Neovim Setup for OCaml

Intro

In previous post, I elaborated on how to set up Neovim from scratch. However, I didn’t answer the following questions:

1. What if an LSP does not support formatting, how to configure Neovim to support a third-party code formatting tool?
2. How to change configuration files when adding a new programming language?

Recently I was learning OCaml, I thought it would be a good chance to answer the questions.

Install OCaml

It is quite handy to install OCaml on a Mac with Homebrew.

$ brew install opam

According to the output of Homebrew, we need to perform additional configurations and modify ~/.zshrc(for Zsh users) or ~/.bashrc(for Bash users).

$ opam init
# press `y` to confirm

To make the modifications work, run the following command in your terminal

$ source ~/.zshrc
# $ source ~/.bashrc # if you use bash

Finally, we can use this command to check if everything goes right

$ opam switch

The output will be like

#   switch   compiler                   description
->  5.0.0    ocaml-base-compiler.5.0.0  5.0.0
    default  ocaml.4.14.0               default

Similar to IPython for Python, the OCaml offers a REPL tool called utop. We can install this to practice programming OCaml

$ opam install utop dune

Just try the utop REPL yourself :)

$ utop

The output would be like

─────────────────────────────────────────────────────────┬─────────────────────────────────────────────────────────────┬──────────────────────────────────────────────────────────
                                                         │ Welcome to utop version 2.13.1 (using OCaml version 5.0.0)! │
                                                         └─────────────────────────────────────────────────────────────┘

Type #utop_help for help about using utop.

─( 10:33:26 )─< command 0 >────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────{ counter: 0 }─
utop #
┌──────────────┬──────────────┬───────────────┬─────┬───────────┬────┬───┬──────────┬─────┬───────────┬──────┬────────────┬───────┬───────────┬──────────────┬──────────┬────────┐
│Afl_instrument│Alias_analysis│Allocated_const│Annot│Applicative│Arch│Arg│Arg_helper│Array│ArrayLabels│Asmgen│Asmlibrarian│Asmlink│Asmpackager│Assert_failure│Ast_helper│Ast_inva│
└──────────────┴──────────────┴───────────────┴─────┴───────────┴────┴───┴──────────┴─────┴───────────┴──────┴────────────┴───────┴───────────┴──────────────┴──────────┴────────┘

OCaml LSP configurations

nvim
├── init.lua
└── lua
    ├── colorscheme.lua
    ├── config
    │   └── nvim-cmp.lua
    ├── keymaps.lua
    ├── lsp.lua
    ├── options.lua
    └── plugins.lua

Previously, we leveraged mason.nvim to set up LSPs. To add a new LSP for a specific programming language, we can also use this. Steps to follow:

1. Open Neovim and type :Mason, found ocaml-lsp in the (2) LSP section. Press i when the cursor stays on the line.
2. Open the configurations file for LSP(lsp.lua in my configurations), add the following lua code

... -- rest of the configurations
lspconfig.ocamllsp.setup({
	on_attach = on_attach,
})

After modifying lsp.lua, we can restart Neovim to make the configurations take effect.

Now you can create a new dune project to see if LSP works.

OCaml Formatter configurations

The ocaml-lsp LSP does not support formatting OCaml code. If you try to format an OCaml code, you will get an error message:

[LSP] Format request failed, no matching language servers.

Now, I will talk about how to set up a formatter for OCaml.

You may have noticed that the Mason tool also offers Linter and Formatter. The formatter for OCaml is ocamlformat. So We can install it just like we install an LSP.

However, ocamlformat is a CLI tool which can not be integrated into Neovim. Luckily, we have none-ls plugin, which can expose linter, formatter, and other CLI tools as LSP for Neovim.

To make it easier, we can use the mason-null-ls.nvim plugin, developed by the same author of mason.nvim. Open plugins.lua file and append these Lua codes. After saving the changes and restarting Neovim you should see lazy.nvim is installing these plugins.

... -- rest of the configurations
require("lazy").setup({
	-- Add hooks to LSP to support Linter && Formatter
	{
		"jay-babu/mason-null-ls.nvim",
		event = { "BufReadPre", "BufNewFile" },
		dependencies = {
			"williamboman/mason.nvim",
			"nvimtools/none-ls.nvim",
		},
		config = function()
			-- Note:
			--     the default search path for `require` is ~/.config/nvim/lua
			--     use a `.` as a path seperator
			--     the suffix `.lua` is not needed
			require("config.mason-null-ls")
		end,
	},
    ... -- rest of the configurations
})

Then, create a new file called mason-null-ls.lua in config folder and put the following code

local mason_ok, mason = pcall(require, "mason")
if not mason_ok then
	return
end

local null_ls_ok, null_ls = pcall(require, "null-ls")
if not null_ls_ok then
	return
end

local mason_null_ls_ok, mason_null_ls = pcall(require, "mason-null-ls")
if not mason_null_ls_ok then
	return
end

mason.setup()

mason_null_ls.setup({
	-- A list of sources to install if they're not already installed.
	-- This setting has no relation with the `automatic_installation` setting.
	ensure_installed = {},
	-- Run `require("null-ls").setup`.
	-- Will automatically install masons tools based on selected sources in `null-ls`.
	-- Can also be an exclusion list.
	-- Example: `automatic_installation = { exclude = { "rust_analyzer", "solargraph" } }`
	automatic_installation = false,
	-- Sources found installed in mason will automatically be set up for null-ls.
	automatic_setup = true,
	handlers = {
		-- Hint: see https://github.com/nvimtools/none-ls.nvim/blob/main/doc/BUILTIN_CONFIG.md
		--       to see what sources are available
		-- Hint: see https://github.com/jose-elias-alvarez/null-ls.nvim/blob/main/doc/BUILTIN_CONFIG.md
		--       to check what we can configure for each source
		ocamlformat = function(source_name, methods)
			null_ls.register(null_ls.builtins.formatting.ocamlformat.with({
				-- Add more arguments to a source's defaults
				-- Default: { "--enable-outside-detected-project", "--name", "$FILENAME", "-" }
				-- Type `ocamlformat --help` in your terminal to check more args
				extra_args = { "--if-then-else", "vertical" },
			}))
		end,
	},
})

null_ls.setup()

Finally, we also need to change the lsp.lua file. The original liens related to code formmating looks like this:

... -- rest of the configurations
vim.keymap.set("n", "<space>f", function()
    vim.lsp.buf.format({ async = true })
end, bufopts)
... -- rest of the configurations

Now we are using none-ls to do formatting. So we need to set the filter argument as follows

... -- rest of the configurations
vim.keymap.set("n", "<space>f", function()
    vim.lsp.buf.format({
        async = true,
		-- Predicate used to filter clients. Receives a client as
		-- argument and must return a boolean. Clients matching the
		-- predicate are included.
        filter = function(client)
            return client.name == "null-ls"
        end,
    })
end, bufopts)
... -- rest of the configurations

Save the changes and restart Neovim to see if it works.

... -- rest of the configurations
vim.keymap.set("n", "<space>f", function()
    vim.lsp.buf.format({
        async = true,
		-- Predicate used to filter clients. Receives a client as
		-- argument and must return a boolean. Clients matching the
		-- predicate are included.
        filter = function(client)
            -- hls stands for Haskell-language-server
			return client.name == "null-ls" or client.name == "hls"
        end,
    })
end, bufopts)
... -- rest of the configurations

Wrap-up

Now let’s summarize how to set up Neovim for a new programming language. If you also use mason.nvim, mason-lspconfig, none-ls.nvim and mason-null-ls.nvim, you can follow the instructions to setup:

1. Read the official pages and install the programming language’s environment on your machine.
2. Open Neovim, type :Mason, and install the corresponding LSP. Refer to the official LSP pages and check if the LSP contains a built-in Formatter. If the answer is no, just use :Mason to install one.
3. Open the lsp.lua file and configure LSP
4. If you install Formatter by yourself, open the mason-null-ls.lua file to configure handlers

For things that are not clearly stated in this post, you can view my dotfiles configurations repo to learn more information.