ocaml-ts-mode

Ocaml mode for emacs using treesitter.

This is beta software and in development.

It implements:

• Syntax highlighting for .ml and .mli files.
• Rudimentary indentation.
• C-c C-a - Switch between .ml and .mli files for a module.

How to Install

You will need a version of Emacs with treesitter enabled.

1. Setup the source list for the ocaml treesitter grammar. Add the following to your ~/.emacs:

   (setq treesit-language-source-alist
     '((ocaml "https://github.com/tree-sitter/tree-sitter-ocaml" "master" "ocaml/src")))

2. Install the treesitter grammar: M-x treesit-install-language-grammar RET ocaml RET.
3. Clone this repository.
4. Add a progmode directory to your emacs path. For this step, there are several ways to accomplish this, these directions show one way to do it. The goal is for emacs to see the ocaml-ts-mode.el file. To do this I have a directory called ~/.emacs-progmode and then I add that path to emacs by adding the following line to my ~/.emacs: (add-to-list 'load-path "~/.emacs-progmode").
5. Symlink ocaml-ts-mode.el into the progmode directory. Given the step above, that would be ~/.emacs-progmode/.
6. Add ocaml-ts-mode to autoload:

   (autoload 'ocaml-ts-mode "ocaml-ts-mode" "Major mode for editing Ocaml code" t)

7. Optional: By default, ocaml-ts-mode sets itself up for .ml and .mli files. However if you are using another ocaml mode, such as Tuareg or camlmode, you will need to either manually switch to ocaml-ts-mode by doing M-x ocaml-ts-mode RET or by remapping the major mode, for example by adding the following to ~/.emacs:

   (setq major-mode-remap-alist
     '((tuareg-mode . ocaml-ts-mode)))

Finally, you can configure ocaml-ts-mode when it load via a hook. For example:

(add-hook 'ocaml-ts-mode-hook
          (lambda ()
            (eglot-ensure)
            (company-mode)
            (flyspell-prog-mode)
            (local-set-key (kbd "C-<tab>") 'company-complete)
            (add-hook 'before-save-hook 'ocamlformat)
            (setq indent-tabs-mode nil)
            (setq truncate-lines t)
            (setq whitespace-line-column 100)
            (setq whitespace-style '(face trailing lines-tail))
            (whitespace-mode)
            (yafolding-mode)
            (rainbow-delimiters-mode)))

Comparison to Tuareg mode

ocaml-ts-mode is a new project and currently only supports .ml and .mli files. It is also much less feature rich then Tuareg mode.

Treesitter is considered the future of programming major modes in Emacs. The power of it is treesitter understands the syntax of a language and thus the syntax highlighting can be more precise. For example, ocaml-ts-mode highlights function calls different than field access. It also has good support for multiple languages in a single buffer.

Future Work & Ways to Contribute

ocaml-ts-mode does not have many features and is beta.

To contribute, just open a pull request!

Some known work that needs to be done:

1. Improve indentation support.
2. Make sure the syntax grammar covers all cases.
3. Figure out if the ocaml interface treesitter grammar should be used, and how. Currently just the ocaml grammar is used.
4. Add more useful features.

Useful links

• How to get started with tree sitter - https://www.masteringemacs.org/article/how-to-get-started-tree-sitter
• Tree Sitter and the Complications of Parsing Languages - https://www.masteringemacs.org/article/tree-sitter-complications-of-parsing-languages
• Let's write a treesitter major mode - https://www.masteringemacs.org/article/lets-write-a-treesitter-major-mode