OCaml-LSP
OCaml-LSP is a language server for OCaml that implements Language Server Protocol (LSP).
If you use Visual Studio Code, see OCaml Platform extension page for detailed instructions on setting up your editor for OCaml development with OCaml-LSP: what packages need to be installed, how to configure your project and get most out of the OCaml editor support, and how to report and debug problems.
- Installation
- Usage
- Features
- Debugging
- Contributing to project
- Tests
- Relationship to Other Tools
- History
- Comparison to other LSP Servers for OCaml
Installation
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
Opam
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.
Esy
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 withocamlformat
(version > 0.21.0). Previously, it was a standalone package.
Usage
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
since OCaml-LSP 1.11.0
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:
-
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.
-
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 actionPromote
.
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.
Features
The server supports the following LSP requests (inexhaustive list):
-
textDocument/completion
-
completionItem/resolve
-
textdocument/hover
-
textDocument/signatureHelp
-
textDocument/declaration
-
textDocument/definition
-
textDocument/typeDefinition
-
textDocument/implementation
-
textDocument/codeLens
-
textDocument/documentHighlight
-
textDocument/documentSymbol
-
textDocument/references
-
textDocument/documentColor
-
textDocument/colorPresentation
-
textDocument/formatting
-
textDocument/rangeFormatting
-
textDocument/onTypeFormatting
-
textDocument/prepareRename
-
textDocument/foldingRange
-
textDocument/selectionRange
-
workspace/symbol
Note that degrees of support for each LSP request are varying.
Semantic highlighting
since OCaml-LSP 1.15.0 (since version
1.15.0-4.14
for OCaml 4,1.15.0-5.0
for OCaml 5)
Semantic highlighting support is enabled by default.
since OCaml-LSP 1.14.0
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": {
"OCAMLLSP_SEMANTIC_HIGHLIGHTING": "full"
},
}
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
since OCaml-LSP 1.0.0
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
since OCaml-LSP 1.8.0
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)
since OCaml-LSP 1.8.0
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
Debugging
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
Changelog
User-visible changes should come with an entry in the changelog under the appropriate part of the unreleased section. PR that doesn't provide an entry will fail CI check. This behavior can be overridden by using the "no changelog" label, which is used for changes that are not user-visible.
Tests
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.
History
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.