Parsing a Literal String

Often you know exactly what you are looking for. E.g. when separating the the symbol for its production we use =>. I would come in handy if there was a parser that could do that job.

    #[test]
    fn parse_literal_arrow() {
        let input = "=> FF";
        let parser = literal("=>");

        let actual = parser.parse(input);

        let expected = Ok(("=>", " FF"));
        assert_eq!(actual, expected);
    }

Literal

The only thing that we need to keep track of is the string that we are interested in. Rust provides various ways to model your data. Tuple structs are a convenient way for our use case. It allows one to name a structure by describing the fields in a tuple.

pub struct Literal<'p>(&'p str);

Note that we have a tuple with only one field. It does not have a name so we need to reference it by index.

Impl Parser

Parsing with a Literal amounts to the following steps

Check if the input starts with our literal.
If it does, chop of the parsed string and return it together with the remaining input
If it does not, report an error.

This translate into code as follows.

impl <'a, 'p> Parser<'a, &'a str> for Literal<'p> {
    fn parse(&self, input: &'a str) -> Result<(&'a str, &'a str), ParseError> {
        if input.starts_with(self.0) {
            let len = self.0.len();
            let substr = &input[..len];
            let rem = &input[len..];
            Ok((substr, rem))
        } else {
            Err(ParseError::ExpectingLiteral(self.0.to_owned()))
        }
    }
}

Take a look at the expected result. It has type &'a str. We will return a slice of the input, so the lifetime should match up.

Constructor

Since tuple structs come with a build int constructor, we do not need to do anything here.

Factory

The factory for the Literal parser can just be a function literal to returns the parser

pub fn literal(match_exactly: &str) -> Literal {
    Literal(match_exactly)
}

Exercises

Implement the Literal parser.
Create some tests to verify your implementation.

L-Systems