Function opt

pub fn opt<I, E, F>(
    f: F,
) -> impl Parser<I, Output = Option<<F as Parser<I>>::Output>, Error = E>
where
    I: Clone,
    E: ParseError<I>,
    F: Parser<I, Error = E>,

Optional parser, will return None on Err::Error.

To chain an error up, see cut.

use nom::combinator::opt;
use nom::character::complete::alpha1;

fn parser(i: &str) -> IResult<&str, Option<&str>> {
  opt(alpha1).parse(i)
}

assert_eq!(parser("abcd;"), Ok((";", Some("abcd"))));
assert_eq!(parser("123;"), Ok(("123;", None)));