Using Dict.update with nested update functions in Elm - function

Question

I'm writing an Elm app where a major part of the state is in a Dict with Records as values. I have an update function for the main state model, and an update function for the individual records in the Dict. Is there a way to use Dict.update with the record state update function?
The issue I'm having is that the state update function for the records returns what update functions usually return: a tuple with the updated object and any commands to trigger (e.g. (newRecord, Cmd.none)). But the Dict.update function needs to take in a record and return a record (e.g. just newRecord), not a tuple with a record and a command object.
Is there a way around this? Right now I've got it working using a combination of Dict.get and Dict.insert, but this seems clumsy.

Answer 1

If update function for record always return Cmd.none, you can simplify it and return only the updated model.
Child modules do not have to follow ( Model, Cmd Msg ) convention if your application architecture does not require that.
If you do need to pass Commands from the lower level, it is also possible to re-structure your child module's update function to simplify those updates.
Example based on example/http
Here is an example of how you can split the update so that you could re-use the same logic in the top-level update without additional trickery.
update : Msg -> Model -> (Model, Cmd Msg)
update msg model =
(updateModel msg model, updateCmd msg model)
updateCmd: Msg -> Model -> Cmd Msg
updateCmd msg model =
case msg of
MorePlease ->
getRandomGif model.topic
_ ->
Cmd.none
updateModel: Msg -> Model -> Model
updateModel msg model =
case msg of
NewGif (Ok newUrl) ->
Model model.topic newUrl
_ ->
model
If you need the updated model in updateCmd, then just pass it instead of current model or even pass both if you desire.
As a bonus, you can entirely omit unused branches of the case expression.
Using Dict.update
It will also be possible to use updateModel in Dict.update without retrieving the record and writing it back.
Dict.update childKey (Maybe.map (updateModel childMsg)) model

Answer 2

Not sure if this is what you are looking for, but if you have a nested Dict structure in your model, like this:
type alias Model =
{ parentsAndChildren : Dict String (Dict String Int) }
Then it is not necessary to make the child update output a Cmd. Your update could look something like this:
update : Msg -> Model -> Model
update msg model =
case msg of
NewChild parentName childName age ->
let
newModel =
{ model
| parentsAndChildren =
model.parentsAndChildren
|> Dict.update
parentName
(Maybe.map insertChild)
}
in
(newModel, Cmd.none)
-- helper function to update a child Dict
insertChild: String -> Int -> Dict (String Int) -> Dict (String Int)
insertChild name age childDict =
Dict.insert name age childDict
The only update function that NEEDS to output a Cmd is the update function in your top component.
So your child update function doesn't have to output a Cmd.

Answer 3

Not that I know of. I have this helper function in my one of my projects
updateDict : comparable -> msg -> Dict comparable b -> (msg -> b -> ( b, Cmd msg )) -> (comparable -> msg -> c) -> ( Dict comparable b, Cmd c )
updateDict uid act dict fn wrapper =
case Dict.get uid dict |> Maybe.map (fn act) of
Just ( m, e ) ->
( Dict.insert uid m dict
, Cmd.map (wrapper uid) e
)
Nothing ->
( dict, Cmd.none )