LSP Server for OCaml

This project contains an implementation of an LSP server for OCaml and a
standalone implementation of the LSP protocol.


We recommend to install the server via a project such as
opam or esy.


To install the lsp server in a particular opam switch:

$ opam pin add ocaml-lsp-server https://github.com/ocaml/ocaml-lsp.git
$ opam install ocaml-lsp-server

Note that you will need to run install the lsp server in every switch where
you'd like to use it.


To add the lsp server to an esy project, add the following lines to your
project's package.json:

  "devDependencies": {
    "@opam/ocaml-lsp-server": "ocaml/ocaml-lsp:ocaml-lsp-server.opam"


This project uses submodules to handle dependencies. This is done so that users
who install ocaml-lsp-server into their sandbox will not share constraints on
the same dependencies that ocaml-lsp-server is using.

$ git clone --recurse-submodules http://github.com/ocaml/ocaml-lsp.git
$ cd ocaml-lsp
$ make build


Once ocaml-lsp-server is installed, the executable is called ocamllsp.
For now, the server can only be used through the standard file descriptors stdin and stdout.

For an example of usage of the server in a VSCode extension, see here.


The server supports the following queries:

  • [x] textDocument/completion

  • [x] completionItem/resolve

  • [x] textdocument/hover

  • [ ] textDocument/signatureHelp

  • [ ] textDocument/declaration

  • [x] textDocument/definition

  • [x] textDocument/typeDefinition

  • [ ] textDocument/implementation

  • [x] textDocument/codeLens

  • [x] textDocument/documentHighlight

  • [x] textDocument/documentSymbol

  • [x] textDocument/references

  • [ ] textDocument/documentColor

  • [ ] textDocument/colorPresentation

  • [x] textDocument/formatting

  • [ ] textDocument/rangeFormatting

  • [ ] textDocument/onTypeFormatting

  • [x] textDocument/prepareRename

  • [x] textDocument/foldingRange

  • [x] textDocument/selectionRange

  • [ ] workspace/symbol

Note that degrees of support for each LSP request are varying.


# clone repo with submodules
git clone --recursive git@github.com:ocaml/ocaml-lsp.git

# if you already cloned, pull submodules
git submodule update --init --recursive

# create local switch (or use global one) and install dependencies
opam switch create . ocaml-base-compiler.4.09.1 --with-test

# don't forget to set your environment to use the local switch
eval ($opam env)

# build
make build

# install ocamllsp
make lsp-server


To run tests execute:

$ make test

Note that tests require Node.js and Yarn installed.

Relationship to Other Tools

The lsp server uses merlin under the hood, but users are not required to have
merlin installed. We vendor merlin because we currently heavily depend on some
implementation details of merlin that make infeasible to upgrade the lsp server
and merlin independently.


The implementation of the lsp protocol itself was taken from facebook's

Previously, this lsp server was a part of merlin, until it was realized that the
lsp protocol covers a wider scope than merlin.

Comparison to other LSP Servers for OCaml

Note that the comparisons below make no claims of being objective and may be
entirely out of date:

  • reason-language-server
    This server supports
    bucklescript &
    reason. However, this project does not
    use merlin which means that it supports less versions of OCaml and offers less
    "smart" functionality - especially in the face of sources that do not yet

  • ocaml-language-server
    This project is extremely similar in the functionality it provides because it
    also reuses merlin on the backend. The essential difference is that this
    project is written in typescript, while our server is in OCaml. We feel that
    it's best to use OCaml to maximize the contributor pool.

15 Oct 2020
Reverse Dependencies
< "0.2.0"