How to break Dependency Cycles?

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Main.elm
1
import User exposing (User)
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​
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type Model =
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    Maybe User
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​
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type Msg =
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    UserSpecific User.Msg
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    Login String String
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​
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update : Msg -> Model -> (Model,Cmd Msg)
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update msg =
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    case msg of
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        UserSpecific userMsg ->
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            User.update userMsg
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        Login name pass ->
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            Debug.todo "login user"
Copied!
User.elm
1
import Main exposing (Msg)
2
​
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type User =
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    ..
5
​
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type Msg =
7
    ..
8
    
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updateUser : Msg -> User -> (Model,Cmd Msg)
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​
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viewUser : User -> Html Msg
Copied!
Main.elm
1
import User exposing (User)
2
​
3
type Model =
4
    Maybe User
5
​
6
type Msg =
7
    UserSpecific User.Msg
8
    Login String String
9
​
10
update : Msg -> Model -> (Model,Cmd Msg)
11
update msg =
12
    case msg of
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        UserSpecific userMsg ->
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            User.update Just UserSpecific userMsg 
15
        Login name pass ->
16
            Debug.todo "login user"
Copied!
User.elm
1
type User =
2
    ..
3
​
4
type Msg =
5
    ..
6
    
7
updateUser : (User -> model) -> (Msg -> msg) -> Msg -> User -> (model,Cmd msg)
Copied!
Main.elm
1
import User exposing (User)
2
​
3
type Model =
4
    Maybe User
5
​
6
type Msg =
7
    UserSpecific User.Msg
8
    Login String String
9
​
10
update : Msg -> Model -> (Model,Cmd Msg)
11
update msg =
12
    case msg of
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        UserSpecific userMsg ->
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            let
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                (user,msg) = User.update userMsg
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            in
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            (Just User,UserSpecific msg)
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        Login name pass ->
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            Debug.todo "login user"
Copied!
User.elm
1
type User =
2
    ..
3
​
4
type Msg =
5
    ..
6
    
7
updateUser : Msg -> User -> (User,Cmd User.Msg)
Copied!
Question
How to break out of the dependency cycles?
Answer
1.Isolate the elements that both of the source modules need access to.
2.Move the shared elements into a new module.
3.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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