I think I've just had an aha moment with this post.
I am in the process of refactoring my monolith MUV into separate modules
with their own MUV for each "page" of my SPA. Up to this point, I have had
a separate Store module with its own Model and Msg types and an update
function (no view, obviously). This has worked well up until now, but
after splitting off the pages of my app, it is getting more cumbersome to
update the Store in a way that looks nice.
e.g. in my Main.elm I'm ending up with something like
update msg model =
case msg of
UserProfilePageMsg msg_ ->
let
( userProfilePageModel, userProfilePageCmd ) =
UserProfilePage.update msg_ model.userProfilePage
in
case msg_ of
UserProfilePage.StoreMsg msg__ ->
let
( storeModel, storeCmd ) =
Store.update msg__ model.store
in
{ model
| userProfilePage = userProfilePageModel
, store = storeModel
}
! [ Cmd.map UserProfilePageMsg
userProfilePageCmd
, Cmd.map StoreMsg storeCmd
]
_ ->
{ model | userProfilePage = userProfilePageModel }
! [ Cmd.map UserProfilePageMsg
userProfilePageCmd ]
and so on for every page that invokes Store.Msg—which is most pages.
I am thinking that there is a better way, and perhaps Kasey's suggestion of
forgoing an in-memory Store on the Model might be it. I'm still not sure—I
do like the snappy feel of a page loading instantly if the data is in
memory—even if it might change after a brief consultation with the server.
Dustin
On Wednesday, April 19, 2017 at 7:28:06 PM UTC-4, Kasey Speakman wrote:
>
> I'm probably slow, but in recent months I've discovered that trying to use
> Elm's Model like a database or cache (as I have previously seen suggested)
> has turned out to be pretty painful for me. An example database-minded
> model where a section could display *either* a list of employees *or* a
> list of courses.
>
> type alias Model =
> { employees : List Employee
> , courses : List Course
> , loadingError : Maybe Http.Error
> , route : MyRoute -- employee or course
> }
>
> The problem this runs into is having to worry about state management. I
> have to remember to "reset" or "turn off" things when they are not active.
> As my application grew, I had a lot of problems that boiled down to tedious
> state management details. My cached data didn't turn out to be all that
> useful because I usually had to reload it anyway, in case something changed.
>
> Instead, I have been moving toward the model only representing the current
> state of my UI. The big difference here is the model representing the
> current *visual* elements and their data. This leads more to using union
> types to represent parts of the UI. When you switch to a different case of
> the union type, the data from the previous case is *dropped on the floor*.
> This leaves nothing to remember to "reset". RemoteData is a good
> micro-example of this. If there was an error fetching the data, when the
> user requests the data again, you switch back to Loading, the error message
> is dropped on the floor. No forgetting to hide it.
>
> type RemoteData e a
> = NotAsked
> | Loading
> | Failure e
> | Success a
>
> If it is really important to cache the data, I prefer to keep that as a
> persistence concern, not on Model. It can be part of the process for
> retrieving the data to first check my chosen cache before making a request
> for fresh data. For instance, first check local storage before making an
> HTTP call. (Currently, this scenario is easier with Native modules for lack
> of Local Storage API or being able to wait on port subscriptions. But it's
> still doable.)
>
> So working towards a Model reflecting the visuals on the page has been an
> interesting challenge. I'm not claiming it's easier, but so far I've found
> it avoids a class of problems, and has led to some interesting discoveries
> in my own apps. One small example: I realized that my LoggedIn and
> NotLoggedIn routes should actually be separate "apps". Attempts to model
> this in a SPA fashion with the LoggedIn and NotLoggedIn routes as siblings
> always came up with the conundrum: how do I make it a compiler error for
> the model to be in LoggedIn mode but I receive a NotLoggedIn message, or
> vice versa? Even using TEA, I could not avoid this situation. Then I
> realized the only way to do that would be as separate apps. And that it was
> entirely possible to separate them. My "login page" turned out to be an
> entirely self-contained process: the user filling in info, obtaining a
> token, and saving it to local storage.
>
> I post this in the slim hope it is helpful to someone.
>
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