure $ Just $ ReceiveTodos todos
    ]
  }

foldp (ReceiveTodos t) st =
  noEffects $ case t of
    Left err -> st { status = "Error fetching todos: " <> show err }
    Right todos -> st { todos = todos, status = "" }

Nesting events

Read the Components section to learn more about organizing your app.

Sometimes you want to compose foldp functions with different event and state types when using external libraries or splitting your foldp function into submodules. To do this, create a parent event type wrapping the child's type and embed the child state in the parent state:

-- | Wrap the child event type
data Event
  = PageView Route
  | ChildEvent Child.Event

-- | Extend the state with the child's state
type State =
  { route :: Route
  , child :: Child.State
  }

Whenever a child event occurs pass it to the child's foldp function, then map over the returned EffModel using Pux's mapEffects and mapState:

foldp :: ∀ fx. Event -> State -> EffModel State Event fx
foldp (PageView r) st = noEffects $ st { route = r }
foldp (ChildEvent e) st =
  Child.foldp e st.child
    # mapEffects ChildEvent
    # mapState \sb -> st { child = sb }

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