Function parser_combinators::optional

pub fn optional<P>(parser: P) -> Optional<P> where
    P: Parser, 

Returns Some(value) and None on parse failure (always succeeds)

 let mut parser = optional(digit());
 let result1 = parser.parse("a");
 assert_eq!(result1, Ok((None, "a")));
 let result2 = parser.parse("1");
 assert_eq!(result2, Ok((Some('1'), "")));