Elm-Submodules

Elm is not a "component based" framework and this is not a problem. There are different strategies for reusing parts of your application and these start out as being way simpler than what you would usually expect from "components".

That being said, sometimes you do need something more "stateful" but you shouldn't need to lose the simplicity of Elm when that time comes. This module aims to help you to do just that.

Creating a Submodule

Let's call those nested "stateful" components "submodules" from now on. When you're working with a submodule you need to define one extra type alongside your usual Model and Msg.

type alias Model
    = { value : String }

type Msg
    = UpdateValue String
    | Submit

type Effect
    = SendValue String

As you can guess, the Effect type serves as a way to "send" a "msg" to the host of your submodule. What is great about it is that this make it a hard requirement for the host module to handle that msg (and, as you will see, there is absolutely no pain in setting it up on the host as well).

To use the Effect you would create a slightly different update function:

update : Msg -> Model -> ( Model, SubCmd Msg Effect )
update msg model =
    case msg of
        UpdateValue value ->
            ( { value = value }, SubCmd.none )

        Submit ->
            ( model, SubCmd.effect (SendValue model.value) )

Done! Your submodule uses the SubCmd module to send both cmd's and effect's and it Just Works®.

Plugging it into a "host" module

Well, how would I actually plug this submodule into my application?

Imagine you have a submodule called Widget. Using it would be as simple as doing this in the host module:

import Widget


type alias Model =
    { widget : Widget.Model
    , valueFromWidget : Maybe String
    }


type Msg
    = GotWidgetMsg Widget.Msg
    | GotWidgetEffect Widget.Effect


update : Msg -> Model -> ( Model, Cmd Msg )
update msg model =
    case msg of
        GotWidgetMsg widgetMsg ->
            Widget.update widgetMsg model.widget
                |> SubModule.updateWithEffect
                    { toMsg = GotWidgetMsg
                    , effectToMsg = GotWidgetEffect
                    , toModel = \widget -> { model | widget = widget }
                    }

        GotWidgetEffect widgetEffect ->
            case widgetEffect of
                Widget.SendValue value ->
                    ( { model | valueFromWidget = Just value }
                    , Cmd.none
                    )


view : Html Msg
view =
    div []
        [ Widget.view
            |> Html.map GotWidgetMsg
        ]

Note that your submodule could also send effects on their init. In fact, they could send effects only on their init or update and that would be ok!

Documentation

• Checkout SubCmd if you're creating a submodule.
• Checkout SubModule if you're plugging a submodule into a host module.