Compiling Scheme to WebAssembly

Summary

One of my oldest open-source projects - Bob - has celebrated 15 a couple of months ago. Bob is a suite of implementations of the Scheme programming language in Python, including an interpreter, a compiler and a VM. Back then I was doing some hacking on CPython internals and was very curious about how CPython-like bytecode VMs work; Bob was an experiment to find out, by implementing one from scratch for R5RS Scheme. Several months later I added a C++ VM to Bob, as an exercise to learn how such VMs are implemented in a low-level language without all the runtime support Python provides; most importantly, without the built-in GC. The C++ VM in Bob implements its own mark-and-sweep GC. After many quiet years (with just a sprinkling of cosmetic changes, porting to GitHub, updates to Python 3, etc), I felt the itch to work on Bob again just before the holidays. Specifically, I decided to add another compiler to the suite - this one from Scheme directly to WebAssembly. The goals of this effort were two-fold: Experiment with lowering a real, high-level language like Scheme to WebAssembly. Experiments like the recent Let's Build a Compiler compile toy languages that are at the C level (no runtime). Scheme has built-in data structures, lexical closures, garbage collection, etc. It's much more challenging. Get some hands-on experience with the WASM GC extension . I have several samples of using WASM GC in the wasm-wat-samples repository, but I really wanted to try it for something "real". Well, it's done now; here's an updated schematic of the Bob project: The new part is the rightmost vertical path. A WasmCompiler class lowers parsed Scheme expressions all the way down to WebAssembly text, which can then be compiled to a binary and executed using standard WASM tools . Highlights The most interesting aspect of this project was working with WASM GC to represent Scheme objects. As long as we properly box/wrap all values in refs, the underlying WASM execution environment will take...