OneOf

Sometimes you want to have a choice. E.g. an expression could either be a number or a variable. So when parsing an expression you want the option to parse one of a number or variable.

This is where the OneOf parser comes in. It will pass the following test.

    #[test]
    fn parse_one_of_a_or_b() {
        let input = "a1";
        let parser = one_of(vec![character('a'), character('b')]);

        let actual = parser.parse(input);

        let expected = Ok(('a', "1"));
        assert_eq!(actual, expected);
    }

Considerations

Since the Parser trait can return only a single thing, all of our options in the OneOf parser should adhere to the same contract.

Impl Parser

Once you have your options our parse strategy would be to try each of our options in turn until one of them succeed. If it succeed return the parsed result and the remaining input. If none of the options succeed, report an error.

Constructor & Factories

Since there is not very much going on with constructors or factories. From now on we will not mention them any more.

Exercises

Implement the OneOf parser.
Write some test to check your implementation.
Think and discuss to do better error handling.