neocaml¶

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), and cram test files (.t) -- 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". ;-)

Why?¶

Because caml-mode is ancient, and tuareg, while very powerful, has grown complex over the years. The time seems ripe for a modern, leaner, tree-sitter-powered mode for OCaml.

There have been two earlier attempts at tree-sitter OCaml modes for Emacs:

Both provided useful early exploration of what a tree-sitter OCaml mode could look like, and helped inspire this project. neocaml aims to take the idea further with a more complete feature set and active maintenance.

They say that third time's the charm, right?

One last thing - we really need more Emacs packages with fun names! :D

Features¶

Tree-sitter based font-locking (4 levels) for .ml and .mli files
Tree-sitter based indentation with cycle-indent support
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)
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)
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+)
Cram test file editing (neocaml-cram-mode) with font-lock for commands, output, modifiers, and prose
Easy installation of ocaml and ocaml-interface tree-sitter grammars via M-x neocaml-install-grammars
Compilation error regexp for M-x compile (errors, warnings, alerts, backtraces)
_build directory awareness (offers to switch to source when opening build artifacts, configurable via neocaml-redirect-build-files)
Eglot integration (with ocaml-eglot support)
Debugging via dape + ocamlearlybird (bytecode)
Prettify-symbols for common OCaml operators

Comparison with Other Modes¶

Feature	neocaml	caml-mode	tuareg
Required Emacs version	29.1+ (30+ recommended)	24+	26+
Font-lock	Tree-sitter (4 levels)	Regex	Regex
Indentation	Tree-sitter + cycle-indent	Custom engine	SMIE
REPL integration	Yes	Yes	Yes
Navigation (defun, sexp)	Yes	Yes	Yes
Imenu	Yes (.ml and .mli)	Yes	Yes
.ml/.mli toggle	Yes	Yes	Yes
LSP (Eglot) integration	Yes (auto-configured)	Yes (manual)	Yes (manual)
Debugger	Yes (dape + ocamlearlybird)	Yes (ocamldebug)	Yes (ocamldebug)
Compilation commands	Error regexp + C-c C-c	Yes	Yes
`_build` directory aware	Yes	No	Yes
opam file support	Yes	No	Yes
dune file support	Yes	No	No
OCamllex support	Yes (with OCaml injection)	No	Yes
Menhir support	Yes (with OCaml injection)	No	Yes
Cram test support	Yes	No	No
Code templates / skeletons	No	Yes	Yes

Keep in mind also that tuareg uses caml-mode internally for some functionality. I think both modes will probably be folded into one down the road.

The impact of LSP on major modes¶

Historically, caml-mode and tuareg bundled features like type display, completion, jump-to-definition, error checking, and refactoring support -- all driven by Merlin. Today, ocamllsp provides all of these through the standard LSP protocol, and Eglot (built into Emacs 29+) acts as the client. There is no reason for a major mode to reimplement any of this.

For OCaml-specific LSP extensions that go beyond the standard protocol -- type enclosing, case analysis (destruct), hole navigation -- ocaml-eglot is the recommended companion package.

Why Tree-sitter?¶

Tree-sitter provides incremental, error-tolerant parsing that is significantly faster and more accurate than regex-based font-lock and SMIE-based indentation. It parses the full syntax tree, so fontification and indentation rules can reference actual language constructs rather than fragile regular expressions. This results in fewer edge-case bugs and simpler, more maintainable code.

Funding¶

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:

License¶

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