Configuring Auto-Indentation For OCaml In Vim

Vim’s default OCaml indentation is limited and doesn’t handle the language’s syntax well. You’ll want to use ocp-indent, a dedicated indentation tool built specifically for OCaml that understands the language’s scoping and structure rules.

Installing ocp-indent

First, install the tool via OPAM (OCaml Package Manager):

opam install ocp-indent

If you don’t have OPAM installed, install it first:

# On Ubuntu/Debian
sudo apt-get install opam

# On macOS
brew install opam

# Initialize OPAM (first time only)
opam init
eval $(opam env)

Verify the installation:

which ocp-indent
ocp-indent --version

Configuring Vim

Create or edit your ~/.vimrc file and add:

" OCaml indentation with ocp-indent
let g:formatprg="ocp-indent"
autocmd FileType ocaml setlocal formatprg=ocp-indent
autocmd FileType ocaml setlocal shiftwidth=2 softtabstop=2 tabstop=2 expandtab

The shiftwidth settings match OCaml’s standard 2-space indentation convention.

Using ocp-indent in Vim

Once configured, you have several ways to apply indentation:

Indent the current line:

:!ocp-indent

Indent a range (for example, lines 10 to 20):

:10,20!ocp-indent

Indent the entire file:

:%!ocp-indent

Or use the standard Vim motion commands:

gg=G    " Indent from start to end of file
=ip     " Indent current paragraph

Better integration with vim-plug

For a more modern setup using vim-plug, add this to your ~/.vimrc:

Plug 'ocaml/vim-ocaml'

autocmd FileType ocaml setlocal formatprg=ocp-indent
autocmd FileType ocaml setlocal shiftwidth=2 softtabstop=2 tabstop=2 expandtab

Install the plugin:

:PlugInstall

Neovim users

If you’re using Neovim with a modern config, you can set this in ~/.config/nvim/init.lua:

vim.api.nvim_create_autocmd("FileType", {
  pattern = "ocaml",
  callback = function()
    vim.opt_local.formatprg = "ocp-indent"
    vim.opt_local.shiftwidth = 2
    vim.opt_local.softtabstop = 2
    vim.opt_local.tabstop = 2
    vim.opt_local.expandtab = true
  end,
})

LSP-based indentation (modern approach)

For OCaml 5.0+, consider using an OCaml language server with Neovim or VS Code. The ocaml-lsp-server provides better indentation and formatting:

opam install ocaml-lsp-server

This gives you real-time indentation feedback and integrates with modern editor configurations.

Common issues

ocp-indent not found: Ensure the OPAM environment is sourced. Add this to your shell profile if needed:

eval $(opam env)

Wrong indentation after paste: After pasting OCaml code, reapply indentation with :%!ocp-indent to fix formatting.

Conflicts with other formatters: If you use formatters like ocamlformat, choose one primary tool to avoid conflicts. ocamlformat is more aggressive about style; ocp-indent focuses on indentation alone.

Essential Editor Configuration

A well-configured editor dramatically improves productivity. For Vim users, invest time in crafting a solid vimrc. For Emacs users, use use-package for cleaner configuration management. Both editors support extensive plugin ecosystems that can transform them into full IDEs.

Learn the navigation commands thoroughly before adding plugins. Mastery of core movement commands (word, paragraph, search motions in Vim; mark, isearch, ace-jump in Emacs) provides more productivity gains than any single plugin.

Performance Tips

Large files can slow down both editors. For Vim, set lazyredraw and disable syntax highlighting for files over a certain size. For Emacs, use so-long-mode for files with long lines. Both editors benefit from disabling line numbers on very large files to improve scrolling performance.

Quick Verification

After applying the changes described above, verify that everything works as expected. Run the relevant commands to confirm the new configuration is active. Check system logs for any errors or warnings that might indicate problems. If something does not work as expected, review the steps carefully and consult the official documentation for your specific version.

Configuring Auto-Indentation for OCaml in Vim