Elm:
Writing to standard error

How to:

Elm is primarily targeted at web development, where the concept of writing directly to stderr doesn’t apply in the same way as it does in traditional command-line environments. However, for Elm programs running in Node.js or similar environments, interop with JavaScript using ports is the key approach to achieve similar functionality. Here is how you might set it up:

Elm Code (Main.elm):

port module Main exposing (main)

import Browser

port errorOut : String -> Cmd msg

-- Dummy example function that sends an error message to JS
generateError : String -> Cmd msg
generateError message =
    errorOut message

main =
    generateError "This is an error message for stderr"

JavaScript Interop (index.js):

const { Elm } = require('./Main.elm');

var app = Elm.Main.init();

app.ports.errorOut.subscribe((message) => {
  console.error(message);
});

This Elm code defines a port errorOut that allows sending messages out of Elm to JavaScript. Then in the JavaScript code, we listen for messages sent through this port and redirect them to stderr using console.error(). This way, you can effectively write to stderr in an environment that supports it, by leveraging Elm’s interop features with JavaScript.

Sample output in Node.js terminal (when index.js is run):

This is an error message for stderr