I used AI agents to build a compiler

I recently bought Claude Pro for a month just to see what the fuss was about. A week ago I decided to spend my leftover credits on building a small language compiler, and surprisingly it worked well enough.

Building a language

I started with a few assumptions:

  • I wanted record types with row polymorphism
  • I wanted full inference, like Haskell or OCaml

Since then I’ve added a few more things:

  • A trait system
  • HKT

The vast majority of the type system code was implemented by models like Sonnet 4.6 and Opus 4.6, and the file that contains the type system is now around 3200 lines of Rust.

There’s no way I would have been able to build something like this in a single week without AI, it’s just too much work. With AI I was able to get a PoC language up quickly and iterate on it easily, even without fully understanding what’s going on in the code.

One could ask: if most of the code was generated, does that mean you don’t actually know anything about it? Did you learn anything at all?

I feel like I am learning something, even though AI is doing the heavy lifting. I know the general layout of the project, I know what each part does roughly. I know what the language can and can’t do, its limitations and so on. By designing the language and having AI implement it, I was able to quickly explore the trade-offs of various designs, see what works and what doesn’t. I was able to architect the tooling around the language, the various pieces that connect together to give you a nice experience writing code.

I now know, for example:

  • How to create an LSP server
  • How to create and use a tree-sitter grammar
  • How to use LuaJIT as an embedded VM
  • …

I feel like even when using AI you can still learn a lot just by trying things you wouldn’t normally attempt on your own. If I had to build this language completely by myself, I don’t think I would ever reach a point where I’d have to genuinely wonder about how row types and traits should interact while staying sound, but here we are.

I think it’s okay to be wary of AI, but I try to stay neutral. Like any technology, it can be used for good or bad. And using AI to learn things you otherwise never would have touched? That feels pretty squarely in the good column.

Where to see it

You can check the online version that just compile code to Lua, or you can compile the full version from the GitHub repo.

By the way, don’t expect all things to work, this is just a language designed and created in a single week! But I made sure to make all the basics work, including LSP, Zed extension and VS Code extension.