Hi HN! We're back with another post describing more detail about how we built Bitrig's interpreter for Swift in Swift, this time going into how we're converting the code into bytecode.
Readit Newsjacobx commented on Bitrig's Swift Interpreter: From Code to Bytecode bitrig.app/blog/interpret... · Posted by u/jacobx
jacobx commented on How we built an interpreter for Swift bitrig.app/blog/swift-int... · Posted by u/jacobx
This is really cool, will definitely try it out. Does it allow you to move this to your computer later and actually publish the apps? What’s the end goal here?
Yep! You can always export your code to take it to your computer.
We also have functionality in bitrig to build your app and send it to App Store Connect so you can deploy it on TestFlight.
jacobx commented on How we built an interpreter for Swift bitrig.app/blog/swift-int... · Posted by u/jacobx
jacobx commented on How we built an interpreter for Swift bitrig.app/blog/swift-int... · Posted by u/jacobx
It looks like it is in the App Store, so I guess my question is already answered - but I wonder if they had trouble or concerns getting it accepted.
Section 2.5.2 in "App Review Guidelines" (https://developer.apple.com/app-store/review/guidelines/):
2.5.2 Apps should be self-contained in their bundles, and may not read or write data outside the designated container area, nor may they download, install, or execute code which introduces or changes features or functionality of the app, including other apps. Educational apps designed to teach, develop, or allow students to test executable code may, in limited circumstances, download code provided that such code is not used for other purposes. Such apps must make the source code provided by the app completely viewable and editable by the user.Yeah there was definitely some back and forth about it before we were eventually approved.
In a sense, this isn't very different from what React Native does (run interpreted code that calls out into native code), just with Swift instead of JavaScript. There used to be JavaScript-specific requirements in the guidelines, but that has been loosened since Swift Playgrounds was released. Now there are Python IDEs, Jupyter Notebooks, and other apps running arbitrary code in the App Store.
jacobx commented on How we built an interpreter for Swift bitrig.app/blog/swift-int... · Posted by u/jacobx
Very very neat.
What's the performance like?
There's definitely a cost: everything is type-erased and there's a lot more indirection than there would be if the code was compiled. But you usually don't hit performance issues because most app code (especially in the UI) is just a thin layer calling into the OS frameworks. The framework code does the heavy lifting and is all compiled.
The places you can hit performance issues are things like when the app itself has a tight loop that's doing a lot of work.
jacobx commented on How we built an interpreter for Swift bitrig.app/blog/swift-int... · Posted by u/jacobx
>Bitrig dynamically generates and runs Swift apps on your phone. Normally this would require compiling and signing with Xcode, and you can’t do that on an iPhone.
>To make it possible to instantly run your app, we built a Swift interpreter. But it’s an unusual interpreter, since it interprets from Swift… to Swift.
I can't understand what they're talking about, and I don't know if it's because I know too little about programming languages or if it's because they're using unusual semantics.
"a compiled language" makes no sense to me because the language itself doesn't determine whether it's compiled or interpreted. It's just a function of what compilers/interpreters exist. I could write an interpreter for C or a compiler for Python, but the languages themselves aren't compiled or interpreted.
I also don't understand what it means to "interpret" Swift to Swift. Do they mean they compile Swift to Swift (or the more modern "transpile" which means the same thing)? But it sounds like they're doing something dynamically at runtime, so it sounds more like decompiling machine code back to Swift, but the rest of the post doesn't match that interpretation.
I refer to Swift as a "compiled language" because no officially provided interpreter exists for it.
Bitrig runs Swift apps which are dynamically generated by an LLM on the iPhone, despite the iPhone strict security provisions (e.g. inability to write executable pages of memory). The way we do this is by parsing the generated Swift code and mapping that to the compiled calls to the libraries that come in the OS. It's pretty weird, but we think it's worth it to get the ability to immediately render the generated code on the phone.
jacobx commented on How we built an interpreter for Swift bitrig.app/blog/swift-int... · Posted by u/jacobx
Do you think you can make it run in Jupyter as a Kernel?
That's not really been something we'd been considering, but yeah I think we probably could. We're primarily using the interpreter to render SwiftUI views, but it supports running arbitrary Swift expressions or statements.
jacobx commented on How we built an interpreter for Swift bitrig.app/blog/swift-int... · Posted by u/jacobx
Your website won't let me scroll to see the content on Firefox Android
I just pushed a change that should fix scrolling on Android browsers.
jacobx commented on How we built an interpreter for Swift bitrig.app/blog/swift-int... · Posted by u/jacobx
Could you use this to support hot module replacement? Replacing a SwiftUI view in a live app without restarting the process?
Yeah you could! The only caveat is that either the whole app, or at least the part of the app showing the view you want to replace, would have to be running via the interpreter.
We're very interested in using the interpreter to improve the Swift developer experience in more ways like that.