I have read this whole subthread and it is quite interesting because it is clear everyone involved knows what they are talking about, but speaking past each other. :)

I agree with you that you have to build a mental model and you have to memorize (or consult) which guards are available, but arguably the `is_foo` vs `foo?` distinction here is a positive one, because it only takes a glance to know if it is a guard or not (and the majority of guards have the `is_*` prefix).

You are correct URI.char_reserved? could have been a guard - but that should not be a confusion point for a user of the library - it is clear it is not a guard function. The confusion only arises as a contributor once you read the source as to why a certain choice was made (in this particular case, char_reserved? was written before defguard existed, but even if I wrote it today I am not sure I would consider the possibility users would invoke it in guards).

Btw, I know you know this. I just hope this clarifies the whole discussion a bit for others!