Expand description
Parses parser
and outputs Some(value)
if it succeeds, None
if it fails without
consuming any input. Fails if parser
fails after having committed some input.
let mut parser = optional(string("hello"));
assert_eq!(parser.parse("hello"), Ok((Some("hello"), "")));
assert_eq!(parser.parse("world"), Ok((None, "world")));
assert!(parser.parse("heya").is_err());