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My first contact with parser combinators was in a parser generator library: https://github.com/engelberg/instaparse

This is (probably weirdly, since I've just learned that the approach in the OP is the original one that introduced parser combinators) the first time I've seen parser combinators where the parsers are just functions like this. If you want to be able to combine parsers and you don't want to write a bunch of mapping/unwrapping code, why not just use combinators over parsers assembled by a parser generator?

by pdpi9 hours ago|

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I'm struggling to see the point in this. Your roll example using Winnow. Not really that much (un)wrapping logic:

    use winnow::{Result, ascii::digit1, combinator::separated_pair, prelude::*};

    #[derive(Debug, PartialEq)]
    struct Roll {
        count: u32,
        sides: u32,
    }

    fn number(input: &mut &str) -> Result<u32> {
        digit1.try_map(str::parse).parse_next(input)
    }

    fn roll(input: &mut &str) -> Result<Roll> {
        let (count, sides) = separated_pair(number, 'd', number).parse_next(input)?;
        Ok(Roll { count, sides })
    }

    #[cfg(test)]
    mod test {
        use super::*;

        #[test]
        fn test_roll() {
            let input = &mut "3d6";
            assert_eq!(roll(input), Ok(Roll { count: 3, sides: 6 }));
        }
    }

by ArneCode8 hours ago|

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In this case yes, but I would argue this is because the roll grammar is a relatively simple grammar, you have no nesting of different combinators etc. (I have used it in the README because it is simple to understand). The winnow code also using seperated_pair to ignore the "d" value, which is also adding some logic to retain only the count and sides result.

One grammar I have been using to make a comparison is a function signature. I tried comparing with nom and chumsky: https://blog.arnedebo.com/posts/a-grammar-first-approach-to-...

These libraries have found ways to replace "unwrapping" code with combinators like separated_by or .ignore_then etc. but this still leaves a lot of code for result handling, in my opinon making the code harder to read and reason about, which is what I attempt to improve.

These are of course very subjective things, so another person might prefer the ways the other libraries do this.

by mrkeen9 hours ago|

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What kind of mapping/unwrapping are you being subjected to with PCs?

by ArneCode9 hours ago|

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When combining two parsers, for example in a sequence like pSeq in this website, you need to think about what to do with their return values. One approach is to put all return values in a tuple. So lets say we are combining parsers p1,p2,..., p_n in a sequence, then the return value of the parser created by pSeq(p1,p2,...,p_n) is (R(p1), R(p2), ..., R(p_n)) if R(p) is the return value of parser p.

When writing parsers you will often use multiple nested combinators. This means you have nested tuples. Getting the values out of these means "unwrapping" these tuples. Additionally you might not need all of the values in the sequence, some of them might only be for parsing whitespace. So when using the results of the parsers to construct values you have to ignore them. This can for example be done by "mapping" the resulting tuples and only leaving relevant fields. There are other approaches as well, for example chaining parsers with p1.then_ignore(p2) which only returns R(p1).

Maybe I could have chosen the words here a bit better, I hope this makes it more clear what I mean.

I have also tried making a side-by-side comparison of parsers for grammars being written using my library and others, maybe this can act as an example: https://blog.arnedebo.com/posts/a-grammar-first-approach-to-...

by Jaxan8 hours ago|

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I’ve always used parser combinaties in Haskell and I’ve never seen the things you describe. I guess in Haskell one always directly apply the parts to the function you want (ie an AST node constructor).

by thedufer2 hours ago|

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I think this is because Haskell allows hiding the tuples construction/unwrapping with do-notation. My primary interaction with parser combinators, in OCaml, uses a similar trick with binding operators. If you don't have some sort of syntactic sugar like that they do become a lot more unwieldy.

by black_knight8 hours ago|

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We are spoiled in so many ways by Haskell!

Parser combinators alone makes monads (or really applicative functors) worth it. But then there are the all the other spectacular things we can do with monads!

by n_plus_1_acc5 hours ago|

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This. The <$ and $> operators are very elegant for parsers.

by n_plus_1_acc5 hours ago|

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This. The <$ and $> operators are very elegant for parsers.

by n_plus_1_acc5 hours ago|

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