How to break Dependency Cycles?

Problem
Solution
Alternative Solution
Main.elm
User.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"
Main.elm
User.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"
Main.elm
User.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"
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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