1. Overview
  2. Docs



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


Below we show how to install OCaml-LSP using opam, esy, and from sources. OCaml-LSP comes in a package called ocaml-lsp-server but the installed program (i.e., binary) is called ocamllsp.

Installing with package managers


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

Installing from sources

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 install

Additional package installations

  • Install ocamlformat
    package if you want source file formatting support.

    Note: To have source file formatting support in your project, there needs to
    be an .ocamlformat file present in your project's root directory.

  • OCaml-LSP also uses a program called ocamlformat-rpc to format code that is
    either generated or displayed by OCaml-LSP, e.g., when you hover over a module
    identifier, you can see its typed nicely formatted. This program comes with
    ocamlformat (version > 0.21.0). Previously, it was a standalone package.


Usually, your code editor, or some extension/plugin that you install on it, is
responsible for launching ocamllsp.

Important: OCaml Language Server has its information about the files from the
last time your built your project. We recommend using the Dune build system and
running it in "watch" mode to always have correctly functioning OCaml-LSP, e.g.,
dune build --watch.

Integration with Dune RPC

OCaml-LSP can communicate wit Dune's RPC system to offer some interesting
features. User can launch Dune's RPC system by running Dune in watch mode.
OCaml-LSP will not launch Dune's RPC for you. But OCaml-LSP will see if there
is an RPC running and will communicate with it automatically.

There are various interesting features and caveats:

  1. Dune's RPC enables new kinds of diagnostics (i.e., warnings and errors) to be
    shown in the editor, e.g., mismatching interface and implementation files.
    You need to save the file to refresh such diagnostics because Dune doesn't
    see unsaved files; otherwise, you may see stale (no longer correct) warnings
    or errors. OCaml-LSP updates diagnostics after each build is complete in
    watch mode.

  2. Dune file promotion support. If you, for example, use ppx_expect and have
    failing tests, you will get a diagnostic when Dune reports that your file can
    be promoted. You can promote your file using the code action Promote.

Merlin configuration (advanced)

If you would like OCaml-LSP to respect your .merlin files, OCaml-LSP needs to
be invoked with --fallback-read-dot-merlin argument passed to it.


The server supports the following LSP requests (inexhaustive list):

  • [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.

Semantic highlighting

Semantic highlighting support is enabled by default.

OCaml-LSP implements experimental semantic highlighting support (also known as
semantic tokens support). The support can be activated by passing an evironment
variable to OCaml-LSP:

  • To enable non-incremental (expectedly slower but more stable) version, pass
    OCAMLLSP_SEMANTIC_HIGHLIGHTING=full environment variable to OCaml-LSP.

  • To enable incremental (potentially faster but more error-prone, at least on VS
    Code) version, pass OCAMLLSP_SEMANTIC_HIGHLIGHTING=full/delta to OCaml-LSP.

Tip (for VS Code OCaml Platform users): You can use ocaml.server.extraEnv
setting in VS Code to pass various environment variables to OCaml-LSP.

    "ocaml.server.extraEnv": {

LSP Extensions

The server also supports a number of OCaml specific extensions to the protocol:

Note that editor support for these extensions varies. In general, the OCaml Platform extension for Visual Studio Code will have the best support.

Unusual features

Destructing a value

OCaml-LSP has a code action that allows to generate an exhaustive pattern
matching for values. For example, placing a cursor near a value (Some 10)|
where | is your cursor, OCaml-LSP will offer a code action "Destruct", which
replaces (Some 10) with (match Some with | None -> _ | Some _ -> _).
Importantly, one can only destruct a value if OCaml-LSP can infer the value's
precise type. The value can be type-annotated, e.g., if it's a function argument
with polymorphic (or yet unknown) type in this context. In the code snippet
below, we type-annotate the function parameter v because when we type let f v = v|, the type of v is polymorphic, so we can't destruct it.

You can also usually destruct the value by placing the cursor on the wildcard
(_) pattern in a pattern-match. For example,

type t = A | B of string option

let f (v : t) = match v with | A -> _ | B _| -> _

invoking destruct near the cursor (|) in the snippet above, you get

type t = A | B of string option

let f (v : t) = match v with | A -> _ | B (None) | B (Some _) -> _

Importantly, note the undescores in place of expressions in each branch of the
pattern match above. The underscores that occur in place of expressions are
called "typed holes" - a concept explained below.

Tip (formatting): generated code may not be greatly formatted. If your project
uses a formatter such as OCamlFormat, you can run formatting and get a
well-formatted document (OCamlFormat supports typed holes formatting).

Tip (for VS Code OCaml Platform users): You can destruct a value using a keybinding
Alt+D or on MacOS Option+D

Typed holes

OCaml-LSP has a concept of a "typed hole" syntactically represented as _
(underscore). A typed hole represents a well-typed "substitute" for an
expression. OCaml-LSP considers these underscores that occur in place of
expressions as a valid well-typed OCaml program: let foo : int = _ (the typed
hole has type int here) or let bar = _ 10 (the hole has type int -> 'a).
One can use such holes during development as temporary substitutes for
expressions and "plug" the holes later with appropriate expressions.

Note, files that incorporate typed holes are not considered valid OCaml by the
OCaml compiler and, hence, cannot be compiled.

Also, an underscore occurring in place of a pattern (for example let _ = 10)
should not be confused with a typed hole that occurs in place of an expression,
e.g., let a = _.

Constructing values by type (experimental)

OCaml-LSP can "construct" expressions based on the type required and offer them
during auto-completion. For example, typing _ (typed hole) in the snippet
below will trigger auto-completion (| is your cursor):

(* file foo.ml *)
type t = A | B of string option

(* file bar.ml *)
let v : Foo.t = _|

The auto-completion offers completions Foo.A and Foo.B _. You can further
construct values by placing the cursor as such: Foo.B _| and triggering code
action "Construct an expression" which offers completions None and Some _.
Trigger the same code action in Some _| will offer "" - one of the possible
expressions to replace the typed hole with.

Constructing a value is thus triggered either by typing _ in place of an
expression or trigger the code action "Construct an Expression". Also, the type
of the value needs to be non-polymorphic to construct a meaningful value.

Tip (for VS Code OCaml Platform users): You can construct a value using a keybinding
Alt+C or on MacOS Option+C


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 git@github.com:ocaml/ocaml-lsp.git

cd ocaml-lsp

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

# create local switch (or use global one)
opam switch --yes create . ocaml-base-compiler.4.14.0

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

# install dependencies
make install-test-deps

# build
make all

# the ocamllsp executable can be found at _build/default/ocaml-lsp-server/bin/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 make no claims of being objective and may be
entirely out of
date. Also, both servers seem deprecated.

  • 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.