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Elm Land aims to be a great fit for web applications with pages that are only available behind a "sign-in" screen. To make it easy to build apps with user authentication, Elm Land provides an Auth module to make it easy to work with signed-in and signed-out users.

If you want a page to be "auth-protected", all you need to do is add Auth.User to the page's type annotation:

page : Shared.Model -> Route () -> Page Model Msg
page shared route =

page : Auth.User -> Shared.Model -> Route () -> Page Model Msg
page user shared route =

By adding that argument, Elm Land will know that this page should only be visible when a user is signed-in. It will look to the Auth module to determine how it should respond when a user visits a page they shouldn't have access to.

Customizing Auth

You can customize the Auth module with the elm-land customize command:

elm-land customize auth

By default, all auth-only pages redirect users to the NotFound_ page when the application starts up. Let's edit our new src/Auth.elm file so it automatically passes the user to any pages that need it, but redirects to /sign-in if there's no user logged in.

module Auth exposing (User, onPageLoad, viewCustomPage)

-- ...

type alias User =
    { token : String

{-| Called before an auth-only page is loaded. -}
onPageLoad : Shared.Model -> Route () -> Auth.Action.Action User
onPageLoad shared route =
    case shared.token of
        Just token ->
                { token = token

        Nothing ->
                { path = Route.Path.SignIn
                , query =
                        [ ( "from", route.url.path )
                , hash = Nothing

{-| Used whenever `Auth.Action.loadCustomPage` is returned. -}
viewCustomPage : Shared.Model -> Route () -> View Msg
viewCustomPage shared route =

In the case that a user isn't signed in, we can even add a query parameter to let us know which page they were on when the sign-in redirect took place.

This means that if the user was signed out and loaded the /settings page, their new URL would be /sign-in?from=/settings.

We can use this query parameter later to make sure we redirect them to the right page after sign-in, rather than always redirecting to the homepage.

Understanding the big picture

There's a dedicated User Auth guide that covers working with the Auth module, setting up a sign-in form, and working with tokens and local storage.

Check that out if you'd like to see a real example of how this API can work for your web application!

Note: You can learn more about the other functions available in the "Auth.Action" module reference.

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