Configuring Your Editor

While the toplevel is great for interactively trying out the language, we will shortly need to write OCaml files in an editor. We already installed the tools required to enhance Merlin, our editor of choice with OCaml support. Merlin provides all features such as "jump to definition," "show type," and ocaml-lsp-server, a server that delivers those features to the editor through the LSP server. OCaml has plugins for many editors, but the most actively maintained are for Visual Studio Code, Emacs, and Vim.

Visual Studio Code

TL;DR Install the packages ocaml-lsp-server and ocamlformat in your opam switch.

For VSCode, install the OCaml Platform Visual Studio Code extension from the Visual Studio Marketplace. The extension depends on OCaml LSP and OCamlFormat. To install them in your switch, you can run:

$ opam install ocaml-lsp-server ocamlformat

Upon first loading an OCaml source file, you may be prompted to select the toolchain in use. Pick the version of OCaml you are using, e.g., 5.1.0 from the list.

Editor Features at Your Disposal

If your editor is setup correctly, here are some important features you can begin using to your advantage:

1) Hovering for Type Information:

VSCode Hovering

This is a great feature that let's you see type information of any OCaml variable or function. All you have to do is place your cursor over the code and it will be displayed in the tooltip.

2) Jump to Definitions With Ctrl + Click:

VSCode Ctrl click

If you hold down the Ctrl key while hovering, the code appears as a clickable link which if clicked takes you to the file where the implementation is. This can be great if you want to understand how a piece of code works under the hood. In this example, hovering and Ctrl + Clicking over the peek method of the Queue module will take you to the definiton of the peek method itself and how it is implemented.

3) OCaml Commands With Ctrl + Shift + P:

VSCode OCaml Commands

Pressing the key combination Ctrl + Shift + P opens a modal dialog at the top. If you type the word ocaml, you will be presented with a list of various OCaml commands at your disposal which can be used for different purposes.

Windows Users

If you used the DkML distribution, you will need to: 1. Go to File > Preferences > Settings view (or press Ctrl ,) 2. Select User > Extensions > OCaml Platform 3. Uncheck OCaml: Use OCaml Env. That's it!

Vim and Emacs

For Vim and Emacs, we won't use the LSP server but rather directly talk to Merlin.

$ opam install merlin

After installing Merlin above, instructions will be printed on how to link Merlin with your editor. If you do not have them visible, just run this command:

$ opam user-setup install

Talking to Merlin

Getting Type Information


Vim Type information

  • In the Vim editor, press the Esc to enter command mode.
  • Place the cursor over the variable.
  • Type :MerlinTypeOf and press Enter.
  • The type information will be displayed in the command bar. Other Merlin commands for Vim are available and you can checkout their usage on the Merlin official documentation for Vim.


Emacs Type information

  • In the Emacs editor, place you cursor over the variable.
  • Use the keyboard shortcut Alt + x followed by merlin-type-enclosing
  • The type information will be displayed in the mini-buffer. Other Merlin commands for Emacs are available and you can checkout their usage on the Merlin Official documentation for Emacs.

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