#Garnet -- what if Rust was small?

#The Pitch

People keep talking about what a small Rust would look like, and how nice it would be, and whether or not Zig or Hare or whatever fits the bill. So, I think it's time to start advertising, at least in a small way: I'm trying to make basically the language these people want, a language that asks "What would Rust look like if it were small?". I call it Garnet. I'm at the point where I am fairly sure my design is gonna do at least something vaguely useful, but I also think it's time to ask for interested parties to talk or help.

Garnet strives for smallness by having three basic features: functions, types, and structs. Somewhat like Zig, structs double as modules when evaluated at compile time. A lot like SML/OCaml, structs also double as your tool for defining interfaces/traits/typeclasses/some kind of way of reasoning about generics. If you make sure your code can be evaluated at compile time, you can treat types mostly like normal values, and "instantiating a generic" becomes literally just a function that returns a function or struct. Again, this is kinda similar to Zig, but I want to avoid the problem where you don't know whether the code will work before it's actually instantiated. Again, this is quite similar to ML languages, but without an extra layer of awkwardness from having a separate module languages. It seems to work out in theory. Whether it can be made actually convenient... well, I hope so.

I also want to solve various things that irk me about Rust, or at least make different design decisions and see what the result looks like. Compile times should be fast, writing/porting the compiler should be easy, ABI should be well-defined, and behavior of low-level code should be easy to reason about (even if it misses some optimization opportunities). The language is intended for low level stuff more than applications: OS's and systems and embedded programming. This makes some Rust features unnecessary, like async/await. Better ergonomics around borrowing would be nice too, though I'm not sure how to do that yet, I just hate that there's no way to abstract over ownership and so we have Fn and FnMut and FnOnce. However, trading some runtime refcounting/etc for better borrowing ergonomics as suggested in Notes on a Smaller Rust and Swift's work is not on the cards; I think there's a lot of potential for a language that does this sort of thing, but Garnet is not that language.

#Code example

fn fib(x I32) I32 =
    if x < 2 then x
    else fib(x-1) + fib(x - 2)
    end
end

-- {} is an empty tuple, "unit"
-- __println() is for now just a compiler builtin.
fn main() {} =
    __println(fib(40))
end

There’s a bunch of small programs in its test suite here: https://hg.sr.ht/~icefox/garnet/browse/tests/programs And some slightly-more-interesting-but-often-still-hypothetical bits of programs here: https://hg.sr.ht/~icefox/garnet/browse/gt Syntax is mostly fixed but there's plenty of details that can still be a WIP.

#Current state

As of spring 2023 he typechecker seems to more or less work but some backend work needs to happen before some sorts of generic type code can be actually compiled.

Just to make sure people have some realistic expectations.

• [✓] = done
• [⛭] = WIP
• [?] = Active design concern, probably in flux
• [ ] = not started

Realistic language goals:

• [✓] Technically Turing complete
• [✓] Basic structs/tuples (did a proof of concept, now rewriting it)
• [✓] Arrays, sum types, other useful basic types
• [✓] Generics and type inference
• [⛭] Full ML-y modules
• [?] Move semantics, references and borrowing
• [?] Stdlib of some kind
• [ ] Pattern matching
• [?] Function properties (const, pure, noalloc, etc)
• [ ] Lots of little ergonomic things

Giant scary tooling goals necessary for Real Use:

• [⛭] Backend support: C or Rust
• [ ] Self-host
• [ ] Basic optimizing backend
• [ ] Debugger/profiler tooling
• [ ] Build/packaging system
• [ ] Language spec
• [ ] ABI spec
• [ ] Documentation generator
• [ ] Semver checker
• [ ] GOOD backend.
• [ ] Backend support: Webassembly
• [ ] Backend support: Actual CPU's

#Assumptions

Things where you go "it's a modern language, of COURSE it has this". If it doesn't have something like this, it's a hard error.

• Unambiguous, context-free syntax
• Good error messages
• Cross-compile everywhere
• Type inference
• Sum types, no null, all that good jazz

#Runtime/language model goals

Things where you might need to need to make explicit design tradeoffs. It concerns the overlap of design and implementation. These are essentially directions explore rather than hard-and-fast rules, and may change with time.

• Simplicity over runtime performance -- Rust and Go are very different places on this spectrum, but I think OCaml demonstrates you should be able to have a bunch of both. There needs to be more points on this spectrum. There haven't been many innovative new low-level languages designed since the 1980's.
• Fast compiler -- This is a pain point for Rust for various reasons, and one of those things where having it work well is real nice.
• Simplicity of compiler -- I'd rather have a GOOD compiler in 50k lines than a FANTASTIC compiler in 500k lines
• I feel like these two things together should combine to (eventually) make compiler-as-library more of a thing, which seems like an overlooked field of study. It can be useful to aid JIT, metaprogramming, powerful dynamic linking, etc. It seems very silly that this remains Dark Magic outside of anything that isn't Lisp or Erlang. (That said, when you don't want this, you REALLY don't want it.)
• As little undefined behavior as possible -- If the compiler is allowed to assume something can't happen, then the language should prevent it from happening if at all feasible. Let's stop calling it "undefined behavior" and call it an out of context problem, since it's often not undefineable, but rather it's something that the compiler doesn't have the information to reason about.
• I am not CONVINCED that a linker is the best way to handle things. This has implications on things like distributing libraries, defining ABI's, using DLL's, and parallelizing the compiler itself. No solid thoughts here yet, but it is an area worth thinking about. Rust, C, Go and Swift present different points in this area to look at.

#C's advantages that I want to have

• Easy to port to new systems
• Easy to use on embedded systems
• Easy to control code size bloat
• Easy to get a (partial) mental model, which is low-level enough to teach you a lot
• Simple and universal ABI for every platform -- easy for higher level stuff to call it, easy for it to call arbitrary stuff.
• Compiles fast
• Everything interoperates with it via FFI

Another way to think about it is "Garnet wants to be the Lua of system programming languages". Small, flexible, made of a few powerful parts that fit together well, easy to port and implement and toy around with, reasonably fast.

#Pain points in Rust to think about

• You can't be generic over mutability and ownership, so for example you end up with iter(), into_iter(), and iter_mut().
• Related, the pile of AsRef, Deref, Borrow, ToOwned etc. traits.
• Related, the pile of various things that look kinda like references/pointers but aren't, and all the hacks that go into making them work. Example: Box. Seems fine, right? Can't pattern match on it. See the box_pattern RFC.
• Rust's hacky generic-ness over length of sequences/tuples is pretty lame
• The slightly-magical relationship between String and &str, and &[] and [] and [T;N], is a little distressing
• Magical AsRef and Deref behavior is a little distressing
• std vs core vs alloc -- it'd be better if std didn't actually re-export core, because then more programs could be no_std implicitly. alloc is kinda a red-headed stepchild in this hierarchy; Zig's approach of explicit allocator objects everywhere may or may not be superior. Talk to some of the stdlib or embedded people about how they'd want to arrange it if they could; papering over weird platforms like wasm is a known annoyance. Maybe something like core for pure computational things, sys for platform-specific low-level stuff like threading and timekeeping primitives that may appear in a microcontroller or low-level VM without a full OS, then os or something for stuff like filesystems, processes, etc. Need better names though. I do like the idea of splitting out specific capabilities into specific parts that may or may not be present on all platforms though, instead of having a strictly additive model.
• Syntax inconsistencies/nuisances: Fiddly match blocks, <>'s for generics (though the turbofish is wonderful), i32 is both a type and a module, -> and => being different is a PITA, you declare values with = in let statements but : in struct constructors, piddly stuff like that.
• Tail call optimization is not guarenteed -- Drop impl's get in the way, but it should be possible to avoid that, or at least make it so the compiler gives a warning if it can't guarentee TCO.
• Lack of construct-on-heap is occasionally kinda awful, though far more often totally unnoticable.
• Rather mediocre support for data type reflection at either compile or run time, such as RTTI in general. Also bites us in trying to make C-like enums, separate enum discriminants from enums or vice versa (which makes them awkward to compose),
• Rust's closures are awful when you try to use them like you would use closures in other functional languages. Passing them down the stack usually works ok, but passing them up the stack or stuffing them into structures is a giant Box-y PITA.
• On the note of boilerplate-y stuff, see https://github.com/rustwasm/walrus/blob/121340d3113e0102707b2b07cab3e764cea1ed6b/crates/macro/src/lib.rs for an example of a giant, complex, heavy proc macro that is used exactly once to generate a huge amount of entirely uninteresting --but nonetheless necessary-- code. It's good that you can use a macro for it, but it's kinda less good that you need to.
• No function currying is rather a pain sometimes, especially when it's really just syntactic sugar for a trivial closure.
• Rust's trait orphan rules are annoying. Getting around them with modules solves that problem but brings up new ones, so we will just have to try it out and see at scale whether the new ones are worse.
• Heckin gorram -> vs => still bleedin' trips me up after five years

#Glory points in Rust to exploit or even enhance

• Move semantics everywhere
• Derive traits
• methods <-> functions
• True, if conservative, constexpr's
• Iterators just return Option
• Math is checked by default
• Stack unwinding without recovery -- very nice compromise of complexity
• UTF-8 everywhere
• Lack of magical constructors

#Functionality we sacrificed for simplicity

• match blocks on function params, like Erlang -- just syntactic sugar
• Cool arbitrary/rational number types -- can be a lib.
• Though it is tempting, we will NOT do arbitrary-precision integer types such as being able to define an integer via an arbitrary range such as [-1, 572) or arbitrary size such as i23. Maybe later.
• Like Rust, we don't need to target architectures smaller than 32 bits

#Wishlist items

• I want to explore having a more orthogonal relationship between tuples, structs, and function args. Enums as well. See https://todo.sr.ht/~icefox/garnet/7
• Better methodology for boilerplate-y things like visitor patterns / big ol' honkin' pattern matches would be very interesting. Some nice sugar here may be very productive, personally. Also https://todo.sr.ht/~icefox/garnet/7
• It would be surprisingly appealing if there were no methods, traits, or anything like that... just data structures, functions and namespaces/modules. Look at OCaml's modules for inspiration perhaps. See https://todo.sr.ht/~icefox/garnet/8
• Consider some sort of templates or macros -- https://dlang.org/spec/template.html? or just use Handlebars :P. Also see https://todo.sr.ht/~icefox/garnet/8

#Goals

• Being effectively finished someday.
• A compilation model that doesn't necessitate a slow compiler
• Being able to reason about what kind of code the compiler will actually output

#Antigoals

• Async, promises, other fanciness for nonblocking I/O or stack juggling
• Ultimate max performance in all circumstances
• Anything requiring a proof solver as part of the type system
• Anything that is an active research project

#Toolchain

• rustc
• logos lexer
• handrolled parser (recursive descent + Pratt)
• output Rust, just to make things work.
• argh for command line opts
• codespan for error reporting
• lang_tester for integration tests

Things to consider:

• rustyline (for repl)
• ryu for parsing floats
• tree-sitter for parsing/formatting tooling (someday?)

#Backend thoughts

Something I need to consider a little is what I want in terms of a compiler backend, since emitting x86_64 opcodes myself basically sounds like the least fun thing ever.

Goals:

• Not huge
• Operates pretty fast
• Outputs pretty good/fast/small code
• Doesn't require binding to C++ code (pure C may be acceptable)
• Produces x86_64, ideally also Aarch64 and WASM, SPIR-V would be a nice bonus

Non-goals:

• Makes best code evar
• Super cool innovative research project
• Supports every platform evar, or anything less than 32-bits (it'd be cool, but it's not a goal)

Options:

• Write our own -- ideal choice in the long run, worst choice in the short run
• LLVM -- Fails at "not huge", "operates fast" and "doesn't require C++ bindings"
• Cranelift -- Might actually be a good choice, but word on the street (as of early 2020) is it's poorly documented and unstable. Investigate more. As of 2023 might be in an ok place?
• QBE -- Fails at "doesn't require C compiler", but initially looks good for everything else. Played around with it a little and it left a bad taste in my mouth for some reasons, but worth revisiting.
• WASM -- Just output straight-up WASM and use wasmtime to run it. Cool idea in the short term, WASM is easy to output and doesn't need us to optimize it much in theory, and would work well enough to let us bootstrap the compiler if we want to. Much easier to output than raw asm, there's good libraries to output it, and I know how to do it.
• C -- Just output C Code. The traditional solution, complicates build process, but will work.
• Rust -- Rust compiles slow but that's the only downside, complicates build process, but will work. Might be useful if we can proof whatever borrow checking type stuff we implement against Rust's

Output Rust for right now, bootstrap the compiler, then think about it.

Trying out QBE and Cranelift both seem reasonable choices, and writing a not-super-sophisticated backend that outputs many targets seems semi-reasonable. Outputting WASM is probably the simplest low-level thing to get started with, but is a little weird since it is kinda an IR itself, so to turn an SSA IR into wasm you need a step such as LLVM's "relooper". Outputting C is similarly fine in the short term and has Problems in the longer term.

#License

LGPL 3.0 only