HPR3068: Keeping track of downloads in Elm

Background

I have page that requests several resources from server. To keep track what is going on, I initially had model like:

type alias Model =
{ availableChassis : List Chassis
, chassisLoaded : Bool
, chassisLoading : Bool
...
}

Problem with this is that I have to remember to check those boolean flags while rendering on screen. And it’s possible to have inconsistent state (both loading and loaded).

Solution

We can model state with algebraic datatypes and we don’t even have to write it by ourselves as there’s RemoteData library.

Now we can change our model to following:

import RemoteData exposing (RemoteData(..), WebData)

type alias Model =
{ availableChassis : WebData (List Chassis)
}

availableChassis has four states it can be in:

NotAsked, data isn’t available and it hasn’t been requested from server
Loading, data isn’t available, but it has been requested from server
Success (List Chassis), data has been loaded from server
Failure Http.Error, there was error while loading data

For example, while rendering the view, you could do

case model.availableChassis of
NotAsked ->
renderEmptyTable

Loading ->
renderLoadingTable

Success chassis ->
renderChassisList chassis

Failure error ->
renderErrorMessage error