Locally scoped updates in elm 0.18 - elm

Question

I have an elm 0.18 web app with a number of pages and routes. In main.elm I define my update function.
update : Msg -> Model -> ( Model, Cmd Msg )
update msg model =
case msg of
FirstUpdateAction ->
...
Every action goes through this function and it's getting big. Is it possible to create an update function to a smaller module that is nested within the overall structure?
For example, I have a settings page that gives the user the ability to change password. There are three fields/states (passwordOld, passwordNew, passwordConfirm) which have update actions associated with onInput and onBlur events. Those states and actions are only relevent to the user settings page, and become irrelevent to the rest of the model when the user leaves the page.
How could I go about setting up a scope for the user settings?

Answer 1

You could break down your code into independent submodules, each with it's own Msg type, update and view functions.
For example you could have a file SubmoduleA.elm looking like this:
module SubmoduleA exposing (Model, Msg, update, view)
type Msg = SubMessageA
| SubMessageB
[..]
type alias model =
{ fieldA : TypeA
, fieldB : TypeB
, [..]
}
update msg model =
case msg of
MessageA ->
{model | fieldA = [..] } ! []
[..]
view model =
div [id "submoduleAView"]
[..]
this module would be connected to your main program like this:
module Main exposing (..)
import SubmoduleA exposing (Model, Msg, update, view)
type Msg = MessageA
| MessageB
| ToSubmoduleA (SubmoduleA.Msg)
[..]
type alias model =
{ fieldA : TypeA
, fieldB : TypeB
, [..]
, subModuleA : SubmoduleA.Model
}
update msg model =
case msg of
MessageA ->
{model | fieldA = [..] } ! []
[..]
ToSubmoduleA msg =
let (newSubmoduleA, newSubmoduleACmd) = SubmoduleA.update msg (.subModuleA model)
in { model | subModuleA = newSubmoduleA } ! [Cmd.map ToSubmoduleA newSubmoduleACmd]
view model =
div [id "mainView"]
[ ..
, Html.map ToSubmoduleA <| SubmoduleA.view (.subModuleA model)
]
this way all the information and state that are relevant to your sub module stay encapsulated in your sub module, and you just have one case in your main update function responsible for the correct routing of messages.