FAWK: LLMs can write a language interpreter

I've been working on something similar, a typed shell scripting language called shady (hehe). haven't shared it because like 99% of the code was written by claude and I'm definitely not a programming language expert. it's a toy really.

but I learned a ton building this thing. it has an LSP server now with autocompletion and go to definition, a type checker, a very much broken auto formatter (this was surprisingly harder to get done than the LSP), the whole deal. all the stuff previously would take months or a whole team to build. there's tons of bugs and it's not something I'd use for anything, nu shell is obviously way better.

the language itself is pretty straightforward. you write functions that manipulate processes and strings, and any public function automatically becomes a CLI command. so like if you write "public deploy $env: str $version: str = ..." you get a ./script.shady deploy command with proper --help and everything. it does so by converting the function signatures into clap commands.

while building it I had lots of process pipelines deadlocking, type errors pointing at the wrong spans, that kind of thing. it seems like LLMs really struggle understanding race conditions and the concept of time, but they seem to be getting better. fixed a 3-process pipeline hanging bug last week that required actually understanding how the pipe handles worked. but as others pointed out, I have also been impressed at how frequently sonnet 4.5 writes working code if given a bit of guidance.

one thing that blew my mind: I started with pest for parsing but when I got to the LSP I realized incremental parsing would be essential. because I was diligent about test coverage, sonnet 4.5 perfectly converted the entire parser to tree-sitter for me. all tests passed. that was wild. earlier versions of the model like 3.5 or 3.7 struggled with Rust quite a bit from my experience.

claude wrote most of the code but I made the design decisions and had to understand enough to fix bugs and add features. learned about tree-sitter, LSP protocol, stuff I wouldn't have touched otherwise.

still feels kinda lame to say "I built this with AI" but also... I did build it? and it works? not sure where to draw the line between "AI did it" and "AI helped me do it"

anyway just wanted to chime in from someone else doing this kind of experiment :)

"because I was diligent about test coverage, sonnet 4.5 perfectly converted the entire parser to tree-sitter for me. all tests passed."

I often suspect that people who complain about getting poor results from agents haven't yet started treating automated tests as a hard requirement for working with them.

If you don't have substantial test coverage your coding agents are effectively flying blind. If you DO have good test coverage prompts like "port this parser to tree-sitter" become surprisingly effective.