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anil madhavapeddy // anil.recoil.org

Intellisense for OCaml with Vim and Merlin

03 October 2013   |   Anil Madhavapeddy   |   tags: ocaml, vim, and realworldocaml   |   all posts

I’m finalizing the installation instructions for Real World OCaml and finally got around to configuring the awesome Merlin editor tool. I pretty much never configure my editor, so here are my notes on getting Merlin working with Vim completely from scratch on MacOS X (I’m on 10.9DP7, but this should work with earlier versions of MacOS X too).

First, install some basic plugin tools by following the Synastic installation instructions.

Install vim-pathogen:

mkdir -p ~/.vim/autoload ~/.vim/bundle; \
curl -so ~/.vim/autoload/pathogen.vim \

Install vim-sensible for useful defaults:

cd ~/.vim/bundle
git clone git://github.com/tpope/vim-sensible.git

Install Syntastic:

cd ~/.vim/bundle
git clone https://github.com/scrooloose/syntastic.git

Follow the Merlin/vim from scratch directions:

opam install merlin

Finally, add this this in your ~/.vimrc to turn everything on.

execute pathogen#infect()
let s:ocamlmerlin=substitute(system('opam config var share'),'\n$','','''') .  "/ocamlmerlin"
execute "set rtp+=".s:ocamlmerlin."/vim"
execute "set rtp+=".s:ocamlmerlin."/vimbufsync"

Trying it out

Read the excellent instructions at the Merlin for Vim from scratch wiki page. A few immediately useful things I did are below.

Displaying types: You can get the local type of something by typing in \t anywhere. This is the <LocalLeader> for anyone who has customized their setup (it is mapped to backslash by default).

Tab completing functions: You can tab-complete module signatures by pressing Ctrl-X-O (make sure you keep Ctrl pressed the whole time). This activates vim’s OmniComplete mode. To get this to work with your external projects, create a .merlin file. I use this for ocaml-dns for example.

PKG lwt cstruct
S lib
S lib_test
S lwt
B _build/lib
B _build/lib_test
B _build/lwt

Also, don’t forget to pass the -bin-annot function to the compiler to generate the typed-AST cmt files. You can add true: bin_annot to your _tags file if you’re using OCamlbuild (in OCaml 4.01.0 or higher, and it’ll be silently ignored on previous versions so you don’t need to worry about breaking older OCaml compilers).

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