In Elm, each component basically has its own internal state (which is
actually all just a slice of one global model). In my app, I also want
global state that is independant of any components; for example a
clientside cache of various API responses (asset details - there could be
many thousands, user authentication status).
I want any component to be able to call methods that make use of this
global state. For example, a method to obtain details for items in the
current view might first look at the global state to see if these items are
cached. If they arent, the call would provide a Cmd to be issued that gets
the items (and puts them in the cache), while simultaneously updating the
state to indicate that they are being loaded (so that the same request
again from another component doesnt trigger another call to the backend).
If they are cached, they can be easily returned from there. A first shot at
a signature might look something like:
getItem : GlobalState -> ID -> Tag -> (GlobalState, Cmd msg)
However we could partially apply functions that exist on some globalState
instantiation to hdie the initial state being passed in and end up with:
state.items.getItem : ID -> Tag -> (GlobalState, Cmd msg)
The downside of this approach is that I have to thread this state through
multiple calls that might make use of it, and thread it back up explicitly
through the update functions to get it back to the top. At the top we'd
then have something like (excuse any mistakes!):
update msg model = case msg of
SubMsg m ->
let (newSubModel, subCmds, newGlobalState) = SubComponent.update m
model.subModel
in ({ model | state = newGlobalState, subModel = newSubModel}, Sub.map
SubMsg subCmds)
...
An alternative approach is to hold this global state in an effect manager,
and so in the app you'd end up using the Cmd/Sub mechanism to ask for
things from the state and internally initiate API requests to update the
state as necessary. We'd end up with an API more like:
getItem : ID -> Tag -> Cmd msg
or
state.items.getItem : ID -> Tag -> Cmd msg
where the returned Cmd would either lead to an item being sent to the
component immediately via a cache (where Tag is a Msg type the component
knows about) or after it was obtained via some backend. This would make all
retrieving of state async but seems to simplify the interface (perhaps at
the cost of more complexity in implementing the effect manager).
Which approach do people think is best for working with global state
(neither is an option if you have a better way!)? Do you get away with not
needing this kind of thing (and if so, how)? I'd love to hear back,
especially from those that have had experience building larger apps in Elm!
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