And the following is a reimplementation of the example in Nom's module documentation, with added support for negative integers:

// https://github.com/utkarshkukreti/munch.rs/blob/62907a57d0d598bd5570c4726bbd0251bd66152f/examples/arithmetic.rs

pub fn expr(str: &str, from: usize) -> munch::Result<i64, munch::str::Error<'static>> {
    use munch::str::*;

    let ws = P(TakeWhile(char::is_whitespace));

    let integer = P(Capture((Optional('-'), TakeWhile1(|ch| ch.is_digit(10))))
        .map(|str| str.parse().unwrap()));

    let factor = (P('(') >> ws >> expr << ws << ')' | integer) << ws;

    let term = factor.repeat(1..)
        .join('*'.or('/') << ws)
        .fold(|| 0, |_, x| x, |acc, op, x| match op {
            '*' => acc * x,
            '/' => acc / x,
            _ => unreachable!(),
        });

    term.repeat(1..)
        .join('+'.or('-') << ws)
        .fold(|| 0, |_, x| x, |acc, op, x| match op {
            '+' => acc + x,
            '-' => acc - x,
            _ => unreachable!(),
        })
        .parse(str, from)
}

pub fn parse(str: &str) -> Result<i64, (usize, munch::str::Error<'static>)> {
    use munch::str::*;

    let ws = P(TakeWhile(char::is_whitespace));
    (ws >> expr << ws << End).parse(str, 0).map(|(_, output)| output)
}

(Yes, it's cheating a bit since Nom doesn't have a built-in combinator for join + fold yet so their example is more verbose than it would have been otherwise.)