Could ErrMode::Incomplete include an instance of E?

[zslayton]

My crate has an error type (MyError) that includes some bits of state/context which are useful for producing friendly error messages. All of my winnow parsers return ErrMode<MyError>. This works great except in the case of the Incomplete variant, which does not hold an instance of the generic E:

...and so cannot store the same state and context as the other variants.

A context-less Incomplete is ok when it leads to "add bytes from input and try again." However, when I need to surface an Incomplete to the application/user as a fatal error, I can't offer the same context I would add for other problems. Even the AddContext trait doesn't seem to work with Incomplete for this reason. Workarounds are possible, but the ones I've tried so far have been rather unsatisfying.

I know this design decision was a holdover from nom, but I wasn't able to find information about their initial rationale. Is this something that could be changed?

[epage]

While complete_err exists, I suspect this is overall a workflow oversight, especially as we've put more emphasis on context.

I suspect there will be some tension in use cases we Incomplete(Needed, E) will be bigger which can be slower. However, I am working on some changes for 0.7 that put all of ErrModes functionality behind a trait (to make it optional) which would allow you to provide your own ErrMode, see #75.

[zslayton]

However, I am working on some changes for 0.7 that put all of ErrModes functionality behind a trait (to make it optional) which would allow you to provide your own ErrMode, see #75.

🤩 This sounds great! Thanks for the link, I'll subscribe to the issue.