That, but also if this should indeed be a global concept I’d still use a protocol, but your own instead of String.Chars. Somethings like MyApp.HumanizedIdentityString.to_string/1. That can be used in your presentation layer (like described before) to replace placeholders in your validation errors.

It is my first question here and the quality of the discussion and of the answers is staggering! Thanks to all those who answered!

Got it. And I think this gets to the crux of the matter: In general the Elixir community thinks that it is very important to be explicit about such things. How to refer to a user varies by context. Sometimes name works, sometimes it’s employee ID, sometimes it’s username. A shipment is referred to by its order number in some cases and it’s tracking number in others.

Defaults like String.Chars create easy pathways, and people tend to default to the pathways that are easiest. As your program grows, a single canonical way won’t suffice so you’ll have several variations anyway eg: #{username(user)} vs #{short_name(user}) vs #{user} and in that last case it requires everyone to simply remember “oh right, the default is long name”. Instead it could just be #{full_name(user)} like the rest and then the code just tells you what it’s doing.