Dr. David J. Pearce

Whiley gets Rusty!

I’ve been learning Rust for a while now but, at the same time, trying to continue developing Whiley. Since Whiley was written entirely in Java, these activities were mutually exclusive and it was frustrating trying to balance things! One day I’d had enough … so I decided to rewrite part of the Whiley compiler in Rust. This is not as dramatic as it sounds, and I’m not planning to rewrite the whole compiler in Rust (yet). That would be a lot of work. However, bringing these things together means I can learn Rust and work on Whiley at the same time. Its actually working out pretty well!


An unexpected benefit of working with Rust is that it makes distributing Whiley quite easy. Installing Whiley through Cargo is really simple:

cargo install whiley

Assuming you have Java installed, you can just run wy --help and see the options for building Whiley programs. For example, we can initialise and then build a new project like so:

> wy init
> wy build
> wy run
Hello World!

This is pretty neat!


An interesting question is how the compiler blends Rust with Java. The compiler has always been structured as a build tool with various plugins implementing core functionality. For example, the WhileyCompiler is actually a plugin responsible for compiling Whiley into its intermediate language, whilst Whiley2JavaScript is another plugin responsible for translating the intermediate language into JavaScript, etc. In principle, we can add plugins to provide more functionality (e.g. compiling to Wasm or EVM bytecode). This is how it looks:

Architectural diagram of Whiley compiler.

What we see in the diagram is that the WhileyBuildTool orchestrates the plugins, and it is this component that I have rewritten in Rust. Roughly speaking, the build tool downloads the jar files representing the plugins from Maven central as needed and fires up a JVM instance to run them. Its pretty simple, and it works surprisingly well!

Another interesting opportunity here is that I can incrementally rewrite the compiler in Rust by rewriting the plugins one by one. Whilst none of the plugins is written in Rust yet, I plan to implement a simple one in the near future as a proof-of-concept.