How to Trigger Multiple Msgs in Elm

2019-01-13

In the Elm Architecture, update function is responsible for changing Model state. Depending on how you structure your Model, Msg, and update, sometimes you may want to call update function again with another Msg after calling update function. Recursively calling update is straightforward.

type Msg
    = FirstMsg
    | SecondMsg

update : Msg -> Model -> ( Model, Cmd Msg )
update msg model =
    case msg of
        FirstMsg -> 
            update SecondMsg model

        SecondMsg -> 
            ( model, Cmd.none )

But sometimes you may want to trigger multiple Msgs. Task can be used here.

type Msg
    = FirstMsg
    | SecondMsg ()
    | ThirdMsg ()

update : Msg -> Model -> ( Model, Cmd Msg )
update msg model =
    case msg of
        FirstMsg -> 
            let
                cmd =
                      Cmd.batch
                          [ Task.perform SecondMsg (Task.succeed ())
                          , Task.perform ThirdMsg (Task.succeed ())
                          ]
            in
                ( model, cmd )

        SecondMsg -> 
            ( model, Cmd.none )

        ThirdMsg ->
            ( model, Cmd.none )

The caveat is that there’s no guaranteed ordering between SecondMsg and ThirdMsg, and that these subsequent Msgs require arguments. Most importantly, I think recursively calling update is a bad practice. Use this approach only when there’s absolutely no other way.