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Some conveniences for implementing the update function with fetch.

The primary purpose of the update function is to take a msg and return an updated model (and possibly a Cmd). So, what one ordinarily does in an update function is run a case statement on the msg, doing whatever is appropriate for the provided msg.

However, there are times when we want to take certain actions based not on the msg we just received, but instead on the state of our model generally ... particularly the state of whatever is controlling our view. For instance, we may need to load some data from the backend in order to support the current view. That will require a Cmd, but it is logically independent of the current msg. Well, I suppose you could try to "catch" every message that creates a requirement to fetch some data, but that is error prone -- it would be nicer to just compute what messages are necessary, given the state of the model.

So, imagine that you have a function like this:

fetch : Model -> List Msg

Its job is to look at the model and decide whether there are any messages that ought to be processed (e.g. to fetch some needed data, but it isn't really limited to that). It's convenient for this to be a separate function (that is, separate from update), because it doesn't depend on the msg ... it will be a function of something in the model which indicates what the view needs.

However, we also need to apply this fetch function. That is where andThenFetch comes in. You provide:

  • your fetch function
  • your update function

What you get back is a function that has the same signature as your update function ... that is, it is also in the form Msg -> Model -> (Model, Cmd Msg). However, after it calls your own update function, it calls your fetch function with the resulting Model, and then feeds those results back into your update function. So, whatever messages your fetch function returns will be processed.

Note that this will run recursively ... that is, when your fetch function returns some messages to process, it will be called again with the results of processing those messages. So, you will need something in your model to keep track of actions in progress (e.g. RemoteData) so that you don't repeatedly kick off the same action in an infinite loop.

To use this function, you would normally just supply the first two parameters ... that is, your fetch function and your normal update function. What you will then get back is a function of the form msg -> model -> ( model, Cmd msg) ... that is, an update function that you can then pass to programWithFlags (or use in another way).

Your main function would look something like:

main =
        { init = App.Update.init

        -- We wire `andThenFetch` here.
        , update = Update.Fetch.andThenFetch App.Fetch.fetch App.Update.update
        , view = App.View.view
        , subscriptions = App.Update.subscriptions

App.Update.update is your normal update function. App.Fetch.fetch will likely look something like this, assuming you have a Page called Items:

{-| Call the needed `fetch` function, based on the active page.
fetch : App.Model.Model -> List App.Model.Msg
fetch model =
    case model.activePage of
        Items ->
            Pages.Items.Fetch.fetch model.backend
                |> (\subMsg -> MsgBackend subMsg)