How to break Dependency Cycles?

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

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