lsp-mode: 30+ languages in one minor mode

Follow along

These slides are generated from an org file with some speaker notes and code blocks

Available at https://gjp.cc/lsp

wget gjp.cc/lsp-mode.org
# or
git clone github.com/gpittarelli/lsp-presentation

What do we want

IDE features:

• auto-complete
• documentation on hover
• jump to definition
• refactoring menu
• error detection
• project-wide diagnostics

Implement it per-language

• R: ESS
• Clojure: CIDER
• Typescript: tsserver and tide-mode
• C#: omnisharp and omnisharp-emacs

Attempts to generalize

• textmate files: syntax highlighting
• tags files (etags, ctags); GNU global
• ycmd YouCompleteMe

LSP: Language Server Protocol

Implement language diagnostics once, as a server

All editors can share the smarts

Overview: https://langserver.org/

Spec: https://microsoft.github.io/language-server-protocol/specification

LSP: Overview

• RPC: JSON with headers
• single connection (stdio or tcp)
• growing fairly fast: https://github.com/Microsoft/language-server-protocol

LSP: Overview (2)

• Synchronize file/project state
• Query for specific information on hover/completion
• Send commands for refactors; get back text changes to apply

lsp-mode

Minor mode:

• implements an LSP client
• hooks into standard Emacs APIs
• provides API for defining clients

Available on MELPA:

(require 'package)
(package-initialize)
(add-to-list 'package-archives
  '("melpa" . "https://melpa.org/packages/") t)
(package-install 'lsp-mode)

lsp-mode provides

• imenu
• xref
• completion-at-point
• eldoc
• flycheck, fancy UI (lsp-ui)
• company (company-lsp)

How to define a client

(lsp-define-stdio-client
 lsp-css
 "css"
 lsp-css--get-root
 '("css-languageserver" "--stdio"))

language packages

(package-install 'lsp-css)

Example setup

(defun my-rust-mode-setup ()
  (company-mode)
  (lsp-rust-enable)
  (eldoc-mode t)
  (flycheck-mode)
  (lsp-ui-mode))
(add-hook 'rust-mode-hook #'my-rust-mode-setup)
(add-to-list 'auto-mode-alist '("\\.rs" . rust-mode))

Testing

• ert
• lsp-java: cucumber tests
• emacs-lsp-acceptance-tests

Protocol parsing in emacs

• frankly, hacky (manual dechunking)
• process filters and lots of manual parsing
• alternative: just use buffers

Future goal: autosetup

Based on major mode and/or file extension, we should be able to automatically:

• download the server
• install the necessary lsp-plugins
• provide good defaults

Alternatives

Note there's a couple defunct "emacs-lsp", etc. packages on GitHub from previous attempts by other people.

eglot is the only reasonably working alternative