M-x Eglot

Emacs Polyglot. Emacs client to Language Server Protocol servers. Scroll down this README for some pretty gifs.

Installation and usage

Eglot is in both ELPA and MELPA. Installation is straightforward:

(package-install 'eglot) ; Requires Emacs 26!
;; Now find some source file, any source file
M-x eglot

That's it. If you're lucky, this guesses the LSP executable to start for the language of your choice. Otherwise, it prompts you to enter one:

M-x eglot can guess and work out-of-the-box with these servers:

• Javascript's javascript-typescript-stdio
• Rust's rls
• Python's pyls
• Ruby's solargraph
• Bash's bash-language-server
• PHP's php-language-server
• C/C++'s cquery
• Haskell's IDE engine
• Kotlin's kotlin-language-server
• Golang's go-langserver

I'll add to this list as I test more servers. In the meantime you can customize eglot-server-programs:

(add-to-list 'eglot-server-programs '(foo-mode . ("foo-language-server" "--args")))

Let me know how well it works and we can add it to the list. You can also enter a server:port pattern to connect to an LSP server. To skip the guess and always be prompted use C-u M-x eglot.

Connecting automatically

You can also do:

  (add-hook 'foo-mode-hook 'eglot-ensure)

, to attempt to start an eglot session automatically everytime a foo-mode buffer is visited.

Connecting via TCP

The examples above use a "pipe" to talk to the server, which works fine on Linux and OSX but in some cases may not work on Windows.

To circumvent this limitation, or if the server doesn't like pipes, you can use C-u M-x eglot and give it server:port pattern to connect to a previously started TCP server serving LSP information.

If you don't want to start it manually every time, you can configure Eglot to start it and immediately connect to it. Ruby's solargraph server already works this way out-of-the-box.

For another example, suppose you also wanted start Python's pyls this way:

(add-to-list 'eglot-server-programs
             `(python-mode . ("pyls" "-v" "--tcp" "--host"
                              "localhost" "--port" :autoport))))

You can see that the element associated with python-mode is now a more complicated invocation of the pyls program, which requests that it be started as a server. Notice the :autoport symbol in there: it is replaced dynamically by a local port believed to be vacant, so that the ensuing TCP connection finds a listening server.

Commands and keybindings

Here's a summary of available commands:

• M-x eglot, as described above;

• M-x eglot-reconnect reconnects to the server;

• M-x eglot-shutdown says bye-bye to the server;

• M-x eglot-rename ask the server to rename the symbol at point;

• M-x eglot-format asks the server to format buffer or the active region;

• M-x eglot-code-actions asks the server for any code actions at point. These may tipically be simple fixes, like deleting an unused variable, or fixing an import. Left click on diagnostics to check if there are any there;

• M-x eglot-help-at-point asks the server for help for symbol at point. Currently this is what eldoc-mode displays in the echo area;

• M-x eglot-events-buffer jumps to the events buffer for debugging communication with the server.

• M-x eglot-stderr-buffer if the LSP server is printing useful debug information in stderr, jumps to a buffer with these contents.

• M-x eglot-signal-didChangeConfiguration updates the LSP server configuration according to the value of the variable eglot-workspace-configuration, which you may be set in a .dir-locals file, for example.

There are no keybindings specific to Eglot, but you can bind stuff in eglot-mode-map, which is active as long as Eglot is managing a file in your project. The commands don't need to be Eglot-specific, either:

(define-key eglot-mode-map (kbd "C-c h") 'eglot-help-at-point)
(define-key eglot-mode-map (kbd "<f6>") 'xref-find-definitions)

How does this work exactly?

M-x eglot starts a server via a shell-command guessed from eglot-server-programs, using the current major-mode (for whatever language you're programming in) as a hint.

If the connection is successful, you see an [eglot:<server>] indicator pop up in your mode-line. More importantly, this means current and future file buffers of that major mode inside your current project automatically become "managed" by the LSP server, i.e. information about their contents is exchanged periodically to provide enhanced code analysis via xref-find-definitions, flymake-mode, eldoc-mode, completion-at-point, among others.

To "unmanage" these buffers, shutdown the server with M-x eglot-shutdown.

Supported Protocol features (3.6)

General

• initialize
• initalized
• shutdown
• exit
• $/cancelRequest

Window

• window/showMessage
• window/showMessageRequest
• window/logMessage
• telemetry/event

Client

• client/registerCapability (but only workspace/didChangeWatchedFiles, like RLS asks)
• client/unregisterCapability (ditto)

Workspace

• workspace/workspaceFolders (3.6.0)
• workspace/didChangeWorkspaceFolders (3.6.0)
• workspace/didChangeConfiguration
• workspace/configuration (3.6.0)
• workspace/didChangeWatchedFiles
• workspace/symbol
• workspace/executeCommand
• workspace/applyEdit

Text Synchronization

• textDocument/didOpen
• textDocument/didChange (incremental or full)
• textDocument/willSave
• textDocument/willSaveWaitUntil
• textDocument/didSave
• textDocument/didClose

Diagnostics

• textDocument/publishDiagnostics

Language features

• textDocument/completion
• completionItem/resolve (works quite well with company-mode)
• textDocument/hover
• textDocument/signatureHelp (fancy stuff with Python's pyls)
• textDocument/definition
• textDocument/typeDefinition (3.6.0)
• textDocument/implementation (3.6.0)
• textDocument/references
• textDocument/documentHighlight
• textDocument/documentSymbol
• textDocument/codeAction
• textDocument/codeLens
• codeLens/resolve
• textDocument/documentLink
• documentLink/resolve
• textDocument/documentColor
• textDocument/colorPresentation (3.6.0)
• textDocument/formatting
• textDocument/rangeFormatting
• textDocument/onTypeFormatting
• textDocument/rename

Obligatory animated gif section

Differences to lsp-mode.el

Eglot and lsp-mode.el share a common goal, which is to bring LSP to Emacs. lsp-mode.el is a more mature extension with a host of plugins for bells and whistles. Eglot may still lag it in some aspects, but the gap is closing as more features make it into Eglot and more servers are supported out-of-the-box.

Conversely, you may find Eglot surpasses lsp-mode.el in other aspects, namely simplicity. Eglot is considerably less code and hassle than lsp-mode.el. In most cases, there's nothing to configure. It's a minimalist approach focused on user experience and performance.

User-visible differences:

• The single most visible difference is the friendly entry point M-x eglot, not M-x eglot-<language>. Also, there are no eglot-<language> extra packages.

• There's no "whitelisting" or "blacklisting" directories to languages. M-x eglot starts servers to handle file of a major mode inside a specific project, using Emacs's built-in project.el library to discover projects. Then it automatically detects current and future opened files under that project and syncs with server;

• Easy way to quit/restart a server, just middle/right click on the connection name;

• Pretty interactive mode-line section for live tracking of server communication;

• Automatically restarts frequently crashing servers (like RLS);

• Slow-to-start servers start asynchronously in the background;

• Server-initiated edits are confirmed with the user;

• Diagnostics work out-of-the-box (no flycheck.el needed);

• Smoother/more responsive (read below).

Under the hood:

• Message parser is much simpler.
• Defers signature requests like textDocument/hover until server is ready.
• Sends textDocument/didChange for groups of edits, not one per each tiny change.
• Easier to read and maintain elisp. Yeah I know, very subjective, so judge for yourself.
• Doesn't require anything other than Emacs, but will automatically upgrade to work with stuff outside Emacs, like company, markdown-mode, if you happen to have these installed.
• Has automated tests that check against actual LSP servers.