Function combine::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'), "")));