winnow::combinator

Function opt

Source 
pub fn opt<Input: Stream, Output, Error, ParseNext>(
    parser: ParseNext,
) -> impl Parser<Input, Option<Output>, Error>
where
    ParseNext: Parser<Input, Output, Error>,
    Error: ParserError<Input>,
Expand description

Apply a Parser, producing None on ErrMode::Backtrack.

To chain an error up, see cut_err.

§Example

use winnow::combinator::opt;
use winnow::ascii::alpha1;

fn parser<'i>(i: &mut &'i str) -> ModalResult<Option<&'i str>> {
  opt(alpha1).parse_next(i)
}

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