Translating non-trivial codebases with Claude

Summary

I was wrong (or was I?)In my last post, I stated:I don’t think it’s [me writing about LLMs] likely to happen anytime soon: I prefer to write about things that I’m excited about.I was wrong. And right at the same time. Here comes another post where LLMs play a prominent role.I've asked Claude Code (Opus 4.6) to “rewrite this non-trivial codebase from C++ to Java”. And it worked quite splendidly. Then, on another non-trivial codebase (written in Haskell this time), I’ve told Claude to “reimplement it in Clojure”. And it worked even better.Yes, this triggered a wow effect. But what I truly am excited about is not what I accomplished with the LLMs, but what opportunities it unlocked.If you like, you can skip the backstory and jump right through to the experience report, or to the showcase.BackstoryI’ve always had an interest in natural language processing. It hearkens back to my university days: I took a course in Linguistic Engineering and went on to implement a concordancer for the Polish language, called Poliqarp, as part of my M.S. thesis. Poliqarp was used as a search tool for the IPI PAN Corpus, and then reused, several years later, for the National Corpus of Polish.These days, I look at Poliqarp with a mixture of embarrassment and pride. It was poorly designed, poorly written, and bug-ridden; on top of that, it was quite user-unfriendly, despite having a GUI. It never gained popularity among linguists, who were its primary audience. (“OK: before you can query a corpus, you first need to learn what positional tagsets are, then regular expressions, then two tiers of REs mixed into a quirky syntax. And if you want to create a corpus, boy, do you need a Ph.D. in Unixology.”) But it also had some sophisticated ideas in it. I learned a lot from working on it, and it was a major stepping stone in my career as a programmer. Poliqarp’s GUI, running a query Shortly after graduation, I’ve started having thoughts about how I’d design Poliqarp if I were to write it again. Mea...