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VSCode OCaml Platform

Main workflow

Visual Studio Code extension for OCaml and relevant tools.

❗️ You are encouraged to read the Getting started section. The rest of the document assumes you have read it. You may also find Important concepts useful.

If you have issues with the extension and you have read the "Getting Started" section, see Debugging the extension and FAQ below.

Getting started

Installation

Below we first install the extension dependencies and then the extension itself. You can reverse the order; it's just that the extension will not work to its full without all of its dependencies.

  1. Installing extension dependencies

    This VS Code for most of its OCaml language support functionality requires OCaml Language Server (often called ocaml-lsp or ocamllsp). Install ocaml-lsp-server package as usual with a package manager of your choice: OPAM or esy. Installation instructions by package manager are available here.

    Make sure to install the packages in the sandbox (usually, OPAM switch or esy sandbox) you use for compiling your project.

    Optionally:

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

      Note: Formatting support requires having .ocamlformat file in your project root directory.

    • When you hover the cursor over OCaml code, the extension shows you the type of the code. Install ocamlformat to get nicely formatted types.

  2. Install this extension from the VSCode Marketplace. VSCode extension installations instructions are available here.

If you're on Mac or Linux, now you should have everything necessary installed and ready. You can skip the sub-section below named "Windows" and proceed to "Setting up the extension for your project."

On Windows

Install OCaml for Windows and make sure the ocaml-env program is accessible on the PATH (ocaml-env is in the usr/local/bin folder relative to the installation directory).

Setting up the extension for your project

  1. Open your OCaml/ReasonML project (File > Add Folder to Workspace...).

  2. Configure the extension to use the desired sandbox (usually, OPAM switch or esy sandbox). You can pick it by

    • either calling VSCode command "OCaml: Select a Sandbox for this Workspace" (one can do this from VSCode Command Palette - Ctrl+P or on MacOS Cmd+Shift+P)

    • or clicking on the package icon at the bottom of VSCode window and picking your sandbox from the menu

      pick sandbox

    What's a sandbox? In short, the main purpose of a sandbox is to specify how this extension should invoke its dependencies such as ocaml-lsp-server or ocamlformat. For more information on what a sandbox is, see "Sandbox" subsection.

  3. Build your project with Dune to get go-to-definition, auto-completion, etc.

    Important note: OCaml Language Server has its information about the files from the last time your built your project.

    Caveat 1: Because of the note above, during active development of your project, we advise building your project with dune in a polling mode using the option --watch. This rebuilds your project whenever a file is changed in your project. For example, run dune build --watch --terminal-persistence=clear-on-rebuild in your VSCode integrated terminal.

    Caveat 2: Save the currently open file to get latest diagnostics (error and warning squiggly underlining). For example, if you created a module A in some file, and you still get an error that it's "unbound" (i.e., not found) in the current file, save the file to get up-to-date diagnostics, assuming you built your project after adding A or are running build in a polling mode, and make sure that error isn't a stale error.

By this point, you should have a working OCaml development editor ready. If you're on Windows or use ReasonML/ReScript/BuckleScript, see subsections below.

ReasonML / ReScript / BuckleScript

ReasonML, as an alternative syntax for OCaml, is supported out-of-the-box, as long as reason is installed in your environment.

The new ReScript syntax (res and resi files) is not supported, you should use rescript-vscode instead.

If you're looking for a way to use OCaml or ReasonML syntax in a ReScript project, it is no longer supported by this extension.

If you need to compile existing OCaml or ReasonML syntax to JS and use this extension, you can use Melange:

  1. Install esy
npm install esy --global
  1. You can use the Melange basic template to add OCaml LSP support. Then modify esy.json to pin ocaml-lsp-server to version 1.8.3 due to lack of Merlin support in newer versions.
{
  "dependencies": {
    "@opam/ocaml-lsp-server": "1.8.3"
  }
}
  1. Install and build packages
esy

Important Concepts

Sandbox

Sandbox defines environment that the extension sees, for example, to launch ocamllsp, detect OCaml compiler version, or use ocamlformat.

The extension supports 4 kinds of sandboxes:

  1. Global

The extension uses the environment that VS Code was opened in.

  1. OPAM Sandbox

The extension uses the environment defined by the OPAM switch that the user picks. Both global and local OPAM switches are supported.

  1. Esy Sandbox

The extension uses the environment defined by the Esy sandbox that the user picks.

  1. Custom

User can define how they would like to run commands using a (templated) command where $prog and $args strings need to be used to denote how to run an extension dependency and how arguments can be passed. One can imitate an OPAM sandbox using a custom sandbox by passing a command opam exec --switch=4.13.1 --set-switch -- $prog $args -- the extension can then replace $prog with ocamllsp and $args with arguments it wants to pass to ocamllsp.

Features

  • Syntax highlighting
    • ATD
    • Cram tests
    • Dune
    • Menhir
    • Merlin
    • META
    • OASIS
    • OCaml
    • OCamlbuild
    • OCamlFormat
    • OCamllex
    • opam
    • ReasonML
    • Eliom
  • Indentation rules
  • Snippets
    • Dune
    • OCaml
    • OCamllex
  • Task Provider
    • Dune
  • Debugger
    • Earlybird (experimental)

Configuration

This extension provides options in VSCode's configuration settings. You can find the settings under File > Preferences > Settings.

Name Description Default
ocaml.sandbox Determines where to find the sandbox for a given project null
ocaml.dune.autoDetect Controls whether dune tasks should be automatically detected. true
ocaml.trace.server Controls the logging output of the language server. Valid settings are off, messages, or verbose. off
ocaml.useOcamlEnv Controls whether to use ocaml-env (if available) for opam commands from OCaml for Windows. true
ocaml.terminal.shell.linux The path of the shell that the sandbox terminal uses on Linux null
ocaml.terminal.shell.osx The path of the shell that the sandbox terminal uses on macOS null
ocaml.terminal.shell.windows The path of the shell that the sandbox terminal uses on Windows null
ocaml.terminal.shellArgs.linux The command line arguments that the sandbox terminal uses on Linux null
ocaml.terminal.shellArgs.osx The command line arguments that the sandbox terminal uses on macOS null
ocaml.terminal.shellArgs.windows The command line arguments that the sandbox terminal uses on Window null
ocaml.repl.path The path of the REPL that the extension uses null
ocaml.repl.args The REPL arguments that the extension uses null
ocaml.repl.useUtop Controls whether to use Utop for the REPL if it is installed in the current switch. true

If ocaml.terminal.shell.* or ocaml.terminal.shellArgs.* is null, the configured VSCode shell and shell arguments will be used instead.

If ocaml.repl.path or ocaml.repl.args is null, the default REPL is used instead. The default REPL used depends on the packages installed in your current sandbox:

  • If dune build passes and the current sandbox has utop installed, the REPL will be dune utop
  • If dune build fails and the current sandbox has utop installed, the REPL will be utop
  • Else, the REPL will be ocaml

If a REPL already exists, it will be used instead, so if you installed utop after openning a REPL, or if you fixed your project compilation, you will need to re-open the REPL to change it.

Commands

An easy way to see what commands are offered by the extension in the currently open file, you can invoke VSCode Command Palette and search for commands with prefix OCaml::

commands

Name Description Keyboard Shortcuts
ocaml.select-sandbox Select sandbox for this workspace
ocaml.server.restart Restart language server
ocaml.open-terminal Open a terminal (current sandbox)
ocaml.open-terminal-select Open a terminal (select a sandbox)
ocaml.current-dune-file Open Dune File (located in the same folder)
ocaml.switch-impl-intf Switch implementation/interface Alt+O
ocaml.open-repl Open REPL
ocaml.evaluate-selection Evaluate Selection Shift+Enter
ocaml.copy-type-under-cursor Copy the type under the cursor

Debugging OCaml programs (experimental)

Experimental support for debugging OCaml programs is provided via earlybird. Problems with the debugger should be reported at https://github.com/hackwaly/ocamlearlybird.

Two steps to set up debugging:

  1. Install earlybird, which provides the ocamlearlybird executable.

    For newer OCaml version support, opam pin the development version from https://github.com/hackwaly/ocamlearlybird.

  2. If you use dune language version 3.0+ to build your project, switch to 3.7+ and make sure that you add:

    (map_workspace_root false)

    to your dune-project file. More info on this here.

  3. Build bytecode version of your OCaml program executable.

    See dune documentation for further information.

There are three ways to launch the debugger in VS Code:

  1. Navigate to the built OCaml bytecode executable in VS Code Explorer panel (a .bc file in _build directory), right click on it and select "Start OCaml Debugging (experimental)".

    The debugger launches immediately.

  2. If no VS Code launch configurations (the .vscode/launch.json file) exist, then navigate to VS Code Run and Debug panel, click on "create a launch.json file" and select "OCaml earlybird (experimental)".

    Run the created "OCaml earlybird (experimental)" launch configuration to launch the debugger. By default, it asks to open an OCaml bytecode executable (a .bc file in _build directory) to debug. You can hard-code a specific program instead of the default ${command:AskProgram}.

  3. If some VS Code launch configurations exist (in .vscode/launch.json), then open the launch.json file and inside configurations press Ctrl+Space to select the "OCaml earlybird (experimental)" snippet. Then fill in the OCaml bytecode executable path and desired launch configuration name.

    Run the created launch configuration to launch the debugger.

Debugging the extension

Problems with code or file formatting support

If you are experiencing problems with OCaml code support, e.g., you invoke Go to Definition on a symbol (for example, a variable name), but nothing happens, or you hover the cursor over a symbol and don't see its type, the problem is likely to be with OCaml-LSP, which is responsible for code support.

Two steps to see if there are reported problems with OCaml-LSP (which you can either act upon yourself if you can or report the problem in Issues/Discussion of this repository):

  1. Set the ocaml.trace.server setting to verbose in VS Code settings.

trace verbose

  1. Invoke command OCaml: Show OCaml Language Server Output from the VS Code Command Palette. This command shows requests and responses exchanged between this extension and OCaml-LSP. If, for example, Go to Definition is not working when you think it should, you may see some exception logged in this Output View:

    [Error - 10:41:51 PM] Locate failed. File_not_found: 'Fiber' seems to originate from 'Fiber' whose ML file could not be found

    You can also sometimes see explicitly an exception being thrown. Such kind of information in an issue report is usually very helpful in detecting and fixing the problem.

If you face such kind of problems that you cannot resolve on your own, please, check that the issue hasn't been reported yet and report in Issues if necessary.

Note: File formatting is performed on the OCaml-LSP side, so OCaml-LSP may be the culprit in a formatting problem. Make sure, however, that you've read Getting Started section, which describes some initial setup required for file formatting to work.

Problems with this Extension or OPAM support in the Extension

One by one, invoke and see outputs for commands OCaml: Show OCaml Platform Extension Output and OCaml: Show OCaml Commands Output. In these Output Views you may see errors and warnings, which can be handy to detect and fix the problem you're facing. Please, check the issue you're facing hasn't been already reported and report in Issues tab of this repository, if necessary.

Things to include in your Issue report

It is helpful to include information such as

  • Operation system information
  • VS Code version
  • OCaml Platform Extension version
  • OCaml-LSP version
  • Reproducible setup (some codebase, for example, where we can see the bug happening) the problem
  • Output View information described above

FAQ

I installed ocaml-lsp-server, but the extension still cannot find it.

Make sure you installed the the language server in the sandbox used by the extension.

OPAM: If you're using opam, make sure that you're using correct switch when installing the extension by running opam switch to see the current switch and check the sandbox set for the current VSCode workspace (see "Setting up the extension for your project" section to learn more about picking a sandbox for the extension).

I am getting Unbound module ... error. What should I do?

  1. Make sure the module should be visible, e.g., there is no typo in the module name, you added the module to libraries stanza in your dune file, etc.

  2. Make sure you have up-to-date diagnostics (error and warning squiggly underlining). Diagnostics are sent when the file open, when file is edited, and when it is saved. Save the file containing the error to make sure the error isn't stale.

  3. Make sure you have built your project after adding that module to your environment. We suggest adhering to Caveat 1 in "Setting up the extension for your project" section. If you haven't built it, build it and go to step 2.

  4. If you are sure there must be a problem with the extension, file an issue.

In case you have a question or problem not listed above:

  • if you don't understand how to the extension works or how to make it work correctly, create a new discussion in the repository Discussions tab.

  • if the extension seems to misbehave:

    • see Debugging section to see if you can see any reported errors
    • file an issue in the repository Issues tab.

If this section doesn't contain the problem you managed to resolve, and you think this may help others, consider adding the problem and its solution here by creating a Pull Request.

For advanced users

This part of README is only for advanced users, who would like more customization.

Disable code lens

Code lens are type information displayed over a symbol. In the screenshot below, code lens is grey text t -> Sandbox.t.

how code lens look like in VSCode

You can disable code lens for all extensions, i.e., in whole VS Code, set this settings in your settings.json:

  "editor.codeLens": false

Or if you only want to disable it for OCaml:

"[ocaml]": {
	"editor.codeLens": false
}

You can also search for "code lens" in the VSCode settings tab and there will be a checkbox you can untick to disable it:

disable code lens in VSCode

(Credit for this answer goes to @mnxn)

Enable only syntax highlighting (No type-on-hover, go-to-definition, etc.)

The extension does not offer such functionality because it is rarely necessary. A workaround is to not install ocamllsp. As a result you will mostly have just syntax highlighting for OCaml source files but also a warning notification that ocamllsp wasn't found. See this issue, feel free to upvote this issue by leaving a thumbs-up reaction. Pull requests are welcome as well.

Persisting sandbox information

Sandbox information is persisted in .vscode/settings.json. Below we show how this settings file's content may look like with different sandbox options.

  1. Global
{
  "ocaml.sandbox": {
    "kind": "global"
  }
}
  1. OPAM

Global switch

{
  "ocaml.sandbox": {
    "kind": "opam",
    "switch": "ocaml-base-compiler.4.13.1"
  }
}

Local switch

{
  "ocaml.sandbox": {
    "kind": "opam",
    "switch": "/Users/ulugbekna/code/olsp"
  }
}
  1. Esy
{
  "ocaml.sandbox": {
    "kind": "esy",
    "root": "${firstWorkspaceFolder}"
  }
}
  1. Custom
{
  "ocaml.sandbox": {
    "kind": "custom",
    "template": "opam exec -- $prog $args"
  }
}