What the elm architecture has to do with turtles

It’s turtles all the way down.

The universal pattern.

So good redux borrowed it.

turtles Pelf at en.Wikipedia was the original creator who placed the work in the PD; I modified the image Pelf placed there, CC0, via Wikimedia Commons

The Elm Architecture

In Elm we usually structure our apps around 3 key parts:

Model, View, and Update

Model describes the shape and structure of our app.

type alias Model = 
  { blogPosts: RemoteData (List BlogPost)
   , session : Session
   , timezome : Time.Zone

View renders it on to the screen

view : Model -> Html Msg
view model =
  Html.div [ Attributes.class "flex flex-col" ]
  [ model.blogPosts
    |> RemoteData.map viewBlogPosts
    |> RemoteData.withDefault viewLoading

viewBlogPosts : List BlogPost -> Html Msg

And Update takes in our State and a message describing how we want to change our State.

update : Msg -> Model -> (Model, Cmd Msg)

Taking a look at Browser - browser 1.0.2 We can see where each piece plugs into the elm package for creating an Elm app, called elm/browser

{ init : flags -> ( model, Cmd msg ) 
-- initialize state and kick off any startup tasks/processes, runs immediately
  , view : model -> Html msg 
-- plug in our view function which will render our app with our initial state, and re-run anytime `update` is called
  , update : msg -> model -> ( model, Cmd msg ) 
-- plug in our update function for when we start receiving Msgs that describe changes to our `model`, returning a new `model` and a `Cmd msg` which may kick off another process or async behaviour.
  , subscriptions : model -> Sub msg 
-- plug in our listeners for background processes like Ports.

These are the top-level functions that every (most) typical Elm apps needs to initialize. Together they make up the Elm Architecture, wiring together each piece.

Its unidirectional data-flow is a robust and reliable pattern for creating different types of apps.

A Simple Ellie example

Most if not all top level update functions in an Elm app will have this type signature, because that is what Browser expects to plugin to its config. update : Msg -> Model -> (Model, Cmd Msg) So when the Architecture calls our update function, it will pass the Msg and Model as arguments and expects to return a new Model and a Cmd msg.

Except for sanbox because it Doesn’t talk to the outside world, its sandboxed. Its type is update : Msg -> Model -> Model It doesn’t return a Cmd msg because it can’’t interact with anything outside of its pure system, like Http requests, or Ports to talk to JavaScript.

Once you start to build anything beyond sandbox things can get a little more confusing trying to interact with the outside world.

Some questions I’ve had and have heard are:

So why does sandbox not return a Cmd msg in our pair of (model, Cmd msg)?

What is a Cmd Msg? And what’s it got to do with Sub Msg and Html Msg?

Cmd comes from the Elm core Platform.Cmd module. core 1.0.5 and as part of the elm/core its imported by default.

A Cmd is how we get things done in Elm, beyond its pure sandbox. It’s a safe, pure way to describe what we want the Browser to do on our behalf. And when it does that thing, it will fire off a Msg which is why its type is often written as Cmd Msg

Like an Http Request, returning a response for a list of blog posts, we need a Msg to handle receiving that response, from that command. Which is why Http.get has a type of

-- Http
get :
  { url : String
      , expect : Expect msg
      -> Cmd msg -- get kicks off a `Cmd` and will run a `msg` with its response

-- App.elm in action
import Http

type Msg
  = GotText (Result Http.Error String)

getPublicOpinion : Cmd Msg -- `Msg` because we're going to run the `GotText` Msg when we receive a response
getPublicOpinion =
    { url = "https://elm-lang.org/assets/public-opinion.txt"
    , expect = Http.expectString GotText

update : Msg -> Model -> (Model, Cmd Msg)
update msg model =
  case msg of 
  GotText res ->
   ({model | text = res}, Cmd.none)

Cmd.none is a Cmd msg that describes a command that does nothing. update always needs a Cmd to run, but sometimes (or lots of times) , we don’t want a command to run after some event. So Cmd.none exists to satisfy the need for a command, but not actually do anything.

Cmd.batch lets us perform one or more, often several commands, in parallel, unordered. Think of a Dashboard that fetches a bunch of data for independent widgets. We can batch them so they all initialize at once and we can then load them independently as they come in.

Ports in Elm are another example where we can kick off a Cmd to send data out to JavaScript. We could send a port out to send a web socket message from JavaScript, since Websockets don’t work in pure Elm. :/

port sendMessage : String -> Cmd msg

We’d call this an “outbound” port.

But what about Sub msg and Html msg?

Sub msg is for when we’re listening to outside events, like I”nbound” Ports from JavaScript to Elm or from Elm modules like Browser.Dom.Events like Browser.Events.onAnimationFrame. Ports let us bring in data from outside, like receiving a response to a WebSocket or interfacing with a 3rd party JavaScript library, or tools like Firebase.

So we can setup a Subscription to listen for these messages coming in and connect them to Msg’s we define and then handle in update. Like Platform.Cmd , Platform.Sub is imported by default in elm/core. And is only available on non-sandbox, or all Browser apps besides sandbox.

Platform.Sub.none is for when we need a Subscription to plugin that does nothing, just like Cmd.none And you might have guessed theres also a Subscription.batch for when we want to listen to several different subscriptions from the outside world.

In order to respond to User events in our app, like form inputs, and button clicks, we have to render Html that emits Msgs that are handled in update. Html events can be transformed into our rich Msg types for better state updates.

view : Model -> Html Msg This type signature means that our view expects a Model and will output Html Msg The Html Msg bit means that its emitting Msgs that are handled by update

type Msg
    = Increment
    | Decrement

update : Msg -> Model -> (Model, Cmd Msg)
update msg model =
    case msg of
        Increment ->
            ({ model | count = model.count + 1 }, Cmd.none)

        Decrement ->
            ({ model | count = model.count - 1 }, Cmd.none)

view : Model -> Html Msg
view model =
    div []
        [ button [ onClick Increment ] [ text "+1" ]
        , div [] [ text <| String.fromInt model.count ]
        , button [ onClick Decrement ] [ text "-1" ]

So here we can see the connection. Events.onClick expects a Msg, so button [ onClick Increment ] [ text "+1" ] wires up a click handler, that will fire the Increment message when this button is clicked.

The elm architecture will wire that button click and message handling to our update function.

From our update function we’re free to update our state and fire off commands in response to events. We could kick off a Cmd to make an http request or simply toggle a boolean on our Model.

It’s that Msg that connects all these parts together.

But as your app grows, it gets harder to scale your modules and organize your code.

One way of handling that scaling is to use Nested TEA. Or Nested Elm Architecture.

It’s turtles all the way down 🐢

So as our application grows, we often need to split our app up by different responsibilities. Often they’re split by Page.

module Main exposing (main)
-- Main.elm
import Page.Dashboard as Dashboard
import Page.Home as Home
import Page.Login as Login

type Page 
= Dashboard Dashboard.Model
| Home Home.Model
| Login Login.Model

type alias Model = {page : Page }

type Msg
  = DashboardMsg Dashboard.Msg 
  | HomeMsg Home.Msg
  | LoginMsg Login.Msg

So each page here is its own Module, And each page will also have 3 key parts:

But why?

Going back to the top level update, it returns a tuple of (Model, Cmd Msg)

So when we want to scale and grow our app, we can use The Elm Architecture to “nest” modules.

Each nested Page mimics that same pattern of function exports and their type signatures match.

We just need a wrapper for them at the appropriate level.

Our top level Msg type wraps nested submodules’ Msgs

And our top level Model will wrap nested Page Models

It’s turtles all the way down.

Wrapping in this way is robust and reliable. It’s easier to reason about when a module at 1 level looks and acts just like a module 1 or 2 levels deeper. The consistency is a huge win for productivity.