“Task and gen_tcp” guide might confuse with `PORT` environment variable

[sonicdoe]

The Task and gen_tcp guide explains how to read the port from an environment variable but then continues with hardcoding the port. However, towards the end it instructs to start a new server with PORT=4040 mix run --no-halt.

In my opinion, this comes across as confusing as the example above this command explicitly shows the port being hardcoded.

Since this change was deliberately made in #994, I didn’t just want to open a pull request reverting it. Evidently, there’s value in explaining how to specify the PORT environment variable when starting the server. I’m not sure how to best incorporate that into the guide – perhaps an example directly after the System.get_env("PORT") code block?

[josevalim]

@sonicdoe great catch. Let's incorporate the code sample into the application. So something like:

port = String.to_integer(System.get_env("PORT") || raise "missing $PORT environment variable")

Thoughts?

[sonicdoe]

I was a bit concerned it would unnecessarily increase the complexity, however, since it would remove the example and would propagate best practices, I’d be fine with that solution 👍