#+TITLE: Scala layer

#+TAGS: general|layer|multi-paradigm|programming

[[file:img/scala.png]] with [[file:img/scalameta.png]]

* Table of Contents                     :TOC_5_gh:noexport:
- [[#description][Description]]
  - [[#features][Features:]]
- [[#layer-installation][Layer Installation]]
- [[#backends][Backends]]
  - [[#metals][Metals]]
- [[#scalastyle][Scalastyle]]
  - [[#use-java-doc-style][Use Java doc-style]]
- [[#enable-debug-adapter-protocol-dap][Enable Debug Adapter Protocol (DAP)]]
- [[#automatically-insert-asterisk-in-multiline-comments][Automatically insert asterisk in multiline comments]]
- [[#enable-gtags-as-a-fallback-navigation-utility][Enable GTags as a fallback navigation utility]]
- [[#key-bindings][Key bindings]]
  - [[#sbt][sbt]]

* Description
This layer adds support for the Scala language to Spacemacs.

** Features:
- Syntax highlighting
- Support for language backend using LSP and [[https://scalameta.org/metals/][Metals]]
- Debugging support via =dap=
- Auto-completion
- Treeview support for viewing project structure and triggering compilation
- Syntax-checking
- Refactoring
- Incremental compilation
- Style linting
- Optional GGTags search

* Layer Installation
To use this configuration layer, add it to your =~/.spacemacs=. You will need to
add =scala= to the existing =dotspacemacs-configuration-layers= list in this
file.
+ To turn off the Metals tree view side bar, set =scala-auto-treeview= to =nil=.
+ To display SBT in a small buffer at the bottom of the frame, set the
  =scala-sbt-window-position= layer variable to =bottom=.

* Backends
The only currently supported language backend is =scala-metals=. Support for
Ensime has been dropped as that project has been dead for some time.

** Metals
Currently, you must manually install the Metals server. It is possible to do so
via coursier; the latest version can be built using the following commands,
where =0.9.8= can be replaced with the current version of [[https://scalameta.org/metals/docs/editors/emacs.html][Metals]]:

#+BEGIN_SRC bash
  ./coursier bootstrap \
    --java-opt -Xss4m \
    --java-opt -Xms100m \
    --java-opt -Dmetals.client=emacs \
    org.scalameta:metals_2.12:0.9.8 \
    -r bintray:scalacenter/releases \
    -r sonatype:snapshots \
    -o /usr/local/bin/metals-emacs -f
#+END_SRC

Notice that the layer by default overwrites Metals-scala tree view to nil
#+begin_example elisp
(defvar scala-auto-treeview nil
#+end_example
This to avoid issues with buffers when rendering VSCode like view, issue described [[https://github.com/syl20bnr/spacemacs/pull/14470][Here]].

You will then have the common LSP key bindings; see
[[https://github.com/syl20bnr/spacemacs/tree/develop/layers/%2Btools/lsp#key-bindings][LSP#key-bindings]] for more details.

* Scalastyle
[[http://www.scalastyle.org/][Scalastyle]] provides style-checking and linting. The Emacs functionality is
provided by Flycheck.

To use scalastyle, it must be present as an executable in your =PATH=.
- macOS users: =brew install scalastyle=
- Linux, please see [[http://www.scalastyle.org/command-line.html]]

To test if =scalastyle= executable is in your path, run =scalastyle= in a new
terminal, it should output something like:

#+BEGIN_SRC bash
  $ scalastyle
  scalastyle 0.8.0
  Usage: scalastyle [options] <source directory>
  ...
#+END_SRC

Finally, enable the =syntax-checking= layer and set the =flycheck-scalastylerc=
variable to a valid location.

#+BEGIN_SRC emacs-lisp
  (setq-default flycheck-scalastylerc "/usr/local/etc/scalastyle_config.xml")
#+END_SRC

See the [[http://www.flycheck.org/en/latest/languages.html?highlight=scala#syntax-checker-scala-scalastyle][flycheck documentation]] and [[http://www.scalastyle.org/configuration.html][scalastyle configuration]] for up-to-date
configuration instructions.

** Use Java doc-style
To enable =java-doc-style=, set the variable =scala-indent:use-javadoc-style= to
=t=

#+BEGIN_SRC emacs-lisp
  (setq-default dotspacemacs-configuration-layers '(
    (scala :variables scala-indent:use-javadoc-style t)))
#+END_SRC

* Enable Debug Adapter Protocol (DAP)
The metals backend enables integration with the DAP layer for debugging support.

* Automatically insert asterisk in multiline comments
To insert a leading asterisk in multiline comments automatically, set the
variable =scala-auto-insert-asterisk-in-comments= to =t=.

#+BEGIN_SRC emacs-lisp
  (setq-default dotspacemacs-configuration-layers '(
    (scala :variables scala-auto-insert-asterisk-in-comments t)))
#+END_SRC

* Enable GTags as a fallback navigation utility
To enable gtags when in =scala-mode= set the variable =scala-enable-gtags= to =t=.

#+BEGIN_SRC emacs-lisp
  (setq-default dotspacemacs-configuration-layers '(
    (scala :variables scala-enable-gtags t)))
#+END_SRC

* Key bindings
Additional major mode key bindings are populated by LSP and DAP.

** sbt

| Key binding | Description         |
|-------------+---------------------|
| ~SPC m b .~ | sbt transient state |
| ~SPC m b b~ | sbt command         |