From the person that brought you calling Rust from Swift comes the thrilling^[1], action^[2]-packed sequel: calling Swift from Rust! For a recent project, I found myself needing to call into Swift from a Rust project (on both macOS and Linux) and so am documenting here in case you, too, are in this unenviable situation.

Hi. It’s been a while. Though the pace of blog posts fell off a cliff last year^[1], I’ve continued working on my toy programming language on and off.

I recently started building a new iOS app and found myself with a need to parse HTML in order to extract some information. My goto tool for this in the past has been SwiftSoup. In this app, I have to deal with larger documents than I’d used it for previously, and unfortunately, its performance leavse something to be desired. Much of the issue comes from the fact that I only want to extract the first paragraph of a document, but SwiftSoup always needs to parse the entire thing—for large documents, potentially a lot of unnecessary work^[1]. And, as far as I could find, there are no streaming HTML parsers written in Swift. One I did find, however, was CloudFlare’s lol-html. It’s specifically designed for speed and low latency, exactly what I want. But it’s written in Rust.

After adding variables, I added boolean values and comparison operators, because why not. With that in place, I figured it would be a good time to add if statements. Parsing them is straightforward—you just look for the if keyword, followed by a bunch of stuff—so I won’t go into the details. But actually evaluating them was a bit more complicated.

Now that the parser can handle multiple statements and the usage of variables, let’s add the ability to actually declare variables.