nom with struct context

parser.rs

use nom::bytes::complete::tag;
use nom::character::complete::multispace0;
use nom::combinator::map;
use nom::error::VerboseError;
use nom::sequence::tuple;
use nom::IResult;
struct Ctx {
    a: i32
}
impl Ctx {
    pub fn new() -> Self {
        Self {a: 0}
    }
    fn unit(&mut self) -> impl FnMut(&str) -> IResult<&str, i32, VerboseError<&str>> + '_{
        move |i| {
            let r = map(tuple((tag("("), multispace0, tag(")"))), |_| self.a)(i);
            self.a = self.a+1;
            r
        }
    }
}
fn main() {
    let mut ctx = Ctx::new();
    for n in 0..10 {
        assert_eq!(Ok(("",n as i32)),ctx.unit()("()"))
    }
}