Exit in Elm

Here’s an idiomatic Elm example that demonstrates the concept of exiting a program:

module Main exposing (main)

import Browser
import Html exposing (Html, div, text)
import Html.Events exposing (onClick)
import Platform.Cmd as Cmd


type alias Model =
    { exitStatus : Maybe Int }


init : () -> ( Model, Cmd Msg )
init _ =
    ( { exitStatus = Nothing }, Cmd.none )


type Msg
    = Exit


update : Msg -> Model -> ( Model, Cmd Msg )
update msg model =
    case msg of
        Exit ->
            ( { model | exitStatus = Just 3 }
            , Cmd.none
            )


view : Model -> Html Msg
view model =
    div []
        [ text <| "Exit status: " ++ Maybe.withDefault "Not exited" (Maybe.map String.fromInt model.exitStatus)
        , div [ onClick Exit ] [ text "Exit with status 3" ]
        ]


main : Program () Model Msg
main =
    Browser.element
        { init = init
        , update = update
        , view = view
        , subscriptions = \_ -> Sub.none
        }

This Elm program demonstrates the concept of exiting a program, although it’s important to note that Elm, being primarily used for web applications, doesn’t have a direct equivalent to os.Exit as seen in the Go example.

Here’s how this Elm code works:

We define a Model that includes an exitStatus of type Maybe Int. This represents whether the program has “exited” and with what status code.
The init function sets up the initial state with exitStatus as Nothing, indicating the program hasn’t exited yet.
We define a Msg type with a single constructor Exit, which will be used to trigger the “exit” action.
In the update function, when an Exit message is received, we update the model to set the exitStatus to Just 3, simulating an exit with status code 3.
The view function displays the current exit status and provides a button to trigger the exit action.
The main function sets up the Elm application using Browser.element.

To run this Elm program:

Save the code in a file named Exit.elm.
Use the Elm compiler to compile the code:

$ elm make Exit.elm --output=exit.js

Create an HTML file that includes the compiled JavaScript:

<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title>Elm Exit Example</title>
</head>
<body>
    <div id="elm"></div>
    <script src="exit.js"></script>
    <script>
        var app = Elm.Main.init({
            node: document.getElementById('elm')
        });
    </script>
</body>
</html>

Open the HTML file in a web browser to see the Elm application running.

This example demonstrates how to handle a concept similar to exiting in Elm, even though web applications don’t typically “exit” in the same way as command-line programs. Instead, we update the application state to reflect an “exited” status.

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Exit in Elm