Setting up Emacs for OCaml Development: Neocaml Edition

A few years ago I wrote about setting up Emacs for OCaml development. Back then the recommended stack was tuareg-mode + merlin-mode, with Merlin providing the bulk of the IDE experience. A lot has changed since then – the OCaml tooling has evolved considerably, and I’ve been working on some new tools myself. Time for an update.

The New Stack

Here’s what I recommend today:

• neocaml instead of tuareg-mode
• ocaml-eglot instead of merlin-mode
• Eglot (built into Emacs 29+) as the LSP client

The key shift is from Merlin’s custom protocol to LSP. ocaml-lsp-server has matured significantly since my original article – it’s no longer a thin wrapper around Merlin. It now offers project-wide renaming, semantic highlighting, Dune RPC integration, and OCaml-specific extensions like pattern match generation and typed holes. ocaml-eglot is a lightweight Emacs package by Tarides that bridges Eglot with these OCaml-specific LSP extensions, giving you the full Merlin feature set through a standardized protocol.

And neocaml is my own TreeSitter-powered OCaml major mode – modern, lean, and built for the LSP era. You can read more about it in the 0.1 release announcement.

The Essentials

First, install the server-side tools:

$ opam install ocaml-lsp-server

You no longer need to install merlin separately – ocaml-lsp-server vendors it internally.

Then set up Emacs:

;; Modern TreeSitter-powered OCaml major mode
(use-package neocaml
  :ensure t)

;; Major mode for editing Dune project files
(use-package dune
  :ensure t)

;; OCaml-specific LSP extensions via Eglot
(use-package ocaml-eglot
  :ensure t
  :hook (neocaml-mode . ocaml-eglot-setup))

That’s it. Eglot ships with Emacs 29+, so there’s nothing extra to install for the LSP client itself. When you open an OCaml file, Eglot will automatically start ocaml-lsp-server and you’ll have completion, type information, code navigation, diagnostics, and all the other goodies you’d expect.

Compare this to the old setup – no more merlin-mode, merlin-eldoc, flycheck-ocaml, or manual Company configuration. LSP handles all of it through a single, uniform interface.

The Toplevel

neocaml includes built-in REPL integration via neocaml-repl-minor-mode. The basics work well:

• C-c C-z – start or switch to the OCaml toplevel
• C-c C-c – send the current definition
• C-c C-r – send the selected region
• C-c C-b – send the entire buffer

If you want utop specifically, you’re still better off using utop.el alongside neocaml. Its main advantage is that you get code completion inside the utop REPL within Emacs – something neocaml’s built-in REPL integration doesn’t provide:

(use-package utop
  :ensure t
  :hook (neocaml-mode . utop-minor-mode))

This will shadow neocaml’s REPL keybindings with utop’s, which is the intended behavior.

That said, as I’ve grown more comfortable with OCaml I find myself using the toplevel less and less. These days I rely more on a test-driven workflow – write a test, run it, iterate. In particular I’m partial to the workflow described in this OCaml Discuss thread – running dune runtest continuously and writing expect tests for quick feedback. It’s a more structured approach that scales better than REPL-driven development, especially as your projects grow.

Give neocaml a Try

If you’re an OCaml programmer using Emacs, I’d love for you to take neocaml for a spin. It’s available on MELPA, so getting started is just an M-x package-install away. The project is still young and I’m actively working on it – your feedback, bug reports, and pull requests are invaluable. Let me know what works, what doesn’t, and what you’d like to see next.

That’s all I have for you today. Keep hacking!