Using LSP in Emacs and Debian

The Language Server Protocol (LSP) is a neat mechanism that provides a common interface to what used to be language-specific lookup mechanisms (like, say, running a Python interpreter in the background to find function definitions).

There is also ctags shipped with UNIX since forever, but that doesn't support looking backwards ("who uses this function"), linting, or refactoring. In short, LSP rocks, and how do I use it right now in my editor of choice (Emacs, in my case) and OS (Debian) please?

Editor (emacs) setup

First, you need to setup your editor. The Emacs LSP mode has pretty good installation instructions which, for me, currently mean:

apt install elpa-lsp-mode

and this .emacs snippet:

(use-package lsp-mode
  :commands (lsp lsp-deferred)
  :hook ((python-mode go-mode) . lsp-deferred)
  :demand t
  :init
  (setq lsp-keymap-prefix "C-c l")
  ;; TODO: https://emacs-lsp.github.io/lsp-mode/page/performance/
  ;; also note re "native compilation": <+varemara> it's the
  ;; difference between lsp-mode being usable or not, for me
  :config
  (setq lsp-auto-configure t))

(use-package lsp-ui
  :config
  (setq lsp-ui-flycheck-enable t)
  (add-to-list 'lsp-ui-doc-frame-parameters '(no-accept-focus . t))
  (define-key lsp-ui-mode-map [remap xref-find-definitions] #'lsp-ui-peek-find-definitions)
  (define-key lsp-ui-mode-map [remap xref-find-references] #'lsp-ui-peek-find-references))

Note: this configuration might have changed since I wrote this, see my init.el configuration for the most recent config.

The main reason for choosing lsp-mode over eglot is that it's in Debian (and eglot is not). (Apparently, eglot has more chance of being upstreamed, "when it's done", but I guess I'll cross that bridge when I get there.)

I already had lsp-mode partially setup in Emacs so I only had to do this small tweak to switch and change the prefix key (because s-l or mod is used by my window manager). I also had to pin LSP packages to bookworm here so that it properly detects pylsp (the older version in Debian bullseye only supports pyls, not packaged in Debian).

This won't do anything by itself: Emacs will need something to talk with to provide the magic. Those are called "servers" and are basically different programs, for each programming language, that provide the magic.

Servers setup

The Emacs package provides a way (M-x lsp-install-server) to install some of them, but I prefer to manage those tools through Debian packages if possible, just like lsp-mode itself. Those are the servers I currently know of in Debian:

package	languages
ccls	C, C++, ObjectiveC
clangd	C, C++, ObjectiveC
elpa-lsp-haskell	Haskell
fortran-language-server	Fortran
gopls	Golang
python3-pyls	Python

There might be more such packages, but those are surprisingly hard to find. I found a few with apt search "Language Server Protocol", but that didn't find ccls, for example, because that just said "Language Server" in the description (which also found a few more pyls plugins, e.g. black support).

Note that the Python packages, in particular, need to be upgraded to their bookworm releases to work properly (here). It seems like there's some interoperability problems there that I haven't quite figured out yet. See also my Puppet configuration for LSP.

Finally, note that I have now completely switched away from Elpy to pyls, and I'm quite happy with the results. lsp-mode feels slower than elpy but I haven't done any of the performance tuning and this will improve even more with native compilation. And lsp-mode is much more powerful. I particularly like the "rename symbol" functionality, which ... mostly works.

Remaining work

Puppet and Ruby

I still have to figure how to actually use this: I mostly spend my time in Puppet these days, there is no server listed in the Emacs lsp-mode language list, but there is one listed over at the upstream language list, the puppet-editor-services server.

But it's not packaged in Debian, and seems somewhat... involved. It could still be a good productivity boost. The Voxpupuli team have vim install instructions which also suggest installing solargraph, the Ruby language server, also not packaged in Debian.

Bash

I guess I do a bit of shell scripting from time to time nowadays, even though I don't like it. So the bash-language-server may prove useful as well.

Other languages

Here are more language servers available:

Created Wednesday après-midi, April 27th, 2022. Edited Wednesday après-midi, April 27th, 2022.

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