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Context sensitive completion for OCaml in Vim and Emacs
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merlin completion in gvim

Installing Merlin manually


Needed: OCaml 4, ocamlfind, ocamlbuild, yojson, menhir

Merlin requires OCaml >=4.00.1 (may work with newer versions, but it is tightly bound to compiler internals).
Then ensures that "yojson" and "menhir" are installed and are registered in ocamlfind.

$ ./configure
$ make


If you don't get any error in step above:

$ make install 

Installation from git

Don't forget to checkout submodules after pulling/cloning:

$ git submodule init
$ git submodule update

Installing Merlin with opam

Needed: opam, OCaml 4.00.1 (can be installed via opam typing opam switch 4.00.1).

Then, just do :

$ opam install merlin

Setting-up vim

Makes sure that ocamlmerlin binary can be found in PATH.

The only setup needed is to have the following directories in vim runtime path (append this to your .vimrc):

:set rtp+=$SHARE_DIR/ocamlmerlin/vim
:set rtp+=$SHARE_DIR/ocamlmerlin/vimbufsync

The default configuration can be seen in:


Now you may be interested by Features and interaction with other plugins, Merlin project and Extensions sections.

Using Vundle

Alternatively you can install vim support using Vundle. Add the following to your .vimrc

Bundle 'def-lkb/merlin', {'rtp': 'vim/merlin/'}
Bundle 'def-lkb/vimbufsync'

Features and interaction with other plugins

Omnicompletion should be available out-of-box.

The documentation is accessible through :h merlin.txt and should provide all the necessary information on how to set-up merlin with other plugins ( Supertab, neocomplcache, syntastic). It also lists, and explain, all the available commands.

Misc: description of plugin's files

  • $SHARE_DIR/ocamlmerlin/vim -- main vim plugin directory

    • plugin/merlin.vim -- sample configuration
    • autoload/
      • merlin.vim -- main vim script
      • -- helper script needed by merlin.vim (has to be in the same directory)
    • syntax_checkers/
      -- integration with syntastic (ocaml or omlet)
      -- set g:syntastic_ocaml_checkers = ['merlin']
      -- or g:syntastic_omlet_checkers = ['merlin']
  • $SHARE_DIR/ocamlmerlin/vimbufsync
    -- library needed by merlin vim mode to keep buffer synchronized

Upgrading vim plugin from merlin 1.0

Merlin plugin now relies on another vim helper called vimbufsync. If you installed from opam and/or archive, just make sure that vimbufsync is in vim runtime path:

:set rtp+=$SHARE_DIR/ocamlmerlin/vimbufsync

Emacs interface

merlin comes with an emacs interface (file: emacs/merlin.el) that implements a minor-mode that is supposed to be used on top of tuareg-mode.

To get it working you only to load the file `emacs/merlin.el' of the distribution.

If you installed through opam, a good thing to do is:

(add-to-list 'load-path "~/.opam/4.00.1/share/emacs/site-lisp/")
(require 'merlin)

To use it, you will need

  • json.el (available by package.el)
  • auto-complete-mode (optional, available by package.el and the MELPA repository)

Once it is done, to enable merlin in a buffer, just type M-x merlin-mode. If you want merlin to be started on every ML buffer issue:

(add-hook 'tuareg-mode-hook 'merlin-mode)


The emacs merlin-mode make all the features of merlin available inside emacs, by synchronizing ML buffers with a merlin instance. At any moment, the instance of merlin knows a part of the buffer that works like locked zones in proof assistant modes (eg. proofgeneral): editing inside this zone retracts it to the last valid phrase, and C-c C-t (merlin-to-point) expands it to the current position. Moreover executing a merlin command also tries to advance the locked zone as much as possible to contain the point.

Main keybindings:

  • C-c C-t (merlin-magic-show-type) shows the type of the expression underpoint. To do so, it tries to compile the current phrase and locate the leaf of the typedtree containing the current position:

    • if it is found, it prints the type of this leaf. Further calls to C-c C-t will print the types of bigger expressions enclosing the position ("go up" in the AST). (C-u C-c C-t will go down). For instance if you have: (fun n -> n + 1) [1; 2; 3]

      and your cursor is on the n, then successive calls to C-c C-t will print (in this order): int (n), int (n+1), int -> int (fun n -> n + 1), and int list (the whole statement)

    • if it is not possible (the current phrase cannot be made to compile), then an approximation is tried to type the expression out of context.

    Moreover, if you use a prefix argument, merlin-mode will try to type the region.

  • C-c C-t (merlin-to-point) will update the locked zone to the current position and report all the errors and warnings found.

  • C-c C-x (merlin-next-error) will jump to the next error and display the error message

  • C-c C-u (merlin-refresh) refreshes merlin's cmis cache (when you recompiled other files)

  • C-c C-r (merlin-rewind) retracts the whole buffer (useful when merlin seems confused)

  • C-c C-l (merlin-locate) tries to find the definition of the ident under point and jumps to it. To go back, use C-c & (merlin-pop-stack)

  • C-c d (merlin-show-type-def) will print the definition of the type of the expression underpoint if any. If the type is compliated (eg. 'a -> 'a option) it will print the definition of the codomain.

  • C-c l (merlin-use) loads a findlib package inside merlin

  • C-c r (merlin-restart-process) restarts merlin process (useful when hung)

Moreover, you have regular auto-completion (M-TAB by default) using completion-at-point. There is auto-complete integration you can enable by setting merlin-use-auto-complete-mode to t:

(setq merlin-use-auto-complete-mode t)

Merlin project

When loading a ml file in your editor, merlin mode will search a file named .merlin in the same directory or in parent directories.

The ".merlin" allows you to integrate merlin with your project. Each line of this file begin with a command name followed by one argument:

  • S <src-dir>: add a source directory, used to find *.ml / *.mli files
  • B <build-dir>: add a build directory, used to find *.cmi files
  • PKG <findlib-pkg>: load a findlib package and its dependencies in merlin

Directory are either absolute or relative to the directory containing ".merlin" file.


Merlin doesn't support (nor plan to support) Camlp4. However, a few common extensions are hardcoded:


Support for lwt, match_lwt, try_lwt / finally, for_lwt, while_lwt, if_lwt and raise_lwt.
You need to add lwt package (with ":Use lwt" or "PKG lwt" in .merlin) for this to work, and it may be necessary to reload buffer for this change to take effect.


A few syntax extensions based on type-conv are supported as well. Namely :

  • sexplib.syntax
  • binprot.syntax


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