For some reason this one feels right to express with if-not to me too.

The one place I’ll often use unless is things like:

    return unless valid

    raise unless h.key?(k)

I think it might have something to do with:

1. How abundantly clear it is that the condition is a Boolean; and

2. The “false” condition indicating a “no work to do” situation.

I think I only use them for postfix return, break, next, and raise.

Fascinating! I find it slightly easier to understand "unless" than "if not". But only oh-so slightly, and probably not enough to justify making it harder for others to read my code.