Roto: A Compiled Scripting Language for Rust
107 comments
·May 21, 2025airstrike
speed_spread
Then also have a look at Mun:
https://github.com/mun-lang/mun
- Ahead of time compilation
- Statically typed
- First class hot-reloading
Not sure how both languages compare though
terts
Hi! Author here. Mun is super cool, but works slightly differently as far as I know. You compile Mun scripts outside of your application to a dynamic library, which can then be (re)loaded by your application. The advantage of that approach is that you can ship the compiled script with your application.
The downside is that it's not really possible to share types between the host application and the script as far as I know. They have something called called `StructRef` which does this a bit, but it's all runtime checks if I understand correctly.
If somebody here knows more about Mun, I'd be happy to be corrected. This is just my understanding from their documentation.
echelon
I have dreamed of this for Rust. It is literally _the_ killer app to have a fast scripting language that pairs with Rust well.
With fast development cycles and a safe scripting language, Rust will find itself in every single programming niche. Server development, game development, desktop application development. If the WASM DOM bridge gets better, maybe even web development. Everything will open up.
You can prototype in the scripting language, write mature application logic in the scripting language, and for the parts that matter, move them to Rust.
I hope Roto can be that. There are lots of other Rust scripting languages that fall short.
A good scripting language will have great Rust interop, similar syntax, and allow simple calling and passing of data structures. It shouldn't weaken Rust in any way.
falcor84
> game development
I've been out of the loop for a while, but last I checked, the running gag was that there are more game engines written in rust than games. Have things changed? Any high profile games that were made in rust?
lunyaskye
The biggest one I know of is Tiny Glade: https://store.steampowered.com/app/2198150/Tiny_Glade/
It's mostly a custom stack, but it's written in Rust and uses some parts of Bevy from what I understand.
kurayashi
Afaik there still no high profile games in rust. But many if the complaints I’ve read of devs switching back from rust were about how hard it was to prototype something in rust. Having a scripting language with solid rust interop could help.
sn9
This might be revealing my ignorance, but if we're going for statically typed "scripting languages", why not implement an ML like SML?
dkarl
We keep saying "scripting language," but given that it's statically typed and JIT compiled, we could also call it an "application language." Relative to Rust, most backend applications can afford to trade off a little bit of performance for better development speed and readability.
airstrike
VBA but good
cess11
What does Rust have that other languages with these properties don't?
mikepurvis
On the game engine side, probably not a lot until there's more ecosystem built up, but across the board you could look at the niches filled by the Java/Groovy and C/Lua pairings to get a pretty good sense of what's possible— it would be the tools stuff that would most excite me: build/CI/automation frameworks, that kind of thing.
Alternatively, look at a project like ruff— hundreds of linter rules statically implemented in native Rust [1], maybe that would make more sense if the rules could be more tersely expressed as runtime-loadable roto scripts that have access to the Python AST and a toolkit of Rust-supplied functions for manipulating and inspecting it?
[1]: https://github.com/astral-sh/ruff/tree/main/crates/ruff_lint...
null
echelon
- The best package manager in the world, bar none. Compiling, linking, and platform difficulties disappear. The devops/devexp is next-level sublime.
- "low defect" software without much work. I'm not talking about memory management, but rather Option<T> and Result<T,E>. For the same amount of effort as writing Golang or Java or C#, you can get defect free code.
- Easy deployable to WASM without packaging a memory management runtime for the most intrusive types. Super portable, super fast.
- If you've used Unreal a lot, you've hit build issues, linker issues, segfault issues. A lot. That'll go away.
ijustlovemath
In case the author comes in, I'm curious about how you designed the registration mechanism. We have a Python application that makes heavy use of decorators to give strong runtime introspection capabilities, and I've always wondered if the same could be done at or near compile time in an equivalent Rust context. I think learning about the drawbacks and boons of the designs you settled on would be really informative!
terts
Hi! So for Roto, our introspection needs are actually fairly limited. We only need the `TypeId`, the type name, the size and the alignment. which Rust can give us without any additional traits. It's not possible currently to - for example - access struct fields and enum variants. That is something that I plan to add, but that might require a crate like `bevy_reflect` or `facet`.
Rust is giving me just enough information to pull the current version of. More powerful introspection/reflection is not possible without derive macros. If you're ok with derive macros though, you could look into the 2 crates I mentioned.
Hope that answers your question!
ijustlovemath
Did you go through many iterations on the API of the registration? If so, which designs did you disqualify and why?
terts
Not many iterations, but a lot of head scratching was involved haha.
I decided against using a trait and a derive macro because I wanted to avoid running into Rust's orphan rule. We have a crate called routecore where most of our types are declared and then a separate crate called Rotonda which uses those types and uses Roto. I wanted Rotonda to be able to register routecore types.
That's also the downside of the current reflection crates; they require each type to have a derive macro.
dkarl
Could you write 80-100% of an application in this language? I'm wondering if it could be a good application language for Rust programmers who want to use the Rust ecosystem and have the option of writing parts of their application in Rust for extra performance, but who also want to experiment and iterate quickly, and who want a simpler, higher-level language for expressing business logic.
terts
Hi! Author here. You could try but there are some fundamental limitations.
The biggest limitation is that we don't have access to the full type system of Rust. I don't think we can ever support registering generic types (e.g. you can register `Vec<u32>` but not `Vec<T>`) and you don't have access to traits. So it would work if you can reduce your API to concrete types.
Otherwise - apart from some missing features - you could probably write big chunks in Roto if you wanted to. You could also prototype things in Roto and then translate to Rust with some simplified types.
Also you'd have to accept that it gets compiled at runtime.
0cf8612b2e1e
That’s my dream. Seamless FFI to Rust with all of the core in a dynamic scripting language. If/when you need to tighten up performance/types, can port more of the code to actual Rust.
dkarl
That's my dream, but statically typed. Let programmers gloss over all the memory management stuff that makes writing Rust painstaking and complex (put everything on the heap by default if necessary.) Trade off runtime performance for faster iteration and simpler code. Still enjoy static typing, the Rust ecosystem, and the option to write parts of your code in Rust for maximum performance.
ori_b
This language looks a lot like Rust. Why not dlopen() a Rust shared library instead? The implementation would be about as complicated, but it would be a well known language that's fully integrated with a large library ecosystem, well defined build and package set, rather than some custom one-off thing with no ecosystem. Going your own way means your users have to re-invent the wheel for themselves every time.
With Rust's safety, it's not even that bad to re-open and re-load the binary when it changes; the big footgun with dlopen is use-after-free.
terts
Hi! Author here. There's a couple of reasons.
First, this language is syntactically a lot like Rust but semantically quite different. It has no references for example. We're trying to keep it simpler than Rust for our users.
Second, using Rust would require compiling the script with a Rust compiler, which you'd then have to install alongside the application. Roto can be fully compiled by the host application.
I think your approach might be preferred when the application author is also the script author. For example, if you're a game developer who is using a script to quickly prototype a game.
erlend_sh
Are you saying Roto has no ambitions to be suitable as a gamedev scripting language?
terts
It depends. I'd love to make a prototype using Bevy with Roto. What I'm trying to say is that if you only want something to make Rust compile faster, then Rust might the better option. If you want something that behaves more like a scripting language and you don't mind that is compiled at startup, then Roto might be good for that purpose (with the caveat that there are missing features of course).
abendstolz
Don't rust shared libraries have the problem of no stable rust ABI? So you either use the C ABI or you use some crate to create a stable rust ABI, because otherwise a shared lib compiled with rust compiler 1.x.y on system a isn't guaranteed to work with the binary compiled on the same system with another compiler... or on another system with the same compiler version.
Right?
ori_b
You'd have a plugin layer that hooks in the right places with a stable ABI on one end, and a native feeling interface on the other.
abendstolz
Very cool!
But I prefer the wasmtime webassembly component model approach these days.
Built a plugin system with that, which has one major upside in my book:
No stringly function invocation.
Instead of run_function("my-function-with-typo") I have a instantiated_plugin.my_function call, where I can be sure that if the plugin has been instantiated, it does have that function.
bobajeff
This sounds like a good approach to overcoming rusts slow compile times and lack of dynamic linking. One thing I'm concerned about with this path is what about hot reloading and fast script running? Doesn't everything in the wasm component model need to be compiled first? I imagine that would remove some of the advantages to using a scripting language like JavaScript or Python.
abendstolz
You're right. Hot reloading isn't done by default.
I manually compile a plugin and in my system I can "refresh" a plugin and even say "activate version 1.1 of the plugin" or "activate version 1.2" of the plugin etc.
But that's something I had to build myself and is not built into wasmtime itself.
zamalek
Alternatively, whenever designing a scripting/plugin host make sure to support plugin-hosting-plugins. That way you could have the Roto plugin host complied to wasm.
abendstolz
Sounds interesting but at the same time a bit complex.
I assume you wouldn't ship the whole plugin runtime for each plugin that wants to host another plugin?!
zamalek
It's not that complex. You basically want to have all plugins support describing themselves (as a list of plugins, which is nearly always 1 item), and "activating" themselves according to one of their descriptors. Bonus points for deactivation for updates (this is often extremely hard to pull off because of how instrusive plugins can be). You could then have a utility header file or whatnot that does the [very minor] plumbing to make the plugin API behave like a single plugin.
Shipping the runtime is another good option, because it means you don't have to worry about runtime versioning.
90s_dev
How is that not also stringly typed?
abendstolz
match Plugin::instantiate_async(&mut store, &component, &linker).await {
Ok(plugin) => {
match plugin
.plugin_guest_oncallback()
.call_ontimedcallback(&mut store, &callback_name)
.await
{
Ok(()) => debug!("Successfully called oncallback for {plugin_path:?}"),
Err(e) => warn!("Failed to call oncallback for {plugin_path:?}: {e}"),
}
}
Err(e) => {
error!("Failed to call oncallback for {plugin_path:?}!: {e}");
}
}
If of course I put a wasm file in the plugin folder that doesn't adhere to that definition, that wasm file isn't considered a plugin.
phickey
Thanks for using wasmtime! I worked on the component bindings generator you’re using and it’s really nice to see it out in the wild.
To elaborate a bit further: wasmtime ships a [bindings generator proc macro](https://docs.wasmtime.dev/api/wasmtime/component/macro.bindg...) that takes a wit and emits all the code wasmtime requires to load a component and use it through those wit interfaces. It doesn’t just check the loaded component for the string names present: it also type checks that all of the types in the component match those given by the wit. So, when you call the export functions above, you can be quite sure all of the bindings for their arguments, and any functions and types they import, all match up to Rust types. And your component can be implemented in any language!
90s_dev
Ah, fair enough. So it is still stringly typed, it's just verified at compile time. Which I guess is true about all compiled functions ever.
breadchris
I like yaegi [1] for go because it is an interpreter for the go language (almost fully supported, generics need some love). The most important part for me is being able to keep all my language tooling when switching between interpreted/compiled code. Also, there is little needed distinction between what is going to be interpreted and compiled. Once you start including libraries it gets dicey and the need for including the libraries in the compiled part is necessary. There is also a blog post that comes along with it describing how it was built! [2]
[1] https://github.com/traefik/yaegi [2] https://marc.vertes.org/yaegi-internals/
90s_dev
> Finally, we want a language that is easy to pick up; it should feel like a statically typed version of scripting languages you're used to.
It looks like Rust. All Rust scripting languages do. Is this true for all other languages? Is this just a property of embeddable scripting languages, they will always resemble the language they're implemented in and meant to be embedded in?
Philpax
People who become proficient in Rust generally enjoy the syntax, so they want to carry it across. (As someone proficient in Rust who has pondered their ideal scripting language, I would have done the same.)
To your more general question: it depends. AngelScript [0] looks very much like C++, while others, like Lua, don't. It's really up to the designer's discretion.
[0]: https://www.angelcode.com/angelscript/, but https://angelscript.hazelight.se/ has better examples of what it actually looks like in use
axegon_
> who has pondered their ideal scripting language
Et tu, brute? :D
nicoburns
I think it's just that a lot of people like Rust syntax, and there is a lot of demand for a Rust-like scripting language (Rust syntax is also very close to JavaScript/TypeScript syntax which many, many people are familiar with)
epage
> It looks like Rust. All Rust scripting languages do.
Not koto (https://koto.dev/) which is one of the reasons I appreciate it. I want an embeddable language targeted at my users, rather than myself which I feel Rust-like ones do. I also want an embeddable language not tied to my implementation language so if I change one, I don't have to change both. Koto only supports Rust atm but I don't see why it couldn't be supported elsewhere.
andsoitis
> Is this just a property of embeddable scripting languages, they will always resemble the language they're implemented in and meant to be embedded in?
No. Think about Lua or Tcl (both implemented in C) or others like Embeddable Common Lisp.
90s_dev
True, but those were designed in the 80s or 90s. I guess I'm thinking of embedded scripting languages designed since ~2015.
pansa2
IIRC Lua deliberately doesn’t resemble C - so if you’re going back-and-forth, editing both the host application code and a script, you can immediately tell which one you’re looking at.
Makes sense to me - which means scripting languages for curly-brace languages should probably use either Lua-style begin-end or Python-style significant indentation.
90s_dev
I don't think that's the reason behind Lua's syntax. I think it was designed specifically to be extraordinarily unambiguous, clean, and simple, which it is.
ordu
It is one of my issues with lua. I'm starting to forget semicolons in C/C++/Rust and write : instead of ::.
I think, that I'd better deal with a language recognition, I could configure emacs to use different highlighting for different languages. Or I could change background color for buffers based on a language.
lynndotpy
Woah, this looks awesome.
One of my favorite things about writing Rust is that it's expression-oriented (i.e. almost everything is an expression), something you almost never see in non-functional languages.
I was wondering if Roto is also expression-oriented?
terts
Hi! Author here. It is indeed expression-oriented, mostly following the same rules as Rust. If-else is an expression, for example.
90s_dev
> Note that nothing in the script is run automatically when the script is loaded, as happens in many other scripting language. The host application decides which functions and filtermaps it extracts from the script and when to run them.
So it's closer to something like C or C++, where it just defined stuff and you can choose what to use? I guess that's fine when there's no initialization for the script to do. Maybe in your domain that's never the case. But many languages end up adding static initialization as a first-class feature eventually.
hannofcart
> Roto has no facilities to create loops. The reason for this is that scripts need to run only for a short time and should not slow down the application. [1]
Wait, what?! Isn't that choice a bit extreme?
There have been plenty of times in scripting where I've needed loops! Am I missing something here?
[1] https://rotonda.docs.nlnetlabs.nl/en/stable/roto/00_introduc...
terts
Hi! That bit of the docs is a bit outdated. We're probably gonna make it optional. The reason for that choice was that the filters for Rotonda need to be very quick and don't really require loops as long as you can do `contains` checks on some lists for example.
So it should probably say "Roto _in Rotonda_ does not have loops". Roto the language as a separate project can then have loops.
lewisjoe
ELI5: Why not use typescript and an embedded v8 engine or deno to run them? What kind of advantages will I miss if I go for typescript as an embedded language for my application?
Also by using v8, I open up new possiblities via wasm (write in any lang and run it here?)
Will be helpful if somebody enlighten me.
duped
Be warned that V8 is a behemoth and adds 100+MB to your binary size, is quite slow to link, and is not practical to compile from source for most projects. If you want a lighter weight JS runtime there's quickjs, if you want WASM there's wasmtime.
Personally I don't think it's a good choice for what it seems Roto is used for (mission critical code that never crashes) since TS doesn't have a sound typesystem and it's still very easy to fuck up without additional tooling around it.
giancarlostoro
There's also Duktape for just JS minus the WASM (at least I don't think they've implemented WASM yet).
ijustlovemath
I think QuickJS wins over Duktape for ES5 compliance, though it's been a few years since I was evaluating embedded JS. They're both extremely easy to integrate into an application, in contrast to V8
giancarlostoro
You are comparing a general purpose scripting language (TypeScript) to a DSL (Domain Specific Language) essentially. They built theirs with a specific purpose.
skybrian
Roto is a very limited scripting language with no loops. You might compare it with the eBPF language used to load filters into the Linux kernel.
airstrike
Why would I want to bundle an entire JS runtime? And why do you think you need that for WASM?
And personally I will go out of my way to not use TS/JS if I can
ijustlovemath
sounds like it's not fast enough for their use case. plus, have you ever tried to integrate v8 into a project? Deno is fine for building binaries, but to date doesn't really have good support out of the box for bundling a script into a library, which this application seems like it would need.
stirfish
Roto means "broken" in Spanish.
diggan
Just add a "Huevos" prefix and a "S" at the end of the name, and suddenly you're thinking of delicious food instead.
darccio
For those who don't speak Spanish, "huevos rotos" are broken eggs (or scrambled eggs).
Edit: I had to double check, because it seems that both translations are valid.
diggan
Ah, well, it's a dish, common name for it is "Huevos estrellados" in most places I think though. https://www.google.com/search?q=huevos+rotos&tbs=imgo:1&udm=...
Scrambled eggs would be "huevos revueltos"
Edit: in Spain at least. YMMV for other Spanish-speaking countries.
juanramos
Either that or an unfortunate accident
> Roto fills this niche for us. In short, it's a statically typed, JIT compiled, hot-reloadable, embedded scripting language. To get good performance, Roto scripts are compiled to machine code at runtime with the cranelift compiler backend.
Statically typed, hot reloadable scripting? Sign me up.