←back to thread

How we built an interpreter for Swift

(www.bitrig.app)
82 points jacobx | 2 comments | 04 Sep 25 16:39 UTC | HN request time: 0.402s | source
Show context
mtlynch ◴[04 Sep 25 19:17 UTC] No.45131147[source]▶
>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.

replies(2): >>45131434 #>>45131746 #
1. jacobx ◴[04 Sep 25 19:43 UTC] No.45131434[source]▶
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.

replies(1): >>45131556 #
2. mtlynch ◴[04 Sep 25 19:55 UTC] No.45131556[source]▶
Thanks for the reply!

I think that's a more accessible explanation. Consider folding that into the article's introduction.

Is the idea that your interpreter is signed, and then you translate the user's arbitrary unsigned Swift code into calls to other already-signed code that ships with iOS?