lean4 VS rakudo

Nice read! Rust has pushed, and will continue to push, the limits of practical, bare metal, memory safe languages. And it's interesting to think about what's next, maybe eventually there will be some form of practical theorem proving "for the masses". Lean 4 looks great and has potential, but it's still mostly a language for mathematicians. There has been some research on AI constructed proofs, which could be the best of both worlds because then the type checker can verify that the AI generated code/proof is indeed correct. Tools like Kani are also a step forward in program correctness.

Yeah, I believe they said intend for it to be used as a general purpose programming language. I used it to complete Advent of Code last year.

There are some really interesting features for general purpose programming in there. For example: you can code updates to arrays in a functional style (change a value, get a new array back), but if the refcount is 1, it updates in place. This works for inductive types and structures, too. So I was able to efficiently use C-style arrays (O(1) update/lookup) while writing functional code. (paper: https://arxiv.org/abs/1908.05647 )

Another interesting feature is that the "do" blocks include mutable variables and for loops (with continue / break / return), that gets compiled down to monad operations. (paper: https://dl.acm.org/doi/10.1145/3547640 )

And I'm impressed that you can add to the syntax of the language, in the same way that the language is implemented, and then use that syntax in the next line of code. (paper: https://lmcs.episciences.org/9362/pdf ). There is an example in the source repository that adds and then uses a JSX-like syntax. (https://github.com/leanprover/lean4/blob/master/tests/playgr... )

My latest inspiration to use code to a) replace my video editor, b) learn the basics of EDM production and c) understand a few topics in higher maths. This might sound very strange given there are specialised tools for these jobs. There's iMovie / Adobe Premier for video, there's GarageBand and FL studio for music and old good pen and pencil for math proofs. But these tools have three big limitations. First they have a lot of idiosyncratic learning, you have to spend quite some time getting used to these tools and my experience is that this time is quite upsetting. In contrast, you only have to learn to code one, maybe spend a few hours getting used to the syntax of another language. I'm not sure if that's true for most people but it was true for me using the tools mentioned above and wanted a place to discuss and see other people ideas and experiments. The second issue is that all these custom-made tools, are not composing easily. I can't search for all math proofs that used a single theorem. I can't create a plugin for iMovie and apply it to all my videos. I can't pick easily pick a rhythm from the internet and build upon for fun. There's also the issue of costs and version control, all tools I'm using today are open source and my work is stored in my repositories. This way I can create branches and test my ideas and I'm also confident that I can work in these projects in years.

Dafny and Whiley are two examples with explicit verification support. Idris and other dependently typed languages should all be rich enough to express the required predicate but might not necessarily be able to accept a reasonable implementation as proof. Isabelle, Lean, Coq, and other theorem provers definitely can express the capability but aren't going to churn out much in the way of executable programs; they're more useful to guide an implementation in a more practical functional language but then the proof is separated from the implementation, and you could also use tools like TLA+.

https://dafny.org/

https://whiley.org/

https://www.idris-lang.org/

https://isabelle.in.tum.de/

https://leanprover.github.io/

https://coq.inria.fr/

http://lamport.azurewebsites.net/tla/tla.html

Lean4 manages to pull off changing the parser on the fly at compile time. You can add new productions, add new syntax node types, and add new tokens. Then define macros or code to process the additional syntax. Here is a sample I found that adds a simple JSX-like syntax starting around line 93 and then uses it at line 169:

https://github.com/leanprover/lean4/blob/master/tests/playgr...

I believe most of the language is defined this way, although it is pre-compiled.

For more details see the lean4 metaprogramming book: https://github.com/arthurpaulino/lean4-metaprogramming-book

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.)