You're right that the "long" type in Java is signed. The JVM uses two's complement representation, so for many operations, unsigned arithmetic is bit-equivalent to signed arithmetic. For the remaining ones, like less-than, divide, shift, etc, Virgil generates more complicated code to do unsigned arithmetic or comparison manually, e.g. by first checking if an input is negative.

> One more question I have is about FFI - I didn't find a mention of it after a quick skim; can I call functions from some thirdparty JARs or JS/WebAPI?

For the Wasm target, Virgil allows you to write an imported component, so the module that gets generated has the imports with the signatures that you want. You can then load the module in JS and supply Web bindings and such. That latter process is quite clunky, but in theory gives you access to any API that can be expressed in terms of wasm (before externref).