Added Error::Term for returning {:error, <term>}

[elbow-jason]

Hi! I added a variant to return any encoder as an error. I think could help ergonomics and help return more meaningful errors from our native Rustler interfaces to Elixir.

Feedback is appreciated. :)

[filmor]

Where would you return something arbitrary (i.e. not {error, _}) to indicate an error?

[elbow-jason]

That's a great question. Idiomatically, I would only ever return {:error, _} as an error.

Perhaps this function would be better as something like ReturnErrorTuple(Box<Encoder>) and return {:error, _}.

It would be pretty strange to return:

ReturnTerm(Box::new((atoms::ok(), "some ok result wrapped in an error")))

@filmor is your concern something like the Rust code above?

[filmor]

In line with the existing options, I think you should just add a Error::Term(X) that results in a returned error {error, X}.

[elbow-jason]

@filmor Agreed; {:error, term} is preferrable. I should have time to get this done on Friday. Thanks for the feedback!

[scrogson]

@elbow-jason...do you have time to finish this? If not, let me know and I can take it to the finish-line.

[scrogson]

I'm good with these changes 👍

I think we should wait to merge this after #264 is merged. There will only be a small number of conflicts to fix in this PR vs the other way round.

[filmor]

Hmm, I just noticed that Error::Atom("some_str") actually returns some_str and this made me wonder whether it wouldn't be good to make this one also stricter by making it return {error, some_str}. The only halfway valid case to return a bare atom is forwarding of a badmatch or badarg and a totally invalid case is returning Error::Atom("ok"). Shouldn't be part of this PR, though.

[elbow-jason]

@filmor Error::Term covers the use case you described for Error::Atom. I think what you are recommending is the deprecation of Error::Atom.

[scrogson]

Given that Error::Term(term) returns wrapped values, I think Error::Atom("error") is still relevant though.

[filmor]

@elbow-jason No, I'm recommending a change of semantics for Error::Atom as I don't like it when libraries return a bare atom instead of {error, Description} as an error, but like @scrogson said, it's quite useful to have the possibility to return something like this without boxing :)

[scrogson]

@elbow-jason now that #264 has been merged, feel free to fix up this PR so we can merge 👍