Use a NoRule sentinel type instead of Nothing?

[nickrobinson251]

Currently if we don't have a rule for something, we return nothing

I do think it is tempting to return nothing here... but I wonder if it is just more explicit to return a NoRule (/NotImplemented) sentinal type, like we already have DNE to distinguish the case when the derivative does not exist (#17). Thoughts?

[simeonschaub]

It does seem consistent with methods from Base like findfirst though, but I don't have a very strong opinion either way

[nickrobinson251]

Yeah, i feel like returning nothing make sense. And I'm not sure a custom sentinal would be better. But just raising it as a (small) user-friendliness question:

There are multiple cases where we don't return the derivative, for example, (1) it does exist, (2) it's not been implemented, (3) you're differentating wrt a function (#22)... probably others. And a maximum of one of these cases should return nothing (to disambiguate them).

Right now only case (2) returns nothing, so we're fine... but it does involve remembering the nothing means "no rule implemented". 🤷‍♂