Setting Up OCaml Indentation in Vim with ocp-indent

Vim’s default OCaml indentation is mediocre at best. The ocp-indent tool provides intelligent, syntax-aware indentation that respects OCaml conventions and integrates cleanly with Vim and Neovim.

Installing ocp-indent

Install via opam:

opam install ocp-indent

This installs the indentation engine and Vim plugin support. Verify the installation:

which ocp-indent

Configuring Vim

Add this to your ~/.vimrc or ~/.config/nvim/init.vim:

augroup ocaml_config
  autocmd!
  autocmd BufNewFile,BufRead *.ml,*.mli setfiletype ocaml
  autocmd FileType ocaml call SetupOCaml()
augroup END

function! SetupOCaml()
  setlocal textwidth=80
  setlocal colorcolumn=80
  setlocal shiftwidth=2
  setlocal tabstop=2
  setlocal softtabstop=2
  setlocal expandtab
  filetype indent on
  filetype plugin indent on

  " Load ocp-indent
  let opamshare = system('opam var share | tr -d "\n"')
  if isdirectory(opamshare)
    let ocp_indent_path = opamshare . '/vim/syntax/ocp-indent.vim'
    if filereadable(ocp_indent_path)
      execute 'source ' . ocp_indent_path
    endif
  endif
endfunction

Key points:

• Sets filetype detection for .ml and .mli files
• Uses 2-space indentation (OCaml standard)
• Expands tabs to spaces with expandtab
• Safely loads ocp-indent only if the file exists
• Uses augroup to prevent duplicate autocommands on reload

For Neovim with plugin managers

If using lazy.nvim:

{
  'ocaml/vim-ocaml',
  ft = { 'ocaml' },
  config = function()
    vim.opt_local.shiftwidth = 2
    vim.opt_local.tabstop = 2
    vim.opt_local.expandtab = true
  end,
}

Or with vim-plug:

Plug 'ocaml/vim-ocaml'

Using ocp-indent

Once configured, indentation works automatically:

Action	Command
Current line	== in normal mode
Visual selection	Select + =
Entire file	gg=G
Paragraph	=ap
Automatic	Triggers when you press Enter or type structural keywords (if, let, match, etc.)

You can also reindent the entire buffer with:

:0,$!ocp-indent

This pipes the buffer through ocp-indent directly, useful for reformatting imported code.

Configuring ocp-indent behavior

Create ~/.ocp/ocp-indent.conf to customize rules:

JaneStreet
indent_with_tabs = false
indent_comments = true
strict_comments = false

Available profiles:

• normal — Standard OCaml indentation
• apprentice — Conservative indentation
• JaneStreet — Jane Street style guide (good default for most projects)

Troubleshooting

ocp-indent not loading:

Check that opam is initialized:

eval $(opam env)

Verify the opam share path exists:

opam var share

If empty, run:

opam init

Conflicting indentation settings:

Remove any duplicate indentation configs in your vimrc. Ensure filetype plugin indent on appears after loading ocp-indent. Check for conflicting plugins with:

:scriptnames

Indentation still looks wrong:

Verify ocp-indent is being sourced:

:echo exists('*ocp_indent#indent')

Should return 1. If 0, the plugin didn’t load. Check the file exists at:

cat $(opam var share)/vim/syntax/ocp-indent.vim

For Neovim specifically:

If using a plugin manager, ensure the vim-ocaml plugin loads before other OCaml-related plugins. Check LSP client settings aren’t overriding indentation behavior.

2026 Best Practices and Advanced Techniques

For Setting Up OCaml Indentation in Vim with ocp-indent, understanding both the fundamentals and modern practices ensures you can work efficiently and avoid common pitfalls. This guide extends the core article with practical advice for 2026 workflows.

Troubleshooting and Debugging

When issues arise, a systematic approach saves time. Start by checking logs for error messages or warnings. Test individual components in isolation before integrating them. Use verbose modes and debug flags to gather more information when standard output is not enough to diagnose the problem.

Performance Optimization

• Monitor system resources to identify bottlenecks
• Use caching strategies to reduce redundant computation
• Keep software updated for security patches and performance improvements
• Profile code before applying optimizations
• Use connection pooling and keep-alive for network operations

Security Considerations

Security should be built into workflows from the start. Use strong authentication methods, encrypt sensitive data in transit, and follow the principle of least privilege for access controls. Regular security audits and penetration testing help maintain system integrity.

Related Tools and Commands

These complementary tools expand your capabilities:

• Monitoring: top, htop, iotop, vmstat for system resources
• Networking: ping, traceroute, ss, tcpdump for connectivity
• Files: find, locate, fd for searching; rsync for syncing
• Logs: journalctl, dmesg, tail -f for real-time monitoring
• Testing: curl for HTTP requests, nc for ports, openssl for crypto

Integration with Modern Workflows

Consider automation and containerization for consistency across environments. Infrastructure as code tools enable reproducible deployments. CI/CD pipelines automate testing and deployment, reducing human error and speeding up delivery cycles.

Quick Reference

This extended guide covers the topic beyond the original article scope. For specialized needs, refer to official documentation or community resources. Practice in test environments before production deployment.