Bonsai: Janestreet's UI Library

A lot of the conversation pushed back on the idea that this somehow makes shared backend and frontend languages newly possible. OCaml-to-JavaScript and same-language full-stack setups have existed for years through tools like js_of_ocaml, Ocsigen, Eliom, Melange, Scala.js, Kotlin/JS, ClojureScript, WebSharper, LiveView, and Fable. What Bonsai changes is the specific programming model and the fact that Jane Street built it deeply inside their own OCaml ecosystem. That came with the usual tradeoff. You get strong types, fast compilation according to users familiar with OCaml, and a compiler that can keep both humans and LLMs on the rails. You also inherit a big dependency footprint through Jane Street’s Core stack and a much thinner bridge into the normal JavaScript world. Several people said that is the real adoption barrier, not whether the library is technically impressive. The screenshots triggered the other big theme. Many people called the UI ugly or dated. That criticism mostly bounced off because the examples look like software for traders, not consumer apps. Dense layouts, low whitespace, keyboard-heavy workflows, and seeing many things at once were treated as deliberate choices for expert users staring at the tool all day. The sharper point was that information density can be good without becoming cramped, and that Bonsai should not be confused with a styling system anyway. Underneath the aesthetic argument, the practical conclusion was clear. This is best read as infrastructure for high-interaction internal tools where performance, state management, and operator throughput matter more than matching the modern web app look. People also noted some rough edges in the open-source package itself. Readme links were broken, examples were hard to find, and docs seemed thin enough that reading the source was still part of the onboarding path. That reinforced the overall read: Bonsai looks credible because Jane Street uses it for serious work, but outside that environment it is still a niche bet that makes the most sense for teams already comfortable living in OCaml.