LSP Server for OCaml

OCaml-LSP is a language server for OCaml that implements Language Server Protocol (LSP).

This project contains an implementation of a language server for OCaml and a standalone
library implementing LSP.


We recommend to install the language server via a package manager such as
opam or esy.


To install the language server in the currently used opam switch:

$ opam install ocaml-lsp-server

Note: you will need to install ocaml-lsp-server in every switch where you would like
to use it.


To add the language server to an esy project, run in terminal:

$ esy add @opam/ocaml-lsp-server


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

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


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

For an example of usage of the server in a VS Code extension, see
OCaml Platform Extension implementation here.


The server supports the following LSP requests:

  • [x] textDocument/completion

  • [x] completionItem/resolve

  • [x] textdocument/hover

  • [ ] textDocument/signatureHelp

  • [x] 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.

Integration with other tools

Formatters: OCamlFormat & Refmt

OCaml-LSP is dependent on external tools (OCamlFormat for OCaml and refmt for Reason)
for formatting source files. You should have the necessary tool (OCamlFormat and/or Refmt)
installed in your opam switch or esy project to have formatting support. Note, however, that
OCaml-LSP requires presence of OCamlFormat configuration file (called .ocamlformat) in
the project root to be able to format source files in your project.

Contributing to project

# 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.4.12.0 --with-test

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

# build
make all

# the ocamllsp executable can be found at _build/default/ocaml-lsp-server/src/main.exe


To run tests execute:

$ make test

Note that tests require Node.js and
[Yarn][https://yarnpkg.com/lang/en/] 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 hack

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 fewer 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.

31 Mar 2021
Reverse Dependencies
< "0.2.0"