Merlin
Context sensitive completion for OCaml in Vim and Emacs
... version of OCaml (between 4.02 and 4.10) you will want to build the 3.4 branch, although it won't contain the most recent features. If you want to work on merlin, you'll want to avoid the -p merlin, to build in dev mode, with some extra warnings enabled. In that case you'll also need an extra dependency.