Emacs and Scala setup with Eglot

Needless to say, the best way to get Scala working with “language-server” compatible integrations is to use Metals LSP. You can read more about it on their website and how it integrates with several editors. I usually install it with coursier doing something along the lines of:

coursier bootstrap --java-opt -XX:+UseG1GC --java-opt -XX:+UseStringDeduplication --java-opt -Xss4m --java-opt -Xms100m --java-opt -Dmetals.client=emacs org.scalameta:metals_2.13:0.11.12 -o $HOME/.local/bin/metals-emacs -f

Note: ensure that $HOME/.local/bin is in your path and Emacs will automagically pick it up.

I am enjoying using Elpaca everyday. It’s the most advanced, reliable and performant package manager ever made for Emacs. It integrates also seamlessly with use-package and pushes you into doing things in a maintainable way. Read more about it here. You can still learn from my config even if you don’t use it, but I do whole-heartedly recommend it.

I whole-heartedly recommend eglot. Eglot, a.k.a. Emacs Polyglot is the Emacs LSP client that stays out of your way. You can read more about it here.

Eglot is to be integrated into Emacs core, and is more performant and minimalistic than lsp-mode. I like to immediately start eglot when opening a Scala file.

(use-package eglot
  :hook (
         (scala-mode . eglot-ensure)
         ;; other modes go here
         ;; e.g.
         ;; (haskell-mode . eglot-ensure)
         ))

Once you install it (or load it, since in new Emacs versions it should be built-in), you will need to configure certain things to ensure it can work together with Scala Metals, the Scala language server.

Here we add a wonderful functionality of optimizing imports and formatting code on save.

(defun my-eglot-organize-imports () (interactive)
        (eglot-code-actions nil nil "source.organizeImports" t))
 (add-hook 'before-save-hook 'my-eglot-organize-imports nil t)
 (add-hook 'before-save-hook 'eglot-format-buffer)

For all my syntax highlighting and syntax tree needs I use tree-sitter, which soon will also be built into Emacs.

(use-package tree-sitter
   :config
   (global-tree-sitter-mode))

(use-package tree-sitter-langs
  :after tree-sitter)

Enable scala-mode for indentation and certain language rules, with Treesitter highlighting.

(use-package scala-mode
  :interpreter ("scala" . scala-mode)
  :hook (scala-mode . tree-sitter-hl-mode)
  )

Flycheck is an enhancement over flymake and helps better visualize and interact with errors and warnings in your Emacs buffers.

(use-package flycheck :config (global-flycheck-mode))

For integrating Flycheck and Eglot, check out the cool flycheck-eglot package.

For all my auto-complete needs I use company which works wonderfully and is highly customizable.

(use-package company
  :custom
  (company-minimum-prefix-length 1)
  (company-idle-delay 0.5)
  (company-tooltip-align-annotations t)
  (company-tooltip-margin 2)
  :config
  (global-company-mode))

The company-quickhelp package will help to visualize available documentation right in the auto-completion tooltip, a feature present in many modern IDEs.

(use-package company-quickhelp
  :after (company)
  :config
  (company-quickhelp-mode))

With all of this setup you will be able to have a productive experience, that surpasses the performance and joy of use of other IDEs. You may want to, on a per project basis, run sbt bloopInstall to get the full experience of Metals LSP. This requires the plugin: addSbtPlugin("ch.epfl.scala" % "sbt-bloop" % "1.5.6").

You may have more success with running the bloopInstall as sbt -Dbloop.export-jar-classifiers=sources bloopInstall.

Also, remember that running M-x eglot* will show all eglot commands available, and teach you some of the shortcuts too.

Feel free to check my dotfiles on SourceHut which might help you get your Emacs setup just right 👌.

Have a nice development day today!