Better code for cut in fail verify?

[tisonkun]

I have a parser like:

#[derive(Debug, Clone, thiserror::Error)]
#[error("{0}")]
struct Context(String);

fn parse_minutes(input: &mut Located<&str>) -> PResult<MinuteExpr> {
    fn match_asterisk(input: &mut Located<&str>) -> PResult<MinuteExpr> {
        literal("*").parse_next(input).map(|_| MinuteExpr::Asterisk)
    }

    fn match_single_number(input: &mut Located<&str>) -> PResult<MinuteExpr> {
        let mut cut = false;
        dec_uint.try_map(|minute| {
            if (0..=59).contains(&minute) {
                Ok(MinuteExpr::PossibleValues(BTreeSet::from([minute])))
            } else {
                cut = true;
                Err(Context(format!("minute must be in the range 0..=59; found: {minute}")))
            }
        }).parse_next(input).map_err(|e| {
            if cut { e.cut() } else { e }
        })
    }

    let failure = fail.context(Label("crontab minute expression"));

    alt((match_asterisk, match_single_number, failure)).parse_next(input)
}

I'd like to cut the parsing when matching an uint but it doesn't fall into the range. Currently I have to use a cut state value but I wonder if there is some better solution.

The current cut_err combinator can only apply on the whole parser and cut for either invalid uint OR not a uint, while I'd like to backtrace and retry when not a uint.

[tisonkun]

I'd expect that verify_map would do backtrace but try_map always fail on error.

[epage]

imo that would be too surprising

[tisonkun]

Somehow work around with:

#[derive(Debug, Clone, thiserror::Error)]
#[error("{0}")]
struct Context(String);

fn parse_minutes(input: &mut Located<&str>) -> PResult<MinuteExpr> {
    fn match_asterisk(input: &mut Located<&str>) -> PResult<MinuteExpr> {
        literal("*").parse_next(input).map(|_| MinuteExpr::Asterisk)
    }

    fn match_single_number(input: &mut Located<&str>) -> PResult<MinuteExpr> {
        let verifier = take_while(0.., |_| true).try_map(|minute: &str| {
            let minute = minute.parse::<u8>().map_err(|err| Context(format!("failed to parse minute {minute}; {err}")))?;
            if (0..=59).contains(&minute) {
                Ok(MinuteExpr::PossibleValues(BTreeSet::from_iter(vec![minute])))
            } else {
                Err(Context(format!("minute must be in the range 0..=59; found: {minute}")))
            }
        });

        digit1.and_then(cut_err(verifier)).parse_next(input)
    }

    let failure = fail.context(StrContext::Label("crontab minute expression"));

    alt((match_asterisk, match_single_number, failure)).parse_next(input)
}

[tisonkun]

Write an extension:

fn parse_single_number_in_range<'a>(
    range: RangeInclusive<u8>,
) -> impl Parser<&'a str, u8, ContextError> {
    dec_uint.try_map_cut(move |n: u64| {
        if n > u8::MAX as u64 {
            return Err(StringContext(format!(
                "value must be in range {range:?}; found {n}"
            )));
        }

        let n = n as u8;
        if range.contains(&n) {
            Ok(n)
        } else {
            Err(StringContext(format!(
                "value must be in range {range:?}; found {n}"
            )))
        }
    })
}

trait ParserExt<I, O, E>: Parser<I, O, E> {
    #[inline(always)]
    fn try_map_cut<G, O2, E2>(self, map: G) -> TryMapCut<Self, G, I, O, O2, E, E2>
    where
        Self: Sized,
        G: FnMut(O) -> Result<O2, E2>,
        I: Stream,
        E: FromExternalError<I, E2>,
    {
        TryMapCut::new(self, map)
    }
}

struct TryMapCut<F, G, I, O, O2, E, E2>
where
    F: Parser<I, O, E>,
    G: FnMut(O) -> Result<O2, E2>,
    I: Stream,
    E: FromExternalError<I, E2>,
{
    parser: F,
    map: G,
    i: core::marker::PhantomData<I>,
    o: core::marker::PhantomData<O>,
    o2: core::marker::PhantomData<O2>,
    e: core::marker::PhantomData<E>,
    e2: core::marker::PhantomData<E2>,
}

impl<F, G, I, O, O2, E, E2> TryMapCut<F, G, I, O, O2, E, E2>
where
    F: Parser<I, O, E>,
    G: FnMut(O) -> Result<O2, E2>,
    I: Stream,
    E: FromExternalError<I, E2>,
{
    #[inline(always)]
    fn new(parser: F, map: G) -> Self {
        Self {
            parser,
            map,
            i: Default::default(),
            o: Default::default(),
            o2: Default::default(),
            e: Default::default(),
            e2: Default::default(),
        }
    }
}

impl<F, G, I, O, O2, E, E2> Parser<I, O2, E> for TryMapCut<F, G, I, O, O2, E, E2>
where
    F: Parser<I, O, E>,
    G: FnMut(O) -> Result<O2, E2>,
    I: Stream,
    E: FromExternalError<I, E2>,
{
    #[inline]
    fn parse_next(&mut self, input: &mut I) -> PResult<O2, E> {
        let start = input.checkpoint();
        let o = self.parser.parse_next(input)?;
        let res = (self.map)(o).map_err(|err| {
            input.reset(&start);
            ErrMode::from_external_error(input, ErrorKind::Verify, err).cut()
        });
        res
    }
}

impl<I, O, E, P> ParserExt<I, O, E> for P where P: Parser<I, O, E> {}

[tisonkun]

@epage What do you think of this new parser combinator try_map_cut?

[epage]

This is being tracked in #180. The problem is with much more than try_map and more than cut. I'd like to look at the problem as a whole and not add variants for every combination.

[tisonkun]

Cool! I'm subscribe that issue then.