How to break Dependency Cycles?

Problem
Solution
Alternative Solution
Problem
Main.elm
import User exposing (User)
​
type Model =
    Maybe User
​
type Msg =
    UserSpecific User.Msg
    Login String String
​
update : Msg -> Model -> (Model,Cmd Msg)
update msg =
    case msg of
        UserSpecific userMsg ->
            User.update userMsg
        Login name pass ->
            Debug.todo "login user"
User.elm
import Main exposing (Msg)
​
type User =
    ..
​
type Msg =
    ..
    
updateUser : Msg -> User -> (Model,Cmd Msg)
​
viewUser : User -> Html Msg
Solution
Main.elm
import User exposing (User)
​
type Model =
    Maybe User
​
type Msg =
    UserSpecific User.Msg
    Login String String
​
update : Msg -> Model -> (Model,Cmd Msg)
update msg =
    case msg of
        UserSpecific userMsg ->
            User.update Just UserSpecific userMsg 
        Login name pass ->
            Debug.todo "login user"
User.elm
type User =
    ..
​
type Msg =
    ..
    
updateUser : (User -> model) -> (Msg -> msg) -> Msg -> User -> (model,Cmd msg)
Alternative Solution
Main.elm
import User exposing (User)
​
type Model =
    Maybe User
​
type Msg =
    UserSpecific User.Msg
    Login String String
​
update : Msg -> Model -> (Model,Cmd Msg)
update msg =
    case msg of
        UserSpecific userMsg ->
            let
                (user,msg) = User.update userMsg
            in
            (Just User,UserSpecific msg)
        Login name pass ->
            Debug.todo "login user"
User.elm
type User =
    ..
​
type Msg =
    ..
    
updateUser : Msg -> User -> (User,Cmd User.Msg)
Question
How to break out of the dependency cycles?
Answer
Isolate the elements that both of the source modules need access to.
Move the shared elements into a new module.
Make the source module depend on the new Module.
Further Reading
📄Article: High-Level Dependency Strategies in Elm by Matthew Buscemi
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Last updated 7 months ago