The elm-pages Architecture

Let's look at the lifecycle of a request in an elm-pages app.

This will walk through 3 different interactions.

1. Open Page
2. Click Button
3. Submit Form

These 3 interactions show the key components of The elm-pages Architecture.

Interaction 1 - Open Page#

If we open our browser to a server-rendered elm-pages Route https://my-app.com/feed, it will render from our Route module app/Route/Feed.elm.

1a - Resolving data#

1. data is resolved first
2. The elm-pages Engine resolves our data, performing HTTP requests, reading data, and running any custom tasks we've defined.
3. BackendTask's BackendTask.Custom.run are executed by running our custom-backend-task Node module, which elm-pages transpiles using esbuild.

1b - Rendering the Page#

Now that we have our data from Step 1, we can render the page.

First, we will server-render the HTML. So the Browser will get an HTML page with all of our initial page content (not an empty shell).

This has some benefits for both performance and SEO. With a fully rendered HTML page, the initial page load allows the browser to start loading all of the images on the page immediately before parsing and hydrating the Elm app. It also means that we have a meaningful and interactive app before hydration.

Our elm-pages Backend takes the data resolved from Step 1, calls our Route Module's init to get any initial page state, and then calls our Route Module's view to render the HTML for the page. Note that update is never called by the Backend during server-side rendering, so we have a predictable view from our Backend.

1c - Hydration#

Now that the browser has received the HTML from Step 1b, it can show the user a meaningful page, and then it's time for the next step - hydration.

Interaction 2 - Client-Side Interaction#

Client-side interactions that trigger a Msg in our Route Module go through the standard Elm Architecture lifecycle, calling update and re-rendering the view in the browser.

Interaction 3 - Form Submission#

The Code#

We could define our Route Module like this:

module Route.Feed exposing (ActionData, Data, Model, Msg, RouteParams, route)

import BackendTask exposing (BackendTask)
import BackendTask.Custom
import Effect
import ErrorPage exposing (ErrorPage)
import FatalError exposing (FatalError)
import Form
import Form.Field
import Form.Validation
import Head
import Html
import Html.Attributes
import Json.Decode as Decode exposing (Decoder)
import Json.Encode as Encode
import PagesMsg exposing (PagesMsg)
import Pages.Form
import RouteBuilder
import Server.Request
import Server.Response
import Server.Session
import Server.SetCookie
import Shared
import View


type alias Model =
    { showMenu : Bool }


type Msg
    = ToggleMenu


type alias RouteParams =
    {}


route : RouteBuilder.StatefulRoute RouteParams Data ActionData Model Msg
route =
    RouteBuilder.serverRender { data = data, action = action, head = head }
        |> RouteBuilder.buildWithLocalState
            { view = view
            , init = init
            , update = update
            , subscriptions = \_ _ _ _ -> Sub.none
            }


init :
    RouteBuilder.App Data ActionData RouteParams
    -> Shared.Model
    -> ( Model, Effect.Effect Msg )
init app shared =
    ( { showMenu = False }, Effect.none )


update :
    RouteBuilder.App Data ActionData RouteParams
    -> Shared.Model
    -> Msg
    -> Model
    -> ( Model, Effect.Effect Msg )
update app shared msg model =
    case msg of
        ToggleMenu ->
            ( { model | showMenu = not model.showMenu }, Effect.none )


type alias Data =
    { user : User
    , posts : List Post
    }


type alias ActionData =
    {}


data :
    RouteParams
    -> Request
    -> BackendTask FatalError (Response Data ErrorPage)
data routeParams request =
    Server.Request.succeed ()
        |> Server.Session.withSession
            { name = "session"
            , options = Server.SetCookie.initOptions
            , secrets = BackendTask.succeed [ "my-secret" ]
            }
            (\() sessionResult ->
                BackendTask.map2 Data
                    (getUser sessionResult)
                    (BackendTask.Custom.run "getPosts" Encode.null postsDecoder |> BackendTask.allowFatal)
                    |> BackendTask.map Server.Response.render
                    |> BackendTask.map (Tuple.pair (sessionResult |> Result.withDefault Server.Session.empty))
            )


head : RouteBuilder.App Data ActionData RouteParams -> List Head.Tag
head app =
    []


view :
    RouteBuilder.App Data ActionData RouteParams
    -> Shared.Model
    -> Model
    -> View.View (PagesMsg Msg)
view app shared model =
    { title = "Feed"
    , body =
        [ navbarView app.data.user ToggleMenu model.showMenu
        , postsView app
        ]
    }


postsView app =
    app.data.posts
        |> List.map (\post -> favoriteFormView app post)
        |> Html.div []


favoriteFormView app post =
    Pages.Form.renderHtml []
        (Form.options ("favorite-" ++ post.id)
            |> Form.withInput post.isFavorited
        )
        app


action :
    RouteParams
    -> Request
    -> BackendTask FatalError (Response ActionData ErrorPage)
action routeParams request =
    case request |> Server.Request.formData formHandlers of
        Just ( _, parsedForm ) ->
                case parsedForm of
                    Ok (ToggleFavorite validatedForm) ->
                        validatedForm.setFavorite
                            |> toggleFavorite
                            |> BackendTask.map (\() -> Server.Response.render {})

                    Err error ->
                        BackendTask.succeed (Server.Response.render {})
	Nothing ->
	    BackendTask.succeed (Server.Response.render {})


toggleFavoriteForm : Form.HtmlForm String ToggleFavoriteForm Bool Msg
toggleFavoriteForm =
    (\setFavorite ->
        { combine =
            ToggleFavoriteForm
                |> Form.Validation.succeed
                |> Form.Validation.andMap setFavorite
        , view =
            \formState -> [ favoriteButton formState ]
        }
    )
        |> Form.init
        |> Form.hiddenKind ( "kind", "regular" ) "Expected kind."
        |> Form.hiddenField "set-favorite" Form.Field.checkbox


type Action
    = ToggleFavorite ToggleFavoriteForm


type alias ToggleFavoriteForm =
    { setFavorite : Bool }


formHandlers : Form.ServerForms String Action
formHandlers =
    Form.Handler.init ToggleFavorite toggleFavoriteForm