A few weeks ago I gave an LLM (Gemini 2.5 something in Cursor) a bunch of examples of a new language, and asked it to write a recursive descent parser in Ruby. The language was nothing crazy, intentionally reminiscent of C/JS style, but certainly the exact definition was new. I didn’t want to use a parser generator because (a) I’d have to learn a new one for Ruby, and (b) I’ve always found it easier to generate useful error messages with a handwritten recursive descent parser.

IIRC, it went like this: I had it first write out the BNF based on the examples, and tweaked that a bit to match my intention. Then I had it write the lexer, and a bunch of tests for the lexer. I had it rewrite the lexer to use one big regex with named captures per token. Then I told it to write the parser. I told it to try again using a consistent style in the parser functions (when to do lookahead and how to do backtracking) and it rewrote it. I told it to write a bunch of parser tests, which I tweaked and refactored for readability (with LLM doing the grunt work). During this process it fixed most of its own bugs based on looking at failed tests.

Throughout this process I had to monitor every step and fix the occasional stupidity and wrong turn, but it felt like using a power tool, you just have to keep it aimed the right way so it does what you want.

The end result worked just fine, the code is quite readable and maintainable, and I’ve continued with that codebase since. That was a day of work that would have taken me more like a week without the LLM. And there is no parser generator I’m aware of that starts with examples rather than a grammar.