The goalposts of this conversation seem to keep moving. It sounds like you don’t like some of Elixir’s choices. That’s fair. I wasn’t hand waving, I was simply

The goalposts of this conversation seem to keep moving. It sounds like you don’t like some of Elixir’s choices. That’s fair. I wasn’t hand waving, I was simply summarizing guards in general. We’ve both provided the same explanation regarding why some things can or cannot be guards —you’ve linked to Erlang’s discussion of guards; I’ve linked to Elixir’s. So we both understand guards and their limitations. I’m not terribly interested in arguing about what should or shouldn’t be “guard-worthy”. The built-in guards don’t strike me as arbitrary choices, but as intentional choices as part of designing core language features—that’s all in Kernel, on which everything else depends. The URI module, while a great part of the stdlib, is not core language guard material. I can say that in 7 years, I’ve never needed or wanted a built-in guard to check if a character was a reserved URI char—but I can believe some problem spaces might have use for enhanced guards. Why, I’ve written my own for having more readable and shorter guards for things like empty lists, maps, and more. The ability to compose those from core language guards and features is one of the many things that makes Elixir a great language for my use cases.