Rune

R ust UN der E macs

This project is an experimental Emacs lisp interpreter written in rust. The project is still at a very early phase and explores using rust as an host language.

Status

The goal of this project is to eventually bootstrap bytecomp.el, which would enable this project to use the same byte compiler as Emacs. This would let me fuzz my vm against emacs byte code to help find issues. The project currently features a elisp interpreter to enable bootstrap. The files bootstrapped so far are:

• byte-run.el
• backquote.el
• subr.el
• pcase.el
• macroexp.el

Development on this project is driven by trying to add new elisp files and seeing where things break. Most of the time, it is just missing builtin functions, but occasionally new functionality needs to be added.

Running

The easiest way to run the interpreter is with cargo run. Running with the load argument (cargo run -- --load) will load the bootstrapped elisp and then exit. Running with the repl argument (cargo run -- --repl) will open an elisp repl. Running with no arguments is equivalent to cargo run -- --load.

Exploring this repo

This project contains one library of derived macros in fn_macros/. This defines the defun proc macro for defining builtin functions. The rest of the code is contained in src/. The modules are described below.

objects

The basic objects used in the interpreter. These are modeled after Emacs objects using tagged pointers with inline fixnums. Conversion between different primitives and object types are also found here.

reader

The emacs lisp reader that translates a string to a cons cell. Due to the simple nature of lisp syntax, the reader is hand rolled and does not rely on any parsing libraries.

symbol

The global obarray. Symbols are always interned and shared between threads. Currently function bindings are global and immutable and value bindings are thread-local and mutable. When the ability is added to share data between threads, this will enable new threads to safely run functions without the need to copy them. Time will tell if this experiment will be successful.

arena

Contains the arena allocator and garbage collector. All code for rooting and managing objects live here as well.

interpreter

The basic elisp interpreter. This is used only to bootstrap the elisp byte-compiler.

fns, data, alloc

These modules contain definitions of builtin in functions. Some of these are just stubbed out until the functionality is actually needed.

Contributing

This project is moved forward by trying to load new elisp files and seeing what breaks. The best way to do that is with cargo run, which will load the currently bootstrapped files. The bootstrapped files are located in main.rs as part of the load function.

Usually what is needed is to implement more primitive functions. This is done with the defun macro. For example if we wanted to implement the substring function, we would first look at the lisp signature.

(substring STRING &optional FROM TO)

Then we would translate the types to their rust equivalent. If the correct type is not known we can use Object. In this example we would write our Rust signature as follows:

#[defun]
fn substring(string: &str, from: Option<i64>, to: Option<i64>) -> String {...}

The defun macro on the function will do the type conversion. After that we just add the function to the runtime with the defsubr macro.

defsubr!(substring);

If you run with cargo run -- --load --repo that will load the current bootstrapped files and then open the REPL. From there you can run (load "/path/to/elisp/file.el") to try loading a new file. The files the need to be bootstrapped next are

• lisp/emacs-lisp/cl-lib.el
• lisp/emacs-lisp/cconv.el
• lisp/progmodes/compile.el
• lisp/emacs-lisp/bytecomp.el

  These files are not yet included in this repo, but are part of Emacs. Once the file is bootstrapped it can be added to the lisp directory.

Blog posts

tagged pointers in Rust

My initial approach to creating tagged pointers in rust. It serves as in intro to this project.

implementing a safe garbage collector

An overview of the garbage collector used in this project.

Further exploration

Remacs

The original rust and Emacs project. Remacs took the approach of enabling interop between Emacs C core and rust, enabling them to replace parts of Emacs piecemeal. The project is currently unmaintained, but is a big inspiration for rune.

emacs-ng

The spiritual successor to remacs. This project integrates the Deno runtime into emacs, allowing you to write extensions in elisp or javascript. Which sounds cool if you happen to be a web developer. It really shows the power of integrating Emacs with a more mature ecosystem (which is part of the promise of rust).

crafting interpreters

This was a big inspiration for this project, and it probably one of the best introductions to programming language implementations.