Pretty nifty. As of now, the code doesn't compile: there's some stray "span" stuff in codegen.rs[1], and it's trying to format `Warning` which doesn't implement `Display` in main.rs[2].
Fixing these, it runs mostly as advertised, but it seems to assume that one-letter types are always generic parameters, so it's impossible to (for example) generate this:
struct X;
enum A {
P(X),
Q
}
Trying this:
(struct X)
(enum A (P X) Q)
produces this:
struct X;
enum A<P, X> { Q }
while using a multi-letter type like `String`:
(enum A (P String) Q)
produces the expected:
enum A { P(String), Q }
One way to solve this would be to always require the generic annotation, and let it be empty when there are no generics, but when I tried that it did something weird:
I think some comments are missing the upside of it being precisely Rust, without any new semantics. If you want lisp that compiles to machine code, Common Lisp can get reasonably efficient. The purpose of bringing Rust into it is to surface Rust-specific semantics -- which many people quite like!
If you already have the ability to express the grammar productions in Rust that allow for optionally-specified types (e.g. variable declaration), then you have the ability to express lifetimes and the turbofish (which is just a curious way to call a generic function with a specific type parameter). The only weird thing would be that Lisp uses the apostrophe character for something very different than Rust, but you could just pick any other way to denote lifetimes.
Type F must be a function that's generic over any possible lifetime 'a, with a single argument that's a reference with lifetime 'a to a tuple of two numbers, and returns a reference with the same lifetime 'a to an 8-bit number.
The full code is usually something like:
fn foo<F>(callback: F) where for<'a> F: ...
Which is a generic function foo that takes the argument of type F, where F must be...
It seems like this is more like writing Rust in an s-expression syntax instead of having a proper lisp dialect that compiles to Rust, which is cool I guess but not very interesting.
It's quite weird-looking for someone who's done any amount of lisp programming.
A let that defines variables that have a lifetime beyond the scope of the expression? Yeah, that's really unusual. And it's not even the oddest looking thing from the first example block of code.
Yeah, it sort of reminds me of the microcode assembly of a few of the lisp machines, that, while in s-expressions were also clearly not lisp themselves. But could be an interesting target for some lisp macros.
I like the ubiquitous type inference. It reminds me a bit of ELSA for Emacs Lisp: https://github.com/emacs-elsa/Elsa. In particular, type aware macros have been on my wishlist forever: there's no good reason I shouldn't be able to write, e.g. an elisp or CL/SBCL compiler-macro that specializes an operation based on its inferred type. In normal lisps, it's hard to get even the declared types.
That said, I wish that part of Loon were less coupled to the allocation model though. What made you opt for mandatory manual memory management in an otherwise high-level language? And effects?
There are two things common in language design that, honestly, strike me as unnecessary:
1. manual allocation and lifetime stacking, and
2. algebraic effects.
On 1: I think we often conflate the benefits of Rust-style mutability-xor-aliased reference discipline with the benefits of using literal malloc and free. You can achieve the former without necessitating the latter, and I think it leads to a nicer language experience.
It's not just true that GC "comes with latency spikes, higher memory usage, and unpredictable pauses" in any meaningful way with modern implementations of the concept. If anything, it leads to more consistent latency (no synchronous Drop of huge trees at unpredictable times) and better memory use (because good GCs use compressed pointers and compaction).
On 2: I get non-algebraic effects for delimited continuations. But lately I've seen people using non-flow-magical effects for everything. If you need to talk to a database, pick a database interface and pass an object implementing the interface to the code that needs it. Effects do basically the same thing, but implicitly.
I always saw algebraic effects as a more-ergonomic alternative to functor/applicative/monad for managing I/O and otherwise impure code. If you aren't particularly concerned with that level of purity then yeah it's "just" an indirect way to write an interface.
I've found that in practice, people use effects systems as dynamic-extent globals, like DEFVAR-ed variables in Lisp.
"Oh, it's not a global. Globals are bad. Effects are typed and blend into the function signature. Totally different and non-bad."
No. Typing the effects doesn't help: oh, sure, in Koka I can say that my function's type signature includes the "database connection" effect. Okay, that's a type. Where does the value backing that type come from? Thin air? No, the value backing an effect comes from the innermost handler, the identity of which, in a large program, is going to be hard to figure out.
Like all global variables, the sorts of "effects" currently in vogue will lead to sadness at scale. Globals don't stop being bad when we call them something else: they're still bits of ambient authority that frustrate local reasoning. It's as if everyone started smoking again but called cigarettes "mist popsicles" and claimed that they didn't cause cancer.
There's no way around writing down names for the capabilities we give a program and propagating these names from one part of the program to another. Every scheme to somehow free us from this chore is just smuggling in ambient authority by another name. Ambient authority is seductive. At small scales, it's fine. Better than fine! Beautiful. Then, one day, as your program scales and its maintainership churns, you find you have no idea who implements what.
Software engineering develops antibodies against these seductions. The problem is that the antibodies are name-based, so when we dress up old, bad ideas with new names, we have to re-learn why they're bad.
P.S. You might object, "You're talking about dynamic-extent effects. What about lexically-scoped effects systems?", you might ask. "These fix the problems with dynamic-extent effects."
Sure. Lexical effects are better. That's why every decent language already has a "lexically-scoped effect system". It's called let-over-lambda, or if you squint, an "object". We've come full circle.
The website seems to have some bugs on mobile, seen on Chrome 147.0.7727.137
- Cannot horizontally scroll the code snippets on homepage when it overflows. The scroll bars appear but swiping the snippet does nothing.
- Footer links are unresponsive (loon, GitHub, MIT Licence links)
- In the changelog page, scrolling makes the hamburger menu hide release dates behind it
- Hamburger close chevron looks misaligned (not sure if this was a deliberate choice)
So if I wanted to actually use this and I write some rust-but-lisp code and there's a compile error, will it show me a nice error message with an arrow pointing to where the error happened in my lisp code?
Can I use the amazing `rust-analyzer` LSP to get cool IDE features?
I suspect the answer is no, but these might be good further prompts to use.
Greenspun's tenth rule was formulated in a time before things like first-class functions were commonplace in industrial languages. Rust supports not just functional programming idioms but outright Scheme-style macros, it's out of scope for Greenspun's.
Reminds me of when I brainstormed an S-expression-based alternative syntax for Rust in the comments on an article about Rust's syntax: <https://news.ycombinator.com/item?id=41399712>.
Yes, but you could do the same by transforming Rust's ASTs. The only downside is that your input format is different from the format you are transforming. But the upside is that readability is much improved, which matters because code is typically read far more often than it is written.
How do you change the syntax to eliminate reverse compatibility? I guess you could change the names of most key functions between releases. But to be compatible with rust you would need to make breaking changes every release.
>S-expression syntax parsers are not hard to write.
I'm not sure I quite understand the point of your comment.
Are you implying that LLMs should be used for very hard to write code? I feel like the best use of LLMs is to automate the easy stuff so that I can focus on the hard to write stuff.
Unfortunately, given the clear LLM basis of this project, s-expressions aren't a great choice. I've found coding agents struggle really hard with s-expression parentheses matching.
Much better to give them something more M-expr styled, I think a grammar that is LL(1) is probably helpful in that regard.
Basically the more you can piggyback on the training data depth for algol-style and pythonic languages the better.
That has definitely not been my experience as of late. I have produced multiple, largeish Clojure projects with AI that have been perfectly formatted and functional. Perhaps you were using an older or possibly smaller model? I am admittedly using Claude with higher end models and mid to high effort but it has been working great for months for me at this point.
Nope, but to be fair when you're working on your own novel S-exprs you don't have LSPs to guide the coding agent. I imagine that it works a lot better in the context of a known and understood language environment like Clojure, CL, scheme, etc. The other option would be to write an LSP in a non-S-expr language to ensure that no turn can end with mismatched parens, for example.
It is absolutely not true, I've vibecoded an app for myself in CL and opus/sonnet had 0 problems with parens and types. Add to it an MCP to work with REPL and it is much more smooth than Go in my experience.
I think this misses the point of LISP macros. LISP macros are just functions written in LISP, so here macros need to be functions written in Rust-but-LISP, but it is not so. In fact, I think this macro language lacks conditionals, so it’s not even Turing-complete.
For everyone who is shaming on the project for "not implementing enough," then you can definitely help me with it.
For everyone who is shaming on the project for being "LLM slop," sure but that's the reason why something like this can exist in the first place. The point isn't to be a finished, production-ready product. The point is to be an interesting work, and just a sly bit silly
Scheme already has hygenic macros, I don't get why you'd vibecode a worse (less battle tested, llm-generated) replacement. I'm not sure why this hit the front-page, to be honest, because it doesn't seem noteworthy or interesting (Anyone and their mother can vibecode something like this in eight hours)
Pretty nifty. As of now, the code doesn't compile: there's some stray "span" stuff in codegen.rs[1], and it's trying to format `Warning` which doesn't implement `Display` in main.rs[2].
Fixing these, it runs mostly as advertised, but it seems to assume that one-letter types are always generic parameters, so it's impossible to (for example) generate this:
Trying this: produces this: while using a multi-letter type like `String`: produces the expected: One way to solve this would be to always require the generic annotation, and let it be empty when there are no generics, but when I tried that it did something weird: produces: I have no idea where the `_` and the comment came from.[1] https://github.com/ThatXliner/rust-but-lisp/blob/70c51a107b2...
[2] https://github.com/ThatXliner/rust-but-lisp/blob/70c51a107b2...
I think some comments are missing the upside of it being precisely Rust, without any new semantics. If you want lisp that compiles to machine code, Common Lisp can get reasonably efficient. The purpose of bringing Rust into it is to surface Rust-specific semantics -- which many people quite like!
So I'm guessing it's sort of like Fennel, which consciously doesn't completely hide its underlying Lua runtime?
https://fennel-lang.org/
this is actually very interesting approach to syntax remixing, while keeping the ontology
[flagged]
Incorrect, and mean-spirited. Come on.
For what it's worth I thought it was a nice comment and I saw no sign of AI in it
Claims to have all the syntax covered, but not a single example of specifying lifetimes or the turbofish, some of the trickiest rust syntax
If you already have the ability to express the grammar productions in Rust that allow for optionally-specified types (e.g. variable declaration), then you have the ability to express lifetimes and the turbofish (which is just a curious way to call a generic function with a specific type parameter). The only weird thing would be that Lisp uses the apostrophe character for something very different than Rust, but you could just pick any other way to denote lifetimes.
Could!
> Everything Rust has … expressed as s-expressions. No semantic gap.
You can also drop into direct Rust with the (rust "...") macro if I forgot to implement anything
It's a vibecoded parser...
Technically it's a transpiler.
The HRTB is probably the trickiest syntax for specifying lifetimes. It looks like `for<'a> F: Fn(&'a (u8, u16)) -> &'a u8`.
Can you translate this for those of us who don't speak rust?
Type F must be a function that's generic over any possible lifetime 'a, with a single argument that's a reference with lifetime 'a to a tuple of two numbers, and returns a reference with the same lifetime 'a to an 8-bit number.
The full code is usually something like:
fn foo<F>(callback: F) where for<'a> F: ...
Which is a generic function foo that takes the argument of type F, where F must be...
It seems like this is more like writing Rust in an s-expression syntax instead of having a proper lisp dialect that compiles to Rust, which is cool I guess but not very interesting.
It's quite weird-looking for someone who's done any amount of lisp programming.
>Rust semantics with LISP syntax. A transparent s-expression frontend that compiles directly to Rust — no runtime, no GC
The first paragraph says literally that.
A let that defines variables that have a lifetime beyond the scope of the expression? Yeah, that's really unusual. And it's not even the oddest looking thing from the first example block of code.
Yeah, it sort of reminds me of the microcode assembly of a few of the lisp machines, that, while in s-expressions were also clearly not lisp themselves. But could be an interesting target for some lisp macros.
Readers may enjoy my lisp, Loon, which takes heavy inspiration from Rust https://loonlang.com/guide/ownership
And if you like Lisp and ownership, there's also Carp [1]. It doesn't mimic Rust's features and naming schemes though.
Carp is about 10 years old and has some cool demo's (like SDL for gamedev).
> The key features of Carp are the following:
> * Automatic and deterministic memory management (no garbage collector or VM)
> * Inferred static types for great speed and reliability
> * Ownership tracking enables a functional programming style while still using mutation of cache-friendly data structures under the hood
> * No hidden performance penalties – allocation and copying are explicit > * Straightforward integration with existing C code
> * Lisp macros, compile time scripting and a helpful REPL
[1]: https://github.com/carp-lang/Carp
I like the ubiquitous type inference. It reminds me a bit of ELSA for Emacs Lisp: https://github.com/emacs-elsa/Elsa. In particular, type aware macros have been on my wishlist forever: there's no good reason I shouldn't be able to write, e.g. an elisp or CL/SBCL compiler-macro that specializes an operation based on its inferred type. In normal lisps, it's hard to get even the declared types.
That said, I wish that part of Loon were less coupled to the allocation model though. What made you opt for mandatory manual memory management in an otherwise high-level language? And effects?
There are two things common in language design that, honestly, strike me as unnecessary:
1. manual allocation and lifetime stacking, and
2. algebraic effects.
On 1: I think we often conflate the benefits of Rust-style mutability-xor-aliased reference discipline with the benefits of using literal malloc and free. You can achieve the former without necessitating the latter, and I think it leads to a nicer language experience.
It's not just true that GC "comes with latency spikes, higher memory usage, and unpredictable pauses" in any meaningful way with modern implementations of the concept. If anything, it leads to more consistent latency (no synchronous Drop of huge trees at unpredictable times) and better memory use (because good GCs use compressed pointers and compaction).
On 2: I get non-algebraic effects for delimited continuations. But lately I've seen people using non-flow-magical effects for everything. If you need to talk to a database, pick a database interface and pass an object implementing the interface to the code that needs it. Effects do basically the same thing, but implicitly.
I always saw algebraic effects as a more-ergonomic alternative to functor/applicative/monad for managing I/O and otherwise impure code. If you aren't particularly concerned with that level of purity then yeah it's "just" an indirect way to write an interface.
I've found that in practice, people use effects systems as dynamic-extent globals, like DEFVAR-ed variables in Lisp.
"Oh, it's not a global. Globals are bad. Effects are typed and blend into the function signature. Totally different and non-bad."
No. Typing the effects doesn't help: oh, sure, in Koka I can say that my function's type signature includes the "database connection" effect. Okay, that's a type. Where does the value backing that type come from? Thin air? No, the value backing an effect comes from the innermost handler, the identity of which, in a large program, is going to be hard to figure out.
Like all global variables, the sorts of "effects" currently in vogue will lead to sadness at scale. Globals don't stop being bad when we call them something else: they're still bits of ambient authority that frustrate local reasoning. It's as if everyone started smoking again but called cigarettes "mist popsicles" and claimed that they didn't cause cancer.
There's no way around writing down names for the capabilities we give a program and propagating these names from one part of the program to another. Every scheme to somehow free us from this chore is just smuggling in ambient authority by another name. Ambient authority is seductive. At small scales, it's fine. Better than fine! Beautiful. Then, one day, as your program scales and its maintainership churns, you find you have no idea who implements what.
Software engineering develops antibodies against these seductions. The problem is that the antibodies are name-based, so when we dress up old, bad ideas with new names, we have to re-learn why they're bad.
P.S. You might object, "You're talking about dynamic-extent effects. What about lexically-scoped effects systems?", you might ask. "These fix the problems with dynamic-extent effects."
Sure. Lexical effects are better. That's why every decent language already has a "lexically-scoped effect system". It's called let-over-lambda, or if you squint, an "object". We've come full circle.
Agreed, a Lisp built around the concept of ownership is much more interesting than just a way to write Rust via S-expressions.
How does ownership work within the Lisp tree structure?, What is the difference between ownership on this setting and ARC?
ARC refcounts everything (many owners) whereas Loon refcounts only inside immutable shared tree nodes (one owner per handle)
I like it but confusing that there's also a similar but different loonlang.org.
That page is beautiful! What ssg / theme are you using to build it?
Thanks :) Check out the code - it's Loon! https://github.com/ecto/loon/blob/main/web/src/pages/guide/o...
The website seems to have some bugs on mobile, seen on Chrome 147.0.7727.137
- Cannot horizontally scroll the code snippets on homepage when it overflows. The scroll bars appear but swiping the snippet does nothing. - Footer links are unresponsive (loon, GitHub, MIT Licence links) - In the changelog page, scrolling makes the hamburger menu hide release dates behind it - Hamburger close chevron looks misaligned (not sure if this was a deliberate choice)
Honestly, that's very cool
That was basically my intent with this project, but I took the laziest way to get there lol
I’ve seen this done with C as well.
https://www.eriksvedang.com/carp
https://github.com/tomhrr/dale
Carp is memory safe via linear types + references, similar to Rust, so I would not describe it as C-like but rather Rust-like.
> ((. dx powf 2.0) + (. dy powf 2.0)) sqrt))
I don't know what this is, but clearly not Lisp...
So if I wanted to actually use this and I write some rust-but-lisp code and there's a compile error, will it show me a nice error message with an arrow pointing to where the error happened in my lisp code?
Can I use the amazing `rust-analyzer` LSP to get cool IDE features?
I suspect the answer is no, but these might be good further prompts to use.
Good idea! I'm adding error messages + spans ala ariadne now
Should be named "Rutht"
Greenspun's tenth rule of programming is an aphorism in computer programming and especially programming language circles that states:[1][2]
Any sufficiently complicated C or Fortran program contains an ad hoc, informally-specified, bug-ridden, slow implementation of half of Common Lisp.
Maybe we should one day include Golang or Rust to it
https://en.wikipedia.org/wiki/Greenspun%27s_tenth_rule
Greenspun's tenth rule was formulated in a time before things like first-class functions were commonplace in industrial languages. Rust supports not just functional programming idioms but outright Scheme-style macros, it's out of scope for Greenspun's.
Reminds me of when I brainstormed an S-expression-based alternative syntax for Rust in the comments on an article about Rust's syntax: <https://news.ycombinator.com/item?id=41399712>.
This is probably what Rust's internal ASTs look like. But why would you want to input programs as ASTs?
so you can do the transformations (see the rlisp macro section)
Yes, but you could do the same by transforming Rust's ASTs. The only downside is that your input format is different from the format you are transforming. But the upside is that readability is much improved, which matters because code is typically read far more often than it is written.
How do you change the syntax to eliminate reverse compatibility? I guess you could change the names of most key functions between releases. But to be compatible with rust you would need to make breaking changes every release.
How is pure unbridled AI slop like this making the front page? Voting rings?
I don't even feel bad saying this because clearly OP is just the front for Claude here.
you ain't seen nothin' yet.
https://www.youtube.com/watch?v=4cia_v4vxfE
I don't understand why this had to be LLM generated. S-expression syntax parsers are not hard to write. That's rather much the point of S-expressions.
>S-expression syntax parsers are not hard to write.
I'm not sure I quite understand the point of your comment.
Are you implying that LLMs should be used for very hard to write code? I feel like the best use of LLMs is to automate the easy stuff so that I can focus on the hard to write stuff.
"no runtime, no GC, just" I am BEGGING every project to not have this LLMism in their docs.
It reads as No X no Y just slop to me every time.
It's completely nonsensical too. Why would a parser for an alternative syntax introduce a GC?!
does there exist something that can do the opposite?
some pre-processor that "compiles into rust" from less awful syntax?
Unfortunately, given the clear LLM basis of this project, s-expressions aren't a great choice. I've found coding agents struggle really hard with s-expression parentheses matching.
Much better to give them something more M-expr styled, I think a grammar that is LL(1) is probably helpful in that regard.
Basically the more you can piggyback on the training data depth for algol-style and pythonic languages the better.
That has definitely not been my experience as of late. I have produced multiple, largeish Clojure projects with AI that have been perfectly formatted and functional. Perhaps you were using an older or possibly smaller model? I am admittedly using Claude with higher end models and mid to high effort but it has been working great for months for me at this point.
Nope, but to be fair when you're working on your own novel S-exprs you don't have LSPs to guide the coding agent. I imagine that it works a lot better in the context of a known and understood language environment like Clojure, CL, scheme, etc. The other option would be to write an LSP in a non-S-expr language to ensure that no turn can end with mismatched parens, for example.
If anyone is curious, I've been making this using DeepSeek v4 Flash with Claude Code as the harness
It is absolutely not true, I've vibecoded an app for myself in CL and opus/sonnet had 0 problems with parens and types. Add to it an MCP to work with REPL and it is much more smooth than Go in my experience.
Opus 4.6 handles elisp just fine. But I suppose YMMV.
Why are we even spending time on this. It's vibe coded slop. The creator probably never even ran it before it got to HN
Hmm, interesting ...
Anyone working on something similar that compiles to go?
I actually like it better than plain Rust…
I think this misses the point of LISP macros. LISP macros are just functions written in LISP, so here macros need to be functions written in Rust-but-LISP, but it is not so. In fact, I think this macro language lacks conditionals, so it’s not even Turing-complete.
> compiles directly to Rust — no runtime, no GC, just (s-expr → .rs → binary).
Can we please write our own READMEs before posting to HN?
"Lust", or "Risp"?
Nah.
It's sort of, but not quite, like "El jefe"
"L rut piss"
For everyone who is shaming on the project for "not implementing enough," then you can definitely help me with it.
For everyone who is shaming on the project for being "LLM slop," sure but that's the reason why something like this can exist in the first place. The point isn't to be a finished, production-ready product. The point is to be an interesting work, and just a sly bit silly
And for why?
To get proper macros.
Scheme already has hygenic macros, I don't get why you'd vibecode a worse (less battle tested, llm-generated) replacement. I'm not sure why this hit the front-page, to be honest, because it doesn't seem noteworthy or interesting (Anyone and their mother can vibecode something like this in eight hours)
Scheme doesn't have Rust semantics, though?
this is not a replacement for scheme, it's simply an alternative syntax for rust
Because Rust syntax is ugly and overcomplicated.
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