A Modular Architecture for Elm programs

This tutorial outlines the general architecture you will see in all Elm programs. It is a simple pattern that is great for modularity, code reuse, and testing. I find it somewhat shocking in its simplicity. We will start with the basic pattern in a small example and slowly build on those core principles.

To follow along with this tutorial, clone this repo and navigate to the root directory.

The Basic Pattern

The logic of every Elm program will break up into three cleanly separated parts: model, update, and view. You can pretty reliably start with the following skeleton and then iteratively fill in details for your particular case.

-- MODEL

type alias Model = { ... }


-- UPDATE

type Action = Reset | ...

update : Action -> Model -> Model
update action model =
  case action of
    Reset -> ...
    ...


-- VIEW

view : Model -> Html
view =
  ...

This tutorial is all about this pattern and small variations and extensions.

Example 1: A Counter

Our first example is a simple counter that can be incremented or decremented. To see it in action, navigate into directory 1/, run elm-reactor, and then open http://localhost:8000/Counter.elm?debug.

This code starts with a very simple model. We just need to keep track of a single number:

type alias Model = Int

When it comes to updating our model, things are relatively simple again. We define a set of actions that can be performed, and an update function to actually perform those actions:

type Action = Increment | Decrement

update : Action -> Model -> Model
update action model =
  case action of
    Increment -> model + 1
    Decrement -> model - 1

Notice that our Action union type does not do anything. It simply describes the actions that are possible. If someone decides our counter should be doubled when a certain button is pressed, that will be a new case in Action. This means our code ends up very clear about how our model can be transformed. Anyone reading this code will immediately know what is allowed and what is not. Furthermore, they will know exactly how to add new features in a consistent way.

Finally, we create a way to view our Model. We are using elm-html to create some HTML to show in a browser. We will create a div that contains: a decrement button, a div showing the current count, and an increment button.

view : Model -> Html
view model =
  div []
    [ button [ onClick (Signal.send actionChannel Decrement) ] [ text "-" ]
    , div [ countStyle ] [ text (toString model) ]
    , button [ onClick (Signal.send actionChannel Increment) ] [ text "+" ]
    ]

countStyle : Attribute
countStyle =
  ...

The first thing I want you to notice about this code is that it is entirely declarative. Take in a Model and produce some Html. That is it. At no point do we mutate the DOM, giving the language and libraries much more freedom to make clever optimizations. Furthermore, view is a plain old function so we can get the full power of Elm’s module system, test frameworks, and libraries when creating views.

(The tricky thing about our `view` function is the `Signal.send actionChannel` part. We will dive into that in the next section.)

This pattern is the essense of architecting Elm programs. Every example we see from now on will be a slight variation on this basic pattern: Model, update, view.

Aside: Driving your App with Signals

So far we have only been talking about pure functions and immutable data. This is great, but we also need to react to events in the world. This is the role of signals in Elm. A signal is a value that changes over time, and it lets us talk about how our Model is going to evolve.

Pretty much all Elm programs will have a small bit of code that drives the whole application. The details are not super important for our purpose, but the code will be some minor variation of what is seen in Example 1:

main : Signal Html
main =
  Signal.map view model

model : Signal Model
model =
  Signal.foldp update 0 (Signal.subscribe actionChannel)

actionChannel : Signal.Channel Action
actionChannel =
  Signal.channel Increment

Rather than trying to figure out exactly what is going on line by line, I think it is enough to visualize what is happening at a high level.

The blue part is our core Elm program which is exactly the model/update/view pattern we have been discussing so far. When programming in Elm, you can mostly think inside this box and make great progress.

The new thing here is how “channels” make it possible for new Actions to be triggered in response to user inputs. These channels are roughly represented by the dotted arrows going from the monitor back to our Elm program. So when we specify certain channels in our view, we are describing how user Actions should come back into our program. Notice we are not performing those actions, we are simply reporting them back to our main Elm program. This separation is a key detail!

I want to reemphasize that this Signal code is pretty much the same in all Elm programs. You can be very productive without diving much deeper than this, and it is not vital to modularity or the specific architecture this tutorial is focused on. All of our subsequent examples will focus on the Model, update, and view so that we do not repeat this signal information again and again.

Example 2: A Pair of Counters

In this example we have two counters, each changing independently. To see it in action, navigate into directory 2/, run elm-reactor, and then open http://localhost:8000/CounterPair.elm?debug.

We wrote a simple counter in example 1, so our goal is to reuse all of that code. We can create a self-contained Counter module that encapsulates all the implementation details. The only change necessary is in the view function, so I have elided all the other definitions which are unchanged:

module Counter (Model, init, Action, update, view) where

type Model = ...

init : Int -> Model
init = ...

type Action = ...

update : Action -> Model -> Model
update = ...

view : LocalChannel Action -> Model -> Html
view channel model =
  div []
    [ button [ onClick (send channel Decrement) ] [ text "-" ]
    , div [ countStyle ] [ text (toString model) ]
    , button [ onClick (send channel Increment) ] [ text "+" ]
    ]

Rather than refering directly to a top-level actionChannel as we did in example 1, we give the channel as an argument so that each counter can be sending messages along different channels. This will let us augment a basic Counter.Action with extra information so that we know which counter is needs to be updated.

Creating modular code is all about creating strong abstractions. We want boundaries which appropriately expose functionality and hide implementation. From outside of the Counter module, we just see a basic set of values: Model, init, Action, update, and view. We do not care at all how these things are implemented. In fact, it is impossible to know how these things are implemented. This means no one can rely on implementation details that were not made public.

So now that we have our basic Counter module, we need to use it to create our CounterPair. As always, we start with a Model:

type alias Model =
    { topCounter : Counter.Model
    , bottomCounter : Counter.Model
    }

init : Int -> Int -> Model
init top bottom =
    { topCounter = Counter.init top
    , bottomCounter = Counter.init bottom
    }

Our Model is a record with two fields, one for each of the counters we would like to show on screen. This fully describes all of the application state. We also have an init function to create a new Model whenever we want.

Next we describe the set of Actions we would like to support. This time our features should be: reset all counters, update the top counter, or update the bottom counter.

type Action
    = Reset
    | Top Counter.Action
    | Bottom Counter.Action

Notice that our union type refers to the Counter.Action type, but we do not know the particulars of those actions. When we create our update function, we are mainly routing these Counter.Actions to the right place:

update : Action -> Model -> Model
update action model =
  case action of
    Reset -> init 0 0

    Top act ->
      { model |
          topCounter <- Counter.update act model.topCounter
      }

    Bottom act ->
      { model |
          bottomCounter <- Counter.update act model.bottomCounter
      }

So now the final thing to do is create a view function that shows both of our counters on screen along with a reset button.

view : Model -> Html
view model =
  div []
    [ Counter.view (LC.create Top actionChannel) model.topCounter
    , Counter.view (LC.create Bottom actionChannel) model.bottomCounter
    , button [ onClick (Signal.send actionChannel Reset) ] [ text "RESET" ]
    ]

Notice that we are able to reuse the Counter.view function for both of our counters. For each counter we create a local-channel. Essentially what we are doing here is saying, “let these counters send messages to the general actionChannel but make sure all of their messages are annotated with Top or Bottom so we can tell the difference.”

That is the whole thing. With the help of local-channel, we were able to nest our pattern model/update/view pattern. The cool thing is that we can keep nesting more and more. We can take the CounterPair module, expose the key values and functions, and create a CounterPairPair or whatever it is we need.

Example 3: A Dynamic List of Counters

A pair of counters is cool, but what about a list of counters where we can add and remove counters as we see fit? Can this pattern work for that too?

To see this example in action, navigate into directory 3/, run elm-reactor, and then open http://localhost:8000/CounterList.elm?debug.

In this example we can reuse the Counter module exactly as it was in example 2.

module Counter (Model, init, Action, update, view)

That means we can just get started on our CounterList module. As always, we begin with our Model:

type alias Model =
    { counters : List ( ID, Counter.Model )
    , nextID : ID
    }

type alias ID = Int

Now our model has a list of counters, each annotated with a unique ID. These IDs allow us to distinguish between them, so if we need to update counter number 4 we have a nice way to refer to it. (This ID also gives us something conveient to key on when we are thinking about optimizing rendering, but that is not the focus of this tutorial!) Our model also contains a nextID which helps us assign unique IDs to each counter as we add new ones.

Now we can define the set of Actions that can be performed on our model. We want to be able to add counters, remove counters, and update certain counters.

type Action
    = Insert
    | Remove
    | Modify ID Counter.Action

Our Action union type is shockingly close to the high-level description. Now we can define our update function.

update : Action -> Model -> Model
update action model =
  case action of
    Insert ->
      let newCounter = ( model.nextID, Counter.init 0 )
          newCounters = model.counters ++ [ newCounter ]
      in
          { model |
              counters <- newCounters,
              nextID <- model.nextID + 1
          }

    Remove ->
      { model | counters <- List.drop 1 model.counters }

    Modify id counterAction ->
      let updateCounter (counterID, counterModel) =
            if counterID == id
                then (counterID, Counter.update counterAction counterModel)
                else (counterID, counterModel)
      in
          { model | counters <- List.map updateCounter model.counters }

Here is a high-level description of each case:

• Insert — First we create a new counter and put it at the end of our counter list. Then we increment our nextID so that we have a fresh ID next time around.

• Remove — Drop the first member of our counter list.

• Modify — Run through all of our counters. If we find one with a matching ID, we perform the given Action on that counter.

All that is left to do now is to define the view.

view : Model -> Html
view model =
  let counters = List.map viewCounter model.counters
      remove = button [ onClick (Signal.send actionChannel Remove) ] [ text "Remove" ]
      insert = button [ onClick (Signal.send actionChannel Insert) ] [ text "Add" ]
  in
      div [] ([remove, insert] ++ counters)

viewCounter : (ID, Counter.Model) -> Html
viewCounter (id, model) =
  Counter.view (LC.create (Modify id) actionChannel) model

The fun part here is the viewCounter function. It uses the same old Counter.view function, but in this case we provide a local-channel that annotates all messages with the ID of the particular counter that is getting rendered.

When we create the actual view function, we map viewCounter over all of our counters and create add and remove buttons that report to the actionChannel directly.

This ID trick can be used any time you want a dynamic number of subcomponents. Counters are very simple, but the pattern would work exactly the same if you had a list of user profiles or tweets or newsfeed items or product details.

Example 4: A Fancier List of Counters

Okay, keeping things simple and modular on a dynamic list of counters is pretty cool, but instead of a general remove button, what if each counter had its own specific remove button? Surely that will mess things up!

Nah, it works.

To see this example in action, navigate into directory 4/, run elm-reactor, and then open http://localhost:8000/CounterList.elm?debug.

In this case our goals mean that we need a new way to view a Counter that adds a remove button. Interestingly, we can keep the view function from before and add a new viewWithRemoveButton function that provides a slightly different view of our underlying Model:

module Counter (Model, init, Action, update, view, viewWithRemoveButton, Context) where

...

type alias Context =
    { actionChan : LocalChannel Action
    , deleteChan : LocalChannel ()
    }

viewWithRemoveButton : Context -> Model -> Html
viewWithRemoveButton context model =
  div []
    [ button [ onClick (send context.actionChan Decrement) ] [ text "-" ]
    , div [ countStyle ] [ text (toString model) ]
    , button [ onClick (send context.actionChan Increment) ] [ text "+" ]
    , div [ countStyle ] []
    , button [ onClick (send context.deleteChan ()) ] [ text "X" ]
    ]

The viewWithRemoveButton function just adds one extra button. The interesting thing here is that we have multiple channels we need to send messages to now. Instead of just updating this counter, we also pass in a way to say “delete me!” How that happens is the responsibility of whoever owns the counter.

This is pretty cool. We do not need to duplicate any code or do any crazy subtyping or overloading. We just add another view function to the public API to expose new functionality!

So now we get to the CounterList module, that actually puts all the individual counters together. The Model is the same as in example 3.

type alias Model =
    { counters : List ( ID, Counter.Model )
    , nextID : ID
    }

type alias ID = Int

Our set of actions is a bit different. Instead of removing any old counter, we want to remove a specific one, so the Remove case now holds an ID.

type Action
    = Insert
    | Remove ID
    | Modify ID Counter.Action

The update function is pretty similar to example 4 as well.

update : Action -> Model -> Model
update action model =
  case action of
    Insert ->
      { model |
          counters <- ( model.nextID, Counter.init 0 ) :: model.counters,
          nextID <- model.nextID + 1
      }

    Remove id ->
      { model |
          counters <- List.filter (\(counterID, _) -> counterID /= id) model.counters
      }

    Modify id counterAction ->
      let updateCounter (counterID, counterModel) =
            if counterID == id
                then (counterID, Counter.update counterAction counterModel)
                else (counterID, counterModel)
      in
          { model | counters <- List.map updateCounter model.counters }

In the case of Remove, we got through the list of counters and keep everything that has a non-matching ID. Otherwise, the cases are quite close to how they were before.

Finally, we put it all together in the view:

view : Model -> Html
view model =
  let insert = button [ onClick (Signal.send actionChannel Insert) ] [ text "Add" ]
  in
      div [] (insert :: List.map viewCounter model.counters)

viewCounter : (ID, Counter.Model) -> Html
viewCounter (id, model) =
  let context =
        Counter.Context
          (LC.create (Modify id) actionChannel)
          (LC.create (always (Remove id)) actionChannel)
  in
      Counter.viewWithRemoveButton context model

In our viewCounter function, we construct the Counter.Context to pass in all the nesessary local channels. In both cases we annotate each Counter.Action so that we know which counter to modify or remove.

Big Lessons So Far

Basic Pattern — Everything is built around a Model, a way to update that model, and a way to view that model. Everything is a variation on this basic pattern.

Nesting Modules — A local-channel makes it easy to nest our basic pattern, hiding implementation details entirely. We can nest this pattern arbitrarily deep, and each level only needs to know about what is going on one level lower.

Adding Context — Sometimes to update or view our model, extra information is needed. We can always add some Context to these functions and pass in all the additional information we need without complicating our Model.

update : Context -> Action -> Model -> Model
view : Context' -> Model -> Html

At every level of nesting we can derive the specific Context needed for each submodule.

Testing is Easy — All of the functions we have created are pure functions. That makes it extremely easy to test your update function. There is no special initialization or mocking or configuration step, you just call the function with the arguments you would like to test.