@nh2, thanks for your PR! By analyzing the history of the files in this pull request, we identified @ericsagnes, @grahamc and @thatdocslady to be potential reviewers.

I can only guess at the author's intentions here, but I read "used" as meaning "declared": failing to declare a value for an option that has no default results in eval failure.

With the proposed change, it reads more like you'll get an error if you refer to an option that has no default.

@joachifm OK, let me first understand what really happens.

For terminology, from the manual:

Thus a module can declare options that can be used by other modules, and conversely can define options provided by other modules in its own implementation.

So declare means to mkOption, and define means to set that option to something, right?

Do I understand correctly that if you declare an option without default, but don't define it to have a certain value, and then some code evaluates the option from config, that is when you get an error?

Or is the behaviour that it fails independent of whether some code evaluates the option?

CC @thatdocslady from the original wording.

@nh2 Yes, that makes sense to me. I should have said "define" in my previous comment, to be consistent with the manual.

I think it'd be an improvement to change the wording to some variation of "define" rather than "use". I agree it's unclear as written.

@joachifm I've just built an example of what error message nix produces when this happens:

error: The option `bla' is used but not defined.

I've also tested what happens when you declare an option without default, but don't set (define) it and don't use (evaluate) it anywhere: In that case, it runs through without complaining.

Should I change the wording to e.g.

value is used (evaluated) anywhere, an error will be thrown.

?

Ah, juxtaposing the manual with the error message makes it clearer why it is worded that way. Taken on its own, I'm not so sure, though. Nix is supposed to be non-strict & it's reasonable to expect similar behavior from its module system, so I think the caveat "if x is evaluated/used" is redundant. I'd say something like "failing to define an option that lacks a default value is an error". When phrased that way it seems kind of self evident, however, suggesting it could be removed outright ...

Anyway, I don't have strong feelings about this; if you find the manual helpful the way it is, then your clarification of what "use" means seems like an improvement to me.

Also, you're absolutely right that it's a typo, so we can ofc merge this as is.

Nix is supposed to be non-strict & it's reasonable to expect similar behavior from its module system, so I think the caveat "if x is evaluated/used" is redundant.

@joachifm Good point. Here's another attempt for maximum clarity:

A default is not required; but if a default is not given, then
users of the module will have to define the value of the
option, otherwise an error will be thrown.

@nh2 makes sense to me :) Perhaps I'm the only one, but the use of "use" really confused me here, so for me this is much better.

PR updated.