Bonsai: Janestreet's UI Library

Jane Street just pulled back the curtain on Bonsai, the in-house toolkit it says powers nearly all of its internal web apps, from office directories to tools used around trading systems. In plain English: it’s a way to build fast, interactive websites using OCaml, a programming language beloved by a very particular kind of software engineer. Jane Street’s pitch is that Bonsai keeps apps speedy, reusable, and easier to test, while also letting teams share the same language between the back end and the front end. Sounds neat! But the real show started when the community wandered into the comments.

The strongest reaction was basically: “Cool, but who is this really for?” One commenter immediately noticed it seems to be web-only, which for some readers made the reveal feel narrower than the hype suggested. Another zeroed in on a technical workaround with a deadpan groan: “Oh it needs a userland trampoline!” That line became the thread’s instant comedy grenade, the kind of hyper-specific nerd joke that lands like a pie to the face if you know, and total chaos if you don’t.

Then came the spiciest take of all: one commenter mockingly rewrote the entire “Why Bonsai?” section as Jane Street basically saying it builds everything in OCaml because it absolutely cannot help itself. That roast carried the mood of the thread: half admiration, half eye-roll, with a side of “this might’ve been exciting in 2014.” Still, not everyone was snarky—some were simply jealous, with one sighing, “I wish there were more OCaml shops out there.” So yes, Bonsai got announced, but the comments turned it into a referendum on niche languages, company culture, and whether this is genius craftsmanship or a very elegant obsession.