Stepping inside the elm runtime

If you have ever wondered how the elm runtime works you may have traced the function calls required to create an application (for example elm/core:Platform.worker or elm/browser:Browser.element). If you do so you will end up at elm/core:Elm.Kernel.Platform.initialize.

The initialize function in the elm/core package is defined like this:

function _Platform_initialize(flagDecoder, args, init, update, subscriptions, stepperBuilder) {
   // ...

flagDecoder sounds a bit scary but we ignore it for now. args is the javascript object you pass to init: Elm.Main.init({ flags: {}, node: {}}). Whilst tracing the functions call you can tell that init, update and subscriptions are the elm functions that you, as a seasoned elm programmer, have come to know and love. But what is the stepperBuilder parameter?

We have the following clues to help us work out the mystery:

  1. When creating a elm/core:Platform.Worker we pass the initialize function this as its stepperBuilder:

    function() { return function() {} }
  2. elm/core:Elm.Kernel.Platform.initialize uses the stepperBuilder parameter like this:

    var initPair = init(result.a);
    var model = initPair.a;
    var stepper = stepperBuilder(sendToApp, model);

    and then uses the created stepper like this:

    var pair = A2(update, msg, model);
    stepper(model = pair.a, viewMetadata);
  3. When creating a HTML element, elm/browser:Elm.Kernel.Browser.element passes this monster as its stepperBuilder:

    function(sendToApp, initialModel) {
       var view = impl.__$view;
       var domNode = args['node'];
       var domNode = args && args['node'] ? args['node'] : __Debug_crash(0);
       var currNode = _VirtualDom_virtualize(domNode);
       return _Browser_makeAnimator(initialModel, function(model)
          var nextNode = view(model);
          var patches = __VirtualDom_diff(currNode, nextNode);
          domNode = __VirtualDom_applyPatches(domNode, currNode, patches, sendToApp);
          currNode = nextNode;

It is crystal clear now? I doubt it, it certainly is not crystal clear for me. Given my three clues I think all we can work out is:

  • complex code without static typing quickly becomes an unintelligible mess that is very hard to read or maintain.
  • the runtime calls stepperBuilder with the model returned by init.
  • stepperBuilder returns a function (here called stepper) that the runtime calls everytime the model is updated.
  • stepperBuilder and its offspring stepper do nothing when the program is headless, but come into their own when the runtime starts needing to manipulate the DOM.

A small step for a man

Let us try to make some sense of this with the help of static typing. A stepperBuilder “builds” a stepper. If were trying to rewrite the runtime in elm we might have [1]:

type alias StepperBuilder =
      SendToApp msg -> model -> Stepper model

We can say that StepperBuilder takes a SendToApp and the initial model and creates Stepper. It is very important to note that this function is not pure. When you call stepperBuilder stuff may happen (concretely the runtime renders your initial view and draws it to the DOM).

“What is Stepper and what is SendToApp msg?” I hear you ask. Well:

type alias Stepper model =
      model -> ViewMetadata -> ()

type alias SendToApp msg =
      msg -> ViewMetadata -> ()

So Stepper takes the model’s new value and some ViewMetadata and returns nothing. We can tell that Stepper must be an impure function (with side effects) otherwise calling stepper would be pointless. Every time the model updates, the elm runtime will call this function and it do its virtual DOM diffing magic and updates the DOM accordingly.

Yet there is another type: ViewMetadata! Before you ask “What is ViewMetadata?” I must confess that I do not really know. I believe it controls whether the runtime instantly renders changes to the DOM or queues the update using requestAnimationFrame. I believe ViewMetadata is the solution to the issues in elm 0.18 where port subscriptions would sometimes fire before the DOM had been updated and other times they would not fire instantly in response to DOM events. For now, we can make ViewMetadata a placeholder type and fill it in future when I have a better idea about what is going on.

type ViewMetadata
   = ViewMetadata ViewMetadata

Finally we come to SendToApp. The DOM may produce events which need to be passed to the application’s update function. The runtime gives the StepperBuilder a SendToApp function to do just that. Event listeners attached to the DOM call SendToApp with a message (and a value for the mysterious ViewMetadata). The runtime will then take care of passing that message to the app via update.

A giant leap for mankind

So what can we say. Well stepperBuilder could be named createOnUpdateHandler and creates a function onUpdateHandler that will be called by the runtime every time the model updates. onUpdateHandler takes care of all the changes to the DOM needed to display your webapp. Tidy.



Elm curries functions. The js definition of stepperBuilder is a function that takes two arguments so this curried version isn’t truely the type ignature of stepperBuilder. We cannot write the true type signature in elm.