How to structure an Elm project?

The answer to this question can be found in the Beginning Elm book. This question will therefore be removed at some time in the future.

Do not overuse: Files are allowed to get quite big without needing to split them.

Todo.elm

type Task =
    Task
        { name : String
        , completed : Bool
        }

createTask : String -> Task

completeTask : Task -> Task

type Form a =
    Valid a
    | Invalid a
    | Partial a

validate : (Form a -> Form ()) -> Form a -> Form a

type alias TaskForm =
    { name : String
    , validated : Bool
    }

validateTaskForm : Form TaskForm -> Form ()

type User =
    User
        { name : String
        , lastOnline : Posix
        }

createUser : String -> Posix -> User

type alias UserForm =
    { name : String
    , password : String
    , validated : Bool
    }

validateUserForm : Form UserForm -> Form ()

type alias TodoPageModel =
    { form :
        Form TaskForm
    , todo : Array Task
    , User : User
    }

type alias LoginPageModel =
    Form UserForm

type Model =
    LoginPage LoginPageModel
    | TodoPage TodoPageModel

type LoginPageMsg =
    NameEntered String
    | PasswordEntered String
    | Submited 
    | GotTime Posix

type TodoPageMsg =
    LoggedOut
    | TaskAdded String
    | TaskCompleted Int
    | TaskDeleted

type Msg =
    LoginSpecific LoginPageMsg
    | TodoSpecific TodoPageMsg

init : () -> Model

updateTodoPage : TodoPageModel -> (TodoPageModel, Cmd TodoPageMsg)

updateLoginPage : LoginPageModel -> (LoginPageModel, Cmd LoginPageMsg)

update : Msg -> (Model, Cmd Msg) -> (Model, Cmd Msg)

viewUserForm : Form UserForm -> Html Msg

viewLoginPage : LoginPageModel -> Html Msg

viewTask : Task -> Html Msg

viewTaskForm : Form TaskForm -> Html Msg

viewTodoPage : TodoPageModel -> Html Msg

view : Model -> Html Msg

subscription : Model -> Sub Msgexpos

Main.elm

importing Todo.Page.Login as LoginPage
importing Todo.Page.Todo as TodoPage

type Model =
    LoginPage LoginPage.Model
    | TodoPage TodoPage.Model

type Msg =
    LoginSpecific LoginPage.Msg
    | TodoSpecific TodoPage.Msg

init : () -> Model

update : Msg -> (Model, Cmd Msg) -> (Model, Cmd Msg)

view : Model -> Html Msg

subscription : Model -> Sub Msg

Page/Login.elm

module Todo.Page.Login exposing (Model,Msg,view,update)

importing Todo.Data.Form as Form exposing (Form)
importing Todo.Data.User as User exposing (User,UserForm)

type alias Model =
    Form UserForm
    
type Msg =
    NameEntered String
    | PasswordEntered String
    | Submited 
    | GotTime Posix

update : Model -> (Model, Cmd Msg)

viewUserForm : Form UserForm -> Html Msg

view : Model -> Html Msg

Page/Todo.elm

module Todo.Page.Login exposing (Model,Msg,view,update)

importing Todo.Data.Task as Task exposing (Task,TaskForm)
importing Todo.Data.User as User exposing (User,UserForm)

type alias Model =
    { form :
        Form TaskForm
    , todo : Array Task
    , User : User
    }

type Msg =
    LoggedOut
    | TaskAdded String
    | TaskCompleted Int
    | TaskDeleted

update : Model -> (Model, Cmd Msg)

viewTask : Task -> Html Msg

viewTaskForm : Form TaskForm -> Html Msg

view : Model -> Html Msg

Data/Task.elm

module Todo.Data.Task exposing (Task,create,complete,validate)

importing Todo.Data.Form as Form exposing (Form)

type Task =

    Task
        { name : String
        , completed : Bool
        }

create : String -> Task

complete : Task -> Task

type alias TaskForm =
    { name : String
    , validated : Bool
    }

validate : Form TaskForm -> Form ()

Data/Form.elm

module Todo.Data.Form exposing (Form(..),validate)

type Form a =
    Valid a
    | Invalid a
    | Partial a

validate : (Form a -> Form ()) -> Form a -> Form a

Data/User.elm

module Todo.Data.User exposing (User,UserForm,create,validate)

importing Todo.Data.Form as Form exposing (Form)

type User =
    User
        { name : String
        , lastOnline : Posix
        }

create : String -> Posix -> User

type alias UserForm =
    { name : String
    , password : String
    , validated : Bool
    }

validate : Form UserForm -> Form ()

Question

How should I structure my Elm project?

Answer

Use the following file structure:

Main.elm - Contains the main code
Data.elm - Contains utility functions for types (like constants)
- Data/.. - Contains types. Group them logically, for example User and UserForm into Data/User.elm.
View.elm - Contains utility functions for views (like view specific constants and very general function.)
- View/.. - Contains different Views. Sometimes a type has different views. A login page might have a special view for a wrong login.
Page.elm - Contains utility functions for pages. For page-transitions its handy to store the different models in here.
- Page/.. - Contains a Model/View/Update for every page.
.. - in the same style you can add your project specific folders like a separate folder for validation.

Question

Answer

Further reading