Function snarkvm_circuit::opt

pub fn opt<I, O, E, F>(f: F) -> impl FnMut(I) where
    I: Clone,
    E: ParseError<I>,
    F: Parser<I, O, E>, 

Optional parser: Will return None if not successful.

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

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

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