I've been using emacs for a long time, and it's mostly pretty unexceptional. A few months ago, I started using LSP mode, and it's been pretty great... mostly. This post reflects on some of the dustier corners, and some of the broader implications.

History and Conflict

Language Server is a cool idea, that (as far as I know) grew out of VS Code, and provides a way to give a text editor "advanced, IDE-like" features without having to implement those features per-language in the text editor. Instead, editor developers implement generic integration for the protocol, and language developers produce a single process that can process source code and supports the editor features. Sounds great!

There are challenges:

  • The LSP protocol uses JSON for encoding data between the server and emacs. This is a reasonable choice, and makes development easy, but it's not a perfect fit. Primarily, in emacs, until version 27, which hasn't reached an official release yet (but is quite usable on Linux), parsed JSON in lisp rather than C, which made everything rather slow, but emacs 27 helps a lot. Second,

  • There is some overhead for the lsp, particularly as you attempt to scale very large numbers of files or very large files. Emacs is really good with large files, as an artifact of having had to support editing larger files on computers 20 or 30 years ago, and lsp feels sluggish in these situations.

    I think some of this overhead is probably depends on the implementation of the language server's themselves, and some percentage is probably protocol itself: http's statelessness is great in big distributed web services, and for the local use cases where responsiveness matters, it's less good. Similarly JSON's transparency and ubiquity is great, but for streaming streaming-type use cases, perhaps less so.

I've not poked around the protocol, or any implementations, extensively, so I'm very prepared to be wrong about these points.

There's also some tension about moving features, like support for adding editor support for a language, out of lisp and into an external process. While I'm broadly sympathetic, and think there are a bunch of high-quality language integrations in emacs lisp, and for basic things, the emacs lisp ones might be faster. Having said that, there's a lot of interface inconsistency between different support for languages in emacs, and LSP support feels spiffy my comparisons. Also at least for python (jedi,) go (gocode), and C/C++ (irony,) lisp (slime,) the non-lsp services were already using external processes for some portion of their functionality.

It's also the case that if people spend their emacs-lisp-writing time doing things other than writing support for various development environments it's all for the better anyway.

Suggestions / Notes

  • things will break or get wedged, call lsp-restart-workspace to restart the server (I think.) It helps.
  • use lsp-file-watch-ignored to avoid having the language servers process emacs process files from vendored code, intermediate/generated code, or similar, in cases where you might have a large number of files that aren't actually material to your development work
  • I believe some (many? all?) servers are sourced from the PATH (e.g. clang/llvm, gopls), which means you are responsible for updating them and their dependencies.
  • I've found it useful to update lsp-mode itself more often than I typically update other emacs packages. I also try and keep the language servers as up to date as possible in coordination with lsp-mode updates.
  • If Emacs 27 can work on your system, it's noticeably faster. Do that.
  • If you have an existing setup for a language environment, the LSP features end up layering over existing functionality, and that can have unexpected results.

My Configuration

None of this is particularly exciting, but if you use use-package, then the following might be an interesting starting point. I stole a lot of this from other places, so shrug.

I must say that I don't really use dap or treemacs, because I tend to keep things pretty barebones.

(use-package helm-lsp
  :ensure t
  :after (lsp-mode)
  :commands (helm-lsp-workspace-symbol)
  :init (define-key lsp-mode-map [remap xref-find-apropos] #'helm-lsp-workspace-symbol))

(use-package lsp-mode
  :diminish (lsp-mode . "lsp")
  :bind (:map lsp-mode-map
    ("C-c C-d" . lsp-describe-thing-at-point))
  :hook ((python-mode . #'lsp-deferred)
    (js-mode . #'lsp-deferred)
    (go-mode-hook . #'lsp-deferred))
  (setq lsp-auto-guess-root t       ; Detect project root
   lsp-log-io nil
   lsp-enable-indentation t
   lsp-enable-imenu t
   lsp-keymap-prefix "C-l"
   lsp-file-watch-threshold 500
   lsp-prefer-flymake nil)      ; Use lsp-ui and flycheck

  (defun lsp-on-save-operation ()
    (when (or (boundp 'lsp-mode)
         (bound-p 'lsp-deferred))

(use-package lsp-clients
  :ensure nil
  :after (lsp-mode)
  :init (setq lsp-clients-python-library-directories '("/usr/local/" "/usr/")))

(use-package lsp-ui
  :ensure t
  :after (lsp-mode)
  :commands lsp-ui-doc-hide
  :bind (:map lsp-ui-mode-map
         ([remap xref-find-definitions] . lsp-ui-peek-find-definitions)
         ([remap xref-find-references] . lsp-ui-peek-find-references)
         ("C-c u" . lsp-ui-imenu))
  :init (setq lsp-ui-doc-enable t
         lsp-ui-doc-use-webkit nil
         lsp-ui-doc-header nil
         lsp-ui-doc-delay 0.2
         lsp-ui-doc-include-signature t
         lsp-ui-doc-alignment 'at-point
         lsp-ui-doc-use-childframe nil
         lsp-ui-doc-border (face-foreground 'default)
         lsp-ui-peek-enable t
         lsp-ui-peek-show-directory t
         lsp-ui-sideline-update-mode 'line
         lsp-ui-sideline-enable t
         lsp-ui-sideline-show-code-actions t
         lsp-ui-sideline-show-hover nil
         lsp-ui-sideline-ignore-duplicate t)
  (add-to-list 'lsp-ui-doc-frame-parameters '(right-fringe . 8))

  ;; `C-g'to close doc
  (advice-add #'keyboard-quit :before #'lsp-ui-doc-hide)

  ;; Reset `lsp-ui-doc-background' after loading theme
  (add-hook 'after-load-theme-hook
       (lambda ()
         (setq lsp-ui-doc-border (face-foreground 'default))
         (set-face-background 'lsp-ui-doc-background
                              (face-background 'tooltip))))

  ;; WORKAROUND Hide mode-line of the lsp-ui-imenu buffer
  ;; @see https://github.com/emacs-lsp/lsp-ui/issues/243
  (defadvice lsp-ui-imenu (after hide-lsp-ui-imenu-mode-line activate)
    (setq mode-line-format nil)))

;; Debug
(use-package dap-mode
  :diminish dap-mode
  :ensure t
  :after (lsp-mode)
  :functions dap-hydra/nil
  :bind (:map lsp-mode-map
         ("<f5>" . dap-debug)
         ("M-<f5>" . dap-hydra))
  :hook ((dap-mode . dap-ui-mode)
    (dap-session-created . (lambda (&_rest) (dap-hydra)))
    (dap-terminated . (lambda (&_rest) (dap-hydra/nil)))))

(use-package lsp-treemacs
  :after (lsp-mode treemacs)
  :ensure t
  :commands lsp-treemacs-errors-list
  :bind (:map lsp-mode-map
         ("M-9" . lsp-treemacs-errors-list)))

(use-package treemacs
  :ensure t
  :commands (treemacs)
  :after (lsp-mode))