elm-action

This module lets you organize your update function for different states.

For example:

    update : Msg -> Model -> (Model, Cmd Msg)
    update msg model =
        case (msg,model) of
            (LoggedIn guestMsg,Nothing) ->
                updateGuest guestMsg
                |> Action.config
                |> Action.withTransition
                    (\name ->
                      (Just <| initUser name,Cmd.none)
                    )
                |> Action.withUpdate (always Nothing) never
                |> Action.apply
            (UserSpecific userMsg,Just user) ->
                updateUser userMsg
                |> Action.config
                |> Action.withUpdate Just UserSpecific
                |> Action.withExit (Nothing,Cmd.none)

Here we have two states: Guest and User. Each of them has there own update function:

    updateGuest : {name:String,pass:String} -> GuestAction
    updateGuest {name,pass} =
        if pass = "password" then
            Action.transitioning name
        else
            Action.updating ((),Cmd.none)


    updateUser : UserMsg -> User -> UserAction
    updateUser msg user =
        case msg of
            Commented string ->
                Debug.todo "send comment to server"
            LoggedOut ->
                Action.exiting

Under the hood, an Action is a state transition of a state machine. The corresponding state machine looks like this:

   LoggedOut?  +--User<-+   !pass == "password"
               |        |
               v        |
               +->Guest-+   ?LoggedIn => pass == "password"?
                   ^    |
                   +----+   !pass /= "password"