Function parser_combinators::try

pub fn try<P>(p: P) -> Try<P> where
    P: Parser, 

Try acts as p except it acts as if the parser hadn't consumed any input if p returns an error after consuming input

 let mut p = try(string("let"))
     .or(string("lex"));
 let result = p.parse("lex").map(|x| x.0);
 assert_eq!(result, Ok("lex"));
 let result = p.parse("aet").map(|x| x.0);
 assert!(result.is_err());