Execing Processes in Elm

Here’s the translation of the Go code to Elm, with explanations in Markdown format suitable for Hugo:

Our example demonstrates how to execute external processes in Elm. While Elm is primarily used for frontend web development and doesn’t have direct system-level access like Go, we can simulate this behavior using ports and JavaScript interop.

First, we’ll create an Elm program that sends a command to execute an external process:

module Main exposing (main)

import Platform exposing (Program)
import Task exposing (Task)
import Port exposing (executeProcess)

type Msg
    = ExecuteProcess
    | ProcessResult (Result String String)

type alias Model =
    { result : Maybe String
    }

init : () -> ( Model, Cmd Msg )
init _ =
    ( { result = Nothing }
    , Task.perform (always ExecuteProcess) (Task.succeed ())
    )

update : Msg -> Model -> ( Model, Cmd Msg )
update msg model =
    case msg of
        ExecuteProcess ->
            ( model
            , executeProcess { command = "ls", args = ["-a", "-l", "-h"] }
            )
        
        ProcessResult result ->
            case result of
                Ok output ->
                    ( { model | result = Just output }, Cmd.none )
                
                Err error ->
                    ( { model | result = Just ("Error: " ++ error) }, Cmd.none )

subscriptions : Model -> Sub Msg
subscriptions _ =
    Port.processResult ProcessResult

main : Program () Model Msg
main =
    Platform.worker
        { init = init
        , update = update
        , subscriptions = subscriptions
        }

In this Elm code, we’re using a Platform.worker program, which doesn’t have a user interface but can perform background tasks. We define a port executeProcess to send the command to JavaScript, and another port processResult to receive the result.

Next, we need to set up the JavaScript side to handle the process execution:

const { exec } = require('child_process');

const app = Elm.Main.init();

app.ports.executeProcess.subscribe(({ command, args }) => {
  exec(`${command} ${args.join(' ')}`, (error, stdout, stderr) => {
    if (error) {
      app.ports.processResult.send({ error: error.message });
    } else {
      app.ports.processResult.send({ ok: stdout });
    }
  });
});

This JavaScript code uses Node.js’s child_process.exec to run the command. It then sends the result back to Elm through the processResult port.

To run this Elm program:

Compile the Elm code to JavaScript:

$ elm make Main.elm --output=main.js

Create an HTML file to load the Elm program:

<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title>Exec'ing Processes in Elm</title>
</head>
<body>
    <script src="main.js"></script>
    <script>
        const app = Elm.Main.init();
        // JavaScript code for port handling goes here
    </script>
</body>
</html>

Run the HTML file using a local server that supports JavaScript modules.

When we run our program, it will execute the ls -a -l -h command and receive the output through the port system.

Note that Elm, being primarily a frontend language, doesn’t offer direct system access like Go does. The approach shown here uses JavaScript interop to achieve similar functionality. In a real-world scenario, executing system commands would typically be handled by a backend service, with Elm focusing on the user interface and application logic.

Http Server in Elm

Learn X By Example

Title here

Execing Processes in Elm