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OCaml-LSP is a language server for OCaml that implements Language Server


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
$ cd ocaml-lsp
$ make install


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


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

  • [x] workspace/symbol

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

Integration with other tools

Source file formatting: 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.

Formatting code on hover

When you hover the cursor over OCaml code, the extension shows you the type of
the symbol. To get nicely formatted types, install
ocamlformat-rpc package.


If you use Visual Studio Code, please see OCaml Platform extension
page for a detailed guide
on how to report and debug problems.

If you use another code editor and use OCaml-LSP, you should be able to set the
server trace to verbose using your editor's LSP client and watch the trace
for errors such as logged exceptions.

Contributing to project

# clone repo with submodules
git clone --recursive

# 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.4.13.0 --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 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 it 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 makes 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.