neocaml is a new Emacs package for programming in OCaml. Built on Tree-sitter, it provides major modes for editing OCaml (.ml) and OCaml Interface (.mli) files with font-locking, indentation, navigation, and toplevel (REPL) integration.

Beyond OCaml source code, neocaml also supports key parts of the OCaml ecosystem: dune build files, opam package definitions, OCamllex lexer definitions (.mll), Menhir parser definitions (.mly), cram test files (.t), and odoc documentation pages (.mld), each with a dedicated major mode.

You can also view compiled OCaml artifacts (.cmi, .cmo, .cmx, etc.) directly in Emacs via ocamlobjinfo.

It's also as cool as Neo from "The Matrix". ;-)

• Tree-sitter based font-locking (4 levels) for .ml and .mli files
• Ocamldoc markup rendering in doc comments: {b bold}, {i italic}, [code], {[...]} and {@ocaml[...]} blocks, headings, {!references} and @tags are displayed with real faces
• Tree-sitter based indentation in three styles (ocp-indent's, and ocamlformat's conventional and janestreet profiles), picked from the project's .ocamlformat or .ocp-indent, with cycle-indent support
• OxCaml syntax: modes, kinds, stack allocation, unboxed types and immutable arrays
• Navigation (beginning-of-defun, end-of-defun, forward-sexp, sentence movement with M-a/M-e)
• Imenu with language-specific categories for .ml and .mli
• Toggling between implementation and interface via ff-find-other-file (C-c C-a)
• OCaml toplevel (REPL) integration (neocaml-repl), with the toplevel's responses highlighted like OCaml code, clickable error locations, and findlib package completion for #require, plus an alternative utop backend (neocaml-utop) that speaks utop's native protocol for precise error highlighting and completion
• Comment support: fill-paragraph (M-q), comment continuation (M-j), and comment-dwim (M-;)
• Electric indentation on delimiter characters
• opam file editing (neocaml-opam-mode) with font-lock, indentation, imenu, and opam lint integration (flymake and flycheck)
• dune file editing (neocaml-dune-mode) for dune, dune-project, and dune-workspace files
• dune build commands (neocaml-dune-interaction-mode): build, test, clean, promote, fmt, exec (with watch mode via prefix arg), plus running the current file's tests and reviewing and promoting their output
• OCamllex file editing (neocaml-ocamllex-mode) with font-lock, indentation, imenu, and OCaml language injection (Emacs 30+)
• Menhir file editing (neocaml-menhir-mode) with font-lock, indentation, imenu, and OCaml language injection (Emacs 30+)
• OCaml/JSX file editing (neocaml-mlx-mode) for .mlx files (Melange/React and similar PPX-driven OCaml-JSX), with embedded JSX highlighting via tsx language injection (Emacs 30+)
• Cram test file editing (neocaml-cram-mode) with font-lock for commands, output, modifiers, and prose, and keys for running the test and promoting its output
• odoc documentation editing (neocaml-odoc-mode) with font-lock, indentation, imenu, and language injection into {@lang[...]} code blocks for OCaml, dune, and opam (Emacs 30+)
• Easy installation of ocaml and ocaml-interface tree-sitter grammars via M-x neocaml-install-grammars (and the tsx/JSX grammar via M-x neocaml-mlx-install-grammar)
• Compilation error regexp for M-x compile (errors, warnings, alerts, backtraces)
• _build directory awareness (offers to switch to source when opening build artifacts)
• Eglot integration (with ocaml-eglot support)
• Debugging via dape + ocamlearlybird (bytecode)
• Prettify-symbols for common OCaml operators

neocaml is available on MELPA:

(use-package neocaml
  :ensure t)

If the required tree-sitter grammars are not installed, run M-x neocaml-install-grammars. Eglot is auto-configured - just run M-x eglot in an OCaml buffer (requires ocaml-lsp-server).

See the documentation for alternative installation methods and detailed configuration.

While neocaml is free software and will always be, the project would benefit from some funding. Consider supporting its ongoing development if you find it useful.

You can support the development of neocaml via:

• GitHub Sponsors
• Patreon
• PayPal

Curious where neocaml is headed? See ROADMAP.md for ideas and planned improvements. Feedback and contributions on any of them are welcome.

See CONTRIBUTING.md for development setup, debugging, and how to run tests. Architecture and design notes live in doc/DESIGN.md.

Copyright (c) 2025-2026 Bozhidar Batsov and contributors.

Distributed under the GNU General Public License, version 3 or later. See LICENSE for details.