How to propagate context

[beeb]

I am trying to attach some context to an error and have it propagate throughout the parser but it seems I don't have the right way.

MRE https://www.rustexplorer.com/b/9v4c7c (refresh if it shows an error)

The final error does not contain the context information when it's the first parser from the alt failing:

Err(ParseError { input: "/***", offset: 0, inner: ContextError { context: [], cause: None } })

Any help would be appreciated! Thanks

[epage]

The problem is alt. It always ends with the last error.

One option is to use cut_err to "commit" to the result of one branch of alt

/*
[dependencies]
winnow = "0.7"
*/
use winnow::{
    combinator::{alt, not, cut_err},
    error::{StrContext, StrContextValue},
    Parser as _, ModalResult,
};

fn multi<'s>(input: &mut &'s str) -> ModalResult<(&'s str, ())> {
    (
        "/**",
        cut_err(not('*'))
            .context(StrContext::Label("delimiter"))
            .context(StrContext::Expected(StrContextValue::Description("/**"))),
    )
        .parse_next(input)
}

fn single<'s>(input: &mut &'s str) -> ModalResult<(&'s str, ())> {
    ("///", cut_err(not('/'))).parse_next(input)
}

fn either<'s>(input: &mut &'s str) -> ModalResult<(&'s str, ())> {
    alt((multi, single)).parse_next(input)
}

fn main() {
    let res = either.parse("/***");
    println!("{res:?}");
}

Another option is to do something like

(
    '/',
    dispatch!{any;
        '/' => not('/'),
        '*' => not('*'),
        _ => fail,
    }
)

[epage]

See also https://docs.rs/winnow/latest/winnow/_tutorial/chapter_7/index.html and https://docs.rs/winnow/latest/winnow/_tutorial/chapter_3/index.html

[beeb]

Thanks a lot! I have read the tutorial and tried something similar but could not really make it work in my more convoluted real use-case. Does this mean I need to change the signature of all my parsers to return a ModalResult? (i.e. a breaking change)

[beeb]

Managed to make it work by changing all the returns to ModalResult. I wish there was a way to mix and match between ModalResult and Result but I get warnings of the sort "couldn't convert the error to winnow::error::ContextError". Since ContextError has more information, I guess it needs to propagate up the chain of parsers and there's no way around it.

[epage]

You could call ErrMode::into_inner().expect("no partial parsers") whenever you are ok with dropping the Cut vs Backtrack error state.

[beeb]

Thanks for your help!