If you add a port in Elm code, then you need to use that port in your Elm code, otherwise the generated Javascript might not even know that your Elm code uses ports at all. As I learn Elm and become accustomed to its compiler’s wonderfully-detailed error messages, encountering error messages like this suddenly feel jarring and I even have trouble understanding them for a moment.
This is a companion discussion topic for the original entry at https://blog.thecodewhisperer.com/permalink/where-the-hell-is-my-port-an-adventure-in-elm