rakudo VS instaparse

Fix IO::Path::parent #4795: merged 2022-02-19 Add more IO::Path::parent tests #801: merged 2022-02-19 Change parent to always just remove the last element #4800: merged 2022-02-26 Change .parent behavior to "stupid" resolving #802: merged 2022-02-26

This then became the Formatter class. And since this was a completely new feature, it only became available for use by opting into the 6.e.PREVIEW language version. And then it went largely unnoticed and uncared for the next 1.5 year. As clearly the time wasn't right for it yet.

The intended audience are those people willing to be early adopters of these exciting new features in the Raku Programming Language. The examples in this blog post will work in the next release of the Rakudo compiler (probably 2023.06), but are now already available in the bleeding edge version.

If you really want to look at this, you can find the code in src/Perl6/Grammar.nqp, src/Perl6/Actions.nqp and src/Perl6/World.nqp.

If you find this very interesting, you probably want to read the RakuAST README. And the actual source code of the RakuAST classes can be found in the same directory. And if you're really feeling adventurous and you have the Rakudo repository checked out, you can have a look at the generated NQP code in gen/moar/ast.nqp.

Yes, it would. But until there was RakuAST, that was virtually impossible to do because there was no proper API for building ASTs. Nor was there an interface to execute those ASTs. And now that there is RakuAST, it is actually possible to do this. And there is actually already an implementation of that idea in the new Formatter class. Although this is definitely not intended as an entry point into grokking RakuAST.

I have opened one. https://github.com/rakudo/rakudo/issues/5205.

Will Coleda has made a Pull Request

There's an issue pertaining to this. This is something I'd like to resolve, but I'm unsure on how to better debug this to see if it really is the deserialization of a setting file triggering it. JDK 11 should at least be capable of building Rakudo, but being an experimental backend people don't always align with MoarVM immediately, I can't make any guarantees about tests. You may be disappointed in its performance at the moment.

(NB. While the PL is just a toy (and just a tiny bit of the toy too), the tech is actually industrial strength, used to power the production Raku compiler, which is written in Raku using its grammar construct. Starting easy doesn't mean you can't go far. Quite the opposite in fact -- you can go as far as you want.)

Caveats: I've used nom in anger, chumsky hardly at all, and tree-sitter only for prototyping. I'm using it for parsing a DSL, essentially a small programming language.

The essential difference between nom/chomsky and tree-sitter is that the former are libraries for constructing parsers out of smaller parsers, whereas tree-sitter takes a grammar specification and produces a parser. This may seem small at first, but is a massive difference in practice.

As far as ergonomics go, that's a rather subjective question. On the surface, the parser combinator libraries seem easier to use. They integrate well with the the host language, so you can stay in the same environment. But this comes with a caveat: parser combinators are a functional programming pattern, and Rust is only kind of a functional language, if you treat it juuuuust right. This will make itself known when your program isn't quite right; I've seen type errors that take up an entire terminal window or more. It's also very difficult to decompose a parser into functions. In the best case, you need to write your functions to be generic over type constraints that are subtle and hard to write. (again, if you get this wrong, the errors are overwhelming) I often give up and just copy the code. I have at times believed that some of these types are impossible to write down in a program (and can only exist in the type inferencer), but I don't know if that's actually true.

deep breath

Tree-sitter's user interface is rather different. You write your grammar in a javascript internal dsl, which gets run and produces a json file, and then a code generator reads that and produces C source code (I think the codegen is now written in rust). This is a much more roundabout way of getting to a parser, but it's worth it because: (1) tree-sitter was designed for parsing programming languages while nom very clearly was not, and (2) the parsers it generates are REALLY GOOD. Tree-sitter knows operator precedence, where nom cannot do this natively (there's a PR open for the next version: https://github.com/Geal/nom/pull/1362) Tree-sitter's parsing algorithm (GLR) is tolerant to recursion patterns that will send a parser combinator library off into the weeds, unless it uses special transformations to accommodate them.

It might sound like I'm shitting on nom here, but that's not the goal. It's a fantastic piece of work, and I've gotten a lot of value from it. But it's not for parsing programming languages. Reach for nom when you want to parse a binary file or protocol.

As for chumsky: the fact that it's a parser combinator library in Rust means that it's going to be subject to a lot of the same issues as nom, fundamentally. That's why I'm targeting tree-sitter next.

There's no reason tree-sitter grammars couldn't be written in an internal DSL, perhaps in parser-combinator style (https://github.com/engelberg/instaparse does this). That could smooth over a lot of the rough edges.

Sure, you can use https://github.com/Engelberg/instaparse to create parser for any language you want, or simply create DSL in basic clojure types, like vectors.

Enter org-parser! It is indeed such a thing implemented already! Remember the magical parser I mentioned above? It is already implemented here Engelberg/instaparse too (in a Lisp)! org-parser is built on top of it by providing a formal specification for org-mode in the EBN form. Any proof that org-parser works? Indeed, there is the celebrated organice which is built on top of it. Kudos for 200ok-ch!

You might enjoy checking out Instaparse, a Clojure library. Its project page reads, “What if context-free grammars were as easy to use as regular expressions?”

It’s not over-promising, either. I went from never having heard of it before to getting complete and correct parse trees of some ancient JSP Expression Language in about 20 minutes. Most of that time was spent typing in the BNF description that I could find only in an image.

https://github.com/Engelberg/instaparse

Instaparse sounds pretty close to what you're looking for assuming you're ok being limited to context-free grammars.

It is in EBNF, with some alternate conventions. It follows the syntax found in here, which I think is pretty easy to get behind.

Another way to solve it is to load the grammar directly into instaparse, specify the start rule :0 and count the successful applications of the parser to the messages: