Update

Our application is still not doing anything as we are ignoring the message we added. We need to do respond to it. Change update to:

update : Msg -> Model -> ( Model, Cmd Msg )
update msg model =
    case msg of
        ChangeCount howMuch ->
            ( { model | count = model.count + howMuch }, Cmd.none )

        NoOp ->
            ( model, Cmd.none )

Here we have introduced a case condition. We pattern-match on the msg argument. Note how we extract the message payload into howMuch in the first pattern.

Update syntax

In the first branch of our case expression, we want to produce an updated model, with the count increased by howMuch. In Elm, you update a record with the following syntax:

{ model | count = model.count + howMuch }

This returns a new version of model where count has been updated. Any other fields that might be present in model will remain unmodified in the new record.

Try this in your browser. Clicking the button should update the count now.