package jsonrpc

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Published: 07 Nov 2022



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

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:

  • [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

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.

Unorthodox 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 *)
type t = A | B of string option

(* file *)
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

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

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 compile.

  • 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.

Dependencies (2)

  1. ocaml >= "4.08"
  2. dune >= "3.0"

Dev Dependencies (1)

  1. odoc with-doc

Used by (2)

  1. goblint >= "2.0.0"
  2. lsp = "1.14.2"




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