A model for scaling Elm apps. (re-write of elm-taco)
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Highly Opinionated Re-Write of elm-taco

After making the elm-donut-tutorials, I decided to take a deep-dive into elm-taco, a pattern for "building larger Single-Page Applications (SPAs) in Elm 0.18".

After spending a few hours looking at it, I realized I totally disagreed with the entire concept of elm-taco, and also realized that this seems to be a common pattern with folks writing about 'big apps' in Elm: the impulse is to make a hierarchy of nested components. This makes sense as many folks are coming from things like React and (more recently) Vue.js, etc. which follow this model.

I would go so far as to say this is an anti-pattern for Elm, and it looks like this:

Parent (model, messages, data, view, update, commands)
  -> Child (childmodel, childmessages, childdata, childview, childupdate, childcommands)
    -> SubChild (subchildmodel, subchildmessages, subchilddata, subchildview, subchildupdate, subchildcommands)
      -> SubSubChild (subsubchildmodel, subsubchildmessages, subsubchilddata, subsubchildview, subsubchildupdate, subsubchildcommands)

I believe this 'nested component' paradigm of organization fights TEA (The Elm Architecture) and makes for code that is difficult to maintain, write and comprehend.

Elm Taco The Second: elm-taco-donut

To run, either use elm reactor or if you have elm-live installed, just type npm run live

Obviously, dive in to the source code, but here's the concept.

At a high level, it strikes me that all Elm apps have the same basic functional requirements and could potentially be broken down as follows:

  1. Models: types and records that maintain state information: the model.
  2. Messages: in order to update the model, you must do that via a message, typically called Msg.
  3. Data: specify api data structures and how to decode them.
  4. Commands: all side-effects go here, e.g. actually fetching data from an api, running a task, etc.
  5. Init Phase: before you can use the app, you need to initialize it to a known state.
  6. Update: all updates to the model are handled here
  7. View: views simply take the model and render a reactive view based on the model.
  8. Subscriptions: usually, this wouldn't need to be a separate file, but could be if required.
  9. Program: wire the app up.

Note that this is different from many Elm examples you see on the web where folks have files like 'Types.elm' that contain all of the type definitions (e.g. Models, Message Data and random types used by other function all lumped together), or a master file named 'Thing.elm' that contains models, messages, data, commands, updates and view functions all in one file related to a 'Thing'.

The goal here is to break an Elm app along 'functional lines' as opposed to 'component lines'

For a super simple app, you could make these eight files and be done, e.g.:


where Main.elm might look something like:

module Main exposing (main)

import Navigation
import Time exposing (Time)
import A_Model exposing (Model)
import B_Message exposing (Msg(TimeChange, UrlChange))
import E_Init exposing (init)
import F_Update exposing (update)
import G_View exposing (view)

main : Program Never Model Msg
main =
    Navigation.program UrlChange
        { init = init
        , update = update
        , view = view
        , subscriptions = \_ -> Time.every Time.second TimeChange

Typically, however, views get fairly hairy quickly, so maybe break those off:

        Home.elm            <-- view helper (pure function, no local state!)
        OtherPage.elm       <-- view helper (pure function, no local state!)
        WidgetOne.elm       <-- view helper (pure function, no local state!)
        WidgetTwo.elm       <-- view helper (pure function, no local state!)
G_View.elm                  <-- view entry point

The next level of complexity would be to break up the update function:

    WidgetOne.elm           <-- update helper function
    WidgetTwo.elm           <-- update helper function
    Home.elm                <-- update helper function
    OtherPage.elm           <-- update helper function
        Home.elm            <-- view helper (pure function, no local state!)
        OtherPage.elm       <-- view helper (pure function, no local state!)
        WidgetOne.elm       <-- view helper (pure function, no local state!)
        WidgetTwo.elm       <-- view helper (pure function, no local state!)
F_Updates.elm               <-- update entry point
G_View.elm                  <-- view entry point

& etc.

The reason for the letter naming (e.g. A_, B_) is that I find it easy to work sequentially either from top to bottom or in reverse from bottom to top.

"Fist, let's add to the model. Now let's add any messages. Do we need anything in Data or Command? No? Okay, add to Init. Now add the update. Finally, make the view." Etc.

Spreadsheet, not a database!

Conceptually, I look at the model, Msg and update as a spreadsheet. Meaning, it's okay to have a lot of stuff in them, even at the top level. There is nothing 'wrong' with a model that looks like this:

type alias Model =
    { appState : AppState
    , location : Location
    , taco : Taco
    , selectedLanguage : Language
    , route : Route
    , home : HomeModel
    , donuts : List DonutModel
    , pageOne : PageModelOne
    , pageTwo : PageModelTwo
    , pageThree : PageModelThree
    , pageFour : PageModelFour
    , pageFive : PageModelFive
    , pageSix : PageModelSix
    , pageSixHundred : PageModelSixHundred
    , widgetOne : List WidgetModelOne
    , widgetTwo : List WidgetModelTwo
    , widgetThree : List WidgetModelThree
    , widgetFour : List WidgetModelFour
    , widgetFive : List WidgetModelFive
    , widgetSix : List WidgetModelSix
    , widgetTwentySix : List WidgetModelTwentySix

There is no need to fully 'embed' a component inside another component.

Often, within a view or an update function, you want to either look at or change part of the model that may not "belong" to the thing you are working on. Great! That's why we have one big model! We can just refer to using dot notation. If I am in a view that mainly uses, say, model.pieChart, I can still happily refer to model.thing.otherthing.globaldatathing and not be concerned I am going to break something somewhere else! Remember, the model is immutable! It is impossible to mess this up!

Back to the spreadsheet metaphor, using 'components' or 'parent-child-subchild' model with Elm is like trying to embed a spreadsheet inside the of a cell of an existing spreadsheet. Super bad idea.

Also, it makes it soooooooooo simple if you don't!


  1. All views have the same signature: Model -> Html Msg. All views take the model and return an Html Msg.
  2. All updates have the same signature: Msg -> Model -> ( Model, Cmd Msg ). All updates take a message + the model, and return a tuple of (model, command)
  3. All commands have the same signature: Cmd Msg
  4. If you want to use a sub-thing, (e.g. sub-view, sub-update or sub-command), that's fine, but it still must return the same return type as above.

Example Update Helper:

type Msg
    = Lots
    | Of
    | Main
    | Messages
    | Go
    | Here
    | Child ChildMsg

type ChildMsg
    = UpdateName String
    | AddCounter
    | ResetCounter

type alias Model =
    { child : ChildModel

type alias ChildModel =
    { counter : Int

-- main update funtion
update : Msg -> Model -> (Model -> Cmd Msg)
update msg model =
    case msg of
        Child childMsg ->
            ( updateChild childMsg model, Cmd.none )


-- child update function
updateChild : ChildMsg -> Model -> Model
updateChild childMsg model =
        resetCounter child =
            { child | counter = 0 }
        case childMsg of
            ResetCounter ->
                { model | child = resetCounter model.child }



  1. By having all messages handled by a single, top level update function (that can have update helper functions), you can avoid using _ wildcard matches. This lets the compiler help you ensure that you've made exactly one response for every possible message.
  2. It is very easy to reuse messages through the app. For example, any page can send any message it wants without having to wire a convoluted mechanism to do so. So if you have a logout button, you can stick it anywhere and not worry that it can cause a problem somewhere else. And if you do break something, the Elm compiler tells you how to fix it!

I believe the instinct to componentize comes from the brittleness of typical software.

"Hey, this main thing works! Please, don't touch it! Maybe write a component, tell me how to access it, and I can stick it somewhere, but just don't touch the main code!" -- Lead Developer to New Hire

Typically, refactoring is terrifying and cognitively very taxing. More often than not, it's just easier to start from scratch than try & re-shape someone else's code.

With Elm, refactoring is a joy. I rebuilt elm-taco in an evening. The first time it compiled, it worked identically to the old elm-taco. I'd have much more confidence with an Elm app that just compiles, with no test coverage at all, vs. a React/Vue.js/Etc. app with tons of tests.

Back to elm-taco-donut

In the version here, I am using the file structure as outlined above.

For the sake of show, I moved update to a separate folder. Personally, I would leave it 'flat' until the level of complexity got much higher, but since this is a model of how to scale apps, here you go.

I also added a component from the elm-donut-tutorial just for fun.

The Larger Goal

One of the biggest challenges to software reuse lies in the difficult of figuring out what someone else has done.

Ultimately, in the same way elm-format rocks (which should be part of the elm core distribution, in my opnion), I am hoping to get something similar for Elm app patterns. Right now, when you looks at someone else's Elm code, the first thing you have to do is map The Elm Architecture:


to whatever they have done. Did they stick the model in that Types.elm file? Or in that file called Global.elm? Or maybe they baked it into the init function, where ever that is... Or maybe they have scattered it around in each component? Etc.

So why not cut to the chase and write the code that way to begin with?

By following this convention, there is no hunting around. No sleuthing.

Also, it makes onboarding new developers a breeze, as they already know what is in each file, and where to look if they need to add or update something. Need to tweak the thing that gets data from an API? Hey, that's a command! So look in D_Command.elm. Also, maybe the format of the API data changed. So update C_Data.elm as well. And if that breaks any of the views or updates, the Elm compiler will tell you! Yippee!

We'll see!

Peace and ❤️

PS I'm sure Ossi and Matias (authors of the original elm-taco) are better programmers than me, so hopefully they aren't offended by this! Finland rocks!

Here is the README from the original elm-taco.

🌮 Taco 🌮

This repository serves as an example for building larger Single-Page Applications (SPAs) in Elm 0.18. The main focus is what we call the Taco model. Taco can be used to provide some application-wide information to all the modules that need it. In this example we have the current time, as well as translations (I18n) in the taco. In a real application, you would likely have the current logged-in user in the taco.

Why the name?

We wanted to avoid names that have strong programming related connotations already. According to Wikipedia:

A taco is a traditional Mexican dish composed of a corn or wheat tortilla folded or rolled around a filling. A taco can be made with a variety of fillings, including beef, pork, chicken, seafood, vegetables and cheese, allowing for great versatility and variety.

What we mean by taco is also a vessel for tasty fillings, allowing for great versatility and variety. Plus, taco is short and memorable.

What's the big 🌮 idea?

Oftentimes in web applications there are some things that are singular and common by nature. The current time is an easy example of this. Of course we could have each module find out the current time on their own, but it does make sense to only handle that stuff in one place. Especially when the shared information is something like the translation files in our example app, it becomes apparent that retrieving the same file in every module would be a waste of time and resources.

How we've solved this in Elm is by introducing an extra parameter in the view functions:

view : Taco -> Model -> Html Msg

That's it, really.

The Taco is managed at the top-most module in the module hierarchy (Main), and its children, and their children, can politely ask for the Taco to be updated.

If need be, the Taco can just as well be given as a parameter to childrens' init and/or update functions. Most of the time it is not necessary, though, as is the case in this example application.

How to try 🌮

There is a live demo here: https://ohanhi.github.io/elm-taco/

To set up on your own computer, you will need git and elm-reactor 0.18 installed.

Simply clone the repository and start up elm-reactor, then navigate your browser to http://localhost:8000/index.html. The first startup may take a moment.

$ git clone https://github.com/ohanhi/elm-taco.git
$ cd elm-taco
$ elm-reactor

File structure

├── api                     # "Mock backend", serves localization files
│   ├── en.json
│   ├── fi-formal.json
│   └── fi.json
├── elm-package.json        # Definition of the project dependencies
├── index.html              # The web page that initializes our app
├── README.md               # This documentation
└── src
    ├── Decoders.elm            # All JSON decoders
    ├── I18n.elm                # Helpers for localized strings
    ├── Main.elm                # Main handles the Taco and AppState
    ├── Pages
    │   ├── Home.elm                # A Page that uses the Taco
    │   └── Settings.elm            # A Page that can change the Taco
    ├── Routing
    │   ├── Helpers.elm             # Definitions of routes and some helpers
    │   └── Router.elm              # The parent for Pages, includes the base layout
    ├── Styles.elm              # Some elm-css
    └── Types.elm               # All shared types

How 🌮 works

Initializing the application

The most important file to look at is Main.elm. In this example app, the default set of translations is considered a prerequisite for starting the actual application. In your application, this might be the logged-in user's information, for example.

Our Model in Main is defined like so:

type alias Model =
    { appState : AppState
    , location : Location

type AppState
    = NotReady Time
    | Ready Taco Router.Model

We can see that the application can either be NotReady and have just a Time as payload, or it can be Ready, in which case it will have a complete Taco as well as an initialized Router.

We are using programWithFlags, which allows us to get the current time immediately from the embedding code. This could be used for initializing the app with some server-rendered JSON, as well.

This is how it works in the Elm side:

type alias Flags =
    { currentTime : Time

init : Flags -> Location -> ( Model, Cmd Msg )
init flags location =
    ( { appState = NotReady flags.currentTime
      , location = location
    , WebData.Http.get "/api/en.json" HandleTranslationsResponse Decoders.decodeTranslations

We are using ohanhi/elm-web-data for the HTTP connections. With WebData, we represent any data that we retrieve from a server as a type like this:

type WebData a
    = NotAsked
    | Loading
    | Failure (Error String)
    | Success a

If you're unsure what the benefit of this is, you should read Kris Jenkins' blog post: How Elm Slays a UI Antipattern.

Now, by far the most interesting of the other functions is updateTranslations, because translations are the prerequisite for initializing the main application.

Let's split it up a bit to explain what's going on.

case webData of
    Failure _ ->
        Debug.crash "OMG CANT EVEN DOWNLOAD."

In this example application, we simply keel over if the initial request fails. In a real application, this case must be handled with e.g. retrying or using a "best guess" default.

    Success translations ->

Oh, jolly good, we got the translations we were looking for. Now all we need to do is either: a) initialize the whole thing, or b) update the current running application.

        case model.appState of

So if we don't have a ready app, let's create one now:

            NotReady time ->
                    initTaco =
                        { currentTime = time
                        , translate = I18n.get translations

                    ( initRouterModel, routerCmd ) =
                        Router.init model.location
                    ( { model | appState = Ready initTaco initRouterModel }
                    , Cmd.map RouterMsg routerCmd

Note that we use the time in the model to set the initTaco's value, and we set the translate function based on the translations we just received. This taco is then set as a part of our AppState.

If we do have an app ready, let's update the taco while keeping the routerModel unchanged.

            Ready taco routerModel ->
                ( { model | appState = Ready (updateTaco taco (UpdateTranslations translations)) routerModel }
                , Cmd.none

Just to make it clear, here's the whole function:

updateTranslations : Model -> WebData Translations -> ( Model, Cmd Msg )
updateTranslations model webData =
    case webData of
        Failure _ ->
            Debug.crash "OMG CANT EVEN DOWNLOAD."

        Success translations ->
            case model.appState of
                NotReady time ->
                        initTaco =
                            { currentTime = time
                            , translate = I18n.get translations

                        ( initRouterModel, routerCmd ) =
                            Router.init model.location
                        ( { model | appState = Ready initTaco initRouterModel }
                        , Cmd.map RouterMsg routerCmd

                Ready taco routerModel ->
                    ( { model | appState = Ready (updateTaco taco (UpdateTranslations translations)) routerModel }
                    , Cmd.none

        _ ->
            ( model, Cmd.none )

Updating the Taco

We now know that the Taco is one half of what makes our application Ready, but how can we update the taco from some other place than the Main module? In Types.elm we have the definition for TacoUpdate:

type TacoUpdate
    = NoUpdate
    | UpdateTime Time
    | UpdateTranslations Translations

And in Pages/Settings.elm we have the update function return one of them along with the typical Model and Cmd Msg:

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

This obviously needs to be passed on also in the parent (Router.elm), which has the same signature for the update function. Then finally, back at the top level of our hierarchy, in the Main module we handle these requests to change the Taco for all modules.

updateRouter : Model -> Router.Msg -> ( Model, Cmd Msg )
updateRouter model routerMsg =
    case model.appState of
        Ready taco routerModel ->
                ( nextRouterModel, routerCmd, tacoUpdate ) =
                    Router.update routerMsg routerModel

                nextTaco =
                    updateTaco taco tacoUpdate
                ( { model | appState = Ready nextTaco nextRouterModel }
                , Cmd.map RouterMsg routerCmd

-- ...

updateTaco : Taco -> TacoUpdate -> Taco
updateTaco taco tacoUpdate =
    case tacoUpdate of
        UpdateTime time ->
            { taco | currentTime = time }

        UpdateTranslations translations ->
            { taco | translate = I18n.get translations }

        NoUpdate ->

And that's it! I know it may be a little overwhelming, but take your time reading through the code and it will make sense. I promise. And if it doesn't, please put up an Issue so we can fix it!

Credits and license

© 2017 Ossi Hanhinen and Matias Klemola

Licensed under BSD (3-clause)