Show HN: Lwt-to-Eio – Automated Migration from Lwt to OCaml 5 Direct Style

Hi everyone,

Like many of you, I’ve been looking into migrating my libraries from Lwt to Eio to take advantage of OCaml 5’s direct-style concurrency. While the manual migration guides are excellent, I found myself making the same mechanical changes over and over—flattening binds, unwrapping promises, and rewriting maps.

So, I built lwt-to-eio, a tool that automates the boring parts of this transition.

Repo: https://github.com/oug-t/lwt-to-eio

What it does (The Magic) It parses your OCaml source code and performs AST-level rewrites to transform monadic Lwt patterns into direct-style Eio code.

Input (Lwt):

let fetch_data id = Lwt.bind (Db.get_user id) (fun user -> Db.get_posts user.id >>= (fun posts -> Lwt_list.map_p process posts ) ) Output (Eio)

let fetch_data id = let user = Lwt_eio.Promise.await_lwt (Db.get_user id) in let posts = Lwt_eio.Promise.await_lwt (Db.get_posts user.id) in Eio.Fiber.List.map process posts Current Status & Help Wanted This is currently an MVP. It handles the most common patterns I encountered (binds, map_p, sleep, return), but there is plenty of surface area left to cover.

I’m looking for contributors to help add rules for other common patterns. I’ve tagged a few issues as “Good First Issues” if anyone wants to dip their toes into ppxlib and AST rewriting:

Lwt.catch → try/with: Transforming monadic error handling to direct style exceptions.

Lwt_io support: Mapping legacy IO functions to Eio buffers.

Feedback, PRs, and “It broke on my file!” reports are all very welcome.