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A new scala-mode for emacs24

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scala-mode2 — A new scala-mode for emacs

This is a new scala major mode for emacs 24. It is a complete rewrite based on the Scala Language Specification 2.9.

The mode intends to provide the basic emacs support, including

  • indenting of code, comments and multi-line strings
  • motion commands
  • highlighting

Currently the indenting of code has been finalized. Highlighting is under work. No scala specific motion commands have been added, but standard emacs motions work ofcourse.

Setting the mode up for use

  1. Make sure you have the latest version of GNU Emacs installed. The mode has been developed on 24.2 and uses features not available in emacs prior to version 24.

  2. There are two mechanisms that can be used for the installation of the mode into Emacs. The preferred manner is to use the built-in package manager of Emacs 24 (i.e. package.el) and the other is to manually clone the git repository, add the path to the mode to the load-path and then to require it. For more information regarding package.el please refer to the EmacsWiki.

    1. Package.el: Using the package.el within Emacs installation is the recommended manner to install scala-mode2 as it allows for continuous, easy updating from within Emacs itself.

      Adding the MELPA or Marmalade repository to your emacs initialization will be required to locate the packages.

      Add the following to your emacs config (.emacs, init.el, etc), and if such a definition already exists, ensure that it contains the MELPA or Marmalade declaration, for example:

      (require 'package)
      (add-to-list 'package-archives
                   '("melpa" . "") t)
      (unless (package-installed-p 'scala-mode2)
        (package-refresh-contents) (package-install 'scala-mode2))

      or you could use customize to add a repository:

      M-x customize-variable [RET] package-archives

      and add MELPA or Marmalade, for example:


      and then use package install to install it:

      M-x package-install [RET] scala-mode2 [RET]
    2. Manual: Download the files to a local directory. You can use the git clone command, this will create a new directory called scala-mode2.

      git clone git://

      Include the following in your Emacs config file. If you have been using the old scala-mode, make sure it is no longer in load-path.

      (add-to-list 'load-path "/path/to/scala-mode2/")
      (require 'scala-mode2)
  3. That's it. Next you can start emacs and take a look at the customization menu for scala-mode (use M-x customize-mode when in scala-mode or use M-x customize-variable to customize one variable). Also be sure to check the customization tips on various keyboard commands and general emacs parameters which cannot be modified from the scala-mode customization menu.

    For scala console (aka REPL) or sbt support, see sbt-mode.

Indenting modes

Where four developers meet, there are four opinions on how code should be indented. Luckily scala-mode already supports 2^4 different ways of indenting.

Run-on lines (scala-indent:default-run-on-strategy)

The indenting engine has three modes for handling run-on lines. The reluctant (default) mode is geared toward a general style of coding and the eager for strictly functional style. A third mode called operators is between the two.

The difference between the modes is how they treat run-on lines. For example, the eager mode will indent map in the following code

val x = List(1, 2, 3)
  map(x => x + 1)

The operators and eager modes will indent the second row in the following code, as the first line ends with an operator character.

val x = 20 +

The reluctant mode (default) will not indent the line in either case. However, all three modes will indent the second line in these examples as it is clear that the first line cannot terminate a statement (see the Scala Language Specification 2.9, section 1.2).

val x = List(0, 1, 2, 3, 4, 5, 6, 7, 8, 9).
  map (x => x + 1) // last token of previous line cannot terminate a statement

val y = (List(0, 1, 2, 3, 4, 5, 6, 7, 8, 9)
           map (x => x + 1)) // inside 'newlines disabled' region

You can use empty lines in the eager mode to stop it from indenting a line. For example

val x = foo("bar")
           ("zot", "kala") // indented as curry

val y = foo("bar")

("zot", "kala") // a tuple

However, in all three modes pressing the tab key repeatedly on a line will toggle between the modes.

Value expressions (scala-indent:indent-value-expression)

When this variable is set to nil (default), body of a value expressions will be indented in the traditional way.

val x = try {
} catch {
  case e => other
} finally {

However, when the variable is set to t, the body will be indented one extra step to make the val, var or def stand out. For example:

val x = try {
  } catch {
    case e => other
  } finally {

Parameter lists (scala-indent:align-parameters)

When this variable is set to nil (default), parameters and run-on lines in parameter lists will not align under or acording to the first parameter.

val y = List( "Alpha", "Bravo",
  "Charlie" )

val x = equals(List(1,2,3) map (x =>
  x + 1))

When the variable is set to t, the same will be indented as:

val y = List( "Alpha", "Bravo",
              "Charlie" )

val x = equals(List(1,2,3) map (x =>
                 x + 1))

Expression forms: if, for, try (scala-indent:align-forms)

When this variable is set to nil (default), if, for and try forms are not aligned specially.

val x = if (kala)
else if (koira)

val x = try "1".toInt
catch { case e => 0}
finally { println("hello") }

val xs = for (i <- 1 to 10)
yield i

When the variable is set to t, the same will be indented as:

val x = if (kala)
        else if (koira)

val x = try "1".toInt
        catch { case e => 0}
        finally { println("hello") }

val xs = for (i <- 1 to 10)
         yield i

Indenting multi-line comments

The start of a multi-line comment is indented to the same level with code.

By default, if a multi-line comment begins with /** it is considered to be a Scaladoc comment. Scaladoc comments are indented according to the Scaladoc style guide.

/** This is a Scaladoc comment.
  * 2nd line.

Alternatively, if the configurable variable scala-indent:use-javadoc-style is set to t, multi-line comments beginning with /** will be indented according to the Javadoc style, wherein all following lines are indented under the first asterisk.

 * This is a Javadoc-style comment.
 * 2nd line.

All other multi-line comments are indented under the first asterisk.

 * Supercalifragilistic-
 * expialidocious!

 A comment

Typing an asterisk in multi-line comment region, at the start of a line, will trigger indent. Furthermore, if the configurable variable scala-indent:add-space-for-scaladoc-asterisk is t (default) and the asterisk was the last character on the line, a space will be inserted after it. If you type a forward slash after the automatically inserted space, the space is deleted again so that you can end the comment without deleting the space manually.

Filling (i.e. word wrap)

Paragraph filling (emacs jargon for word wrap) is supported for comments and multi-line strings. Auto-fill is not supported yet.

To re-fill a paragraph, use the fill-paragraph command ( M-q ). As always, the column at which to wrap is controlled by the fill-column variable, which you set it with the set-fill-column command. To set the default, you use the customize-variable command or a mode-hook.

Joinin lines (delete indentation)

Scala-mode defines its own scala-indent:join-line' function. Besides doing what the normal *join-line (aka delete-indentation) function does, it also removes comment marks (asterisks and slashes) when comment lines are joined and space when code lines are joined and the uppper line ended with a dot.

In scala-mode2 buffers scala-indent:join-line replaces delete-indentation in your key bindings. The default binding is M-^.


Basic emacs motion will work as expected.

Text paragraph motion (i.e. forward-paragraph, backward-paragraph) works inside comments and multi-line strings, and it respect Scaladoc's wiki-style markup.

The commands forward-sexp and backward-sexp ( M-C-f, M-C-b ) motion commands will move over reserved words, literals, ids and lists.

Keymap and other commands

For the sake of customizability, scala-mode does not alter the default emacs keymap beyond what is absolutely necessary. However, you can customize the mode through the scala-mode-hook. Here are some suggested modification you may want to try. Just copy-paste it to your .emacs file.

(add-hook 'scala-mode-hook '(lambda ()

  ;; Bind the 'newline-and-indent' command to RET (aka 'enter'). This
  ;; is normally also available as C-j. The 'newline-and-indent'
  ;; command has the following functionality: 1) it removes trailing
  ;; whitespace from the current line, 2) it create a new line, and 3)
  ;; indents it.  An alternative is the
  ;; 'reindent-then-newline-and-indent' command.
  (local-set-key (kbd "RET") 'newline-and-indent)

  ;; Alternatively, bind the 'newline-and-indent' command and
  ;; 'scala-indent:insert-asterisk-on-multiline-comment' to RET in
  ;; order to get indentation and asterisk-insertion within multi-line
  ;; comments.
  ;; (local-set-key (kbd "RET") '(lambda ()
  ;;   (interactive)
  ;;   (newline-and-indent)
  ;;   (scala-indent:insert-asterisk-on-multiline-comment)))

  ;; Bind the backtab (shift tab) to
  ;; 'scala-indent:indent-with-reluctant-strategy command. This is usefull
  ;; when using the 'eager' mode by default and you want to "outdent" a
  ;; code line as a new statement.
  (local-set-key (kbd "<backtab>") 'scala-indent:indent-with-reluctant-strategy)

  ;; and other bindings here


Emacs has a very nice minor mode for highlighting bad whitespace and removing any unwanted whitespace when you save a file. To use it, add this to your .emacs file.

(add-hook 'scala-mode-hook '(lambda ()
  (require 'whitespace)

  ;; clean-up whitespace at save
  (make-local-variable 'before-save-hook)
  (add-hook 'before-save-hook 'whitespace-cleanup)

  ;; turn on highlight. To configure what is highlighted, customize
  ;; the *whitespace-style* variable. A sane set of things to
  ;; highlight is: face, tabs, trailing

Since scala is indented with two spaces, scala-mode2 does not use tabs at all for indents by default. If you want to turn on tabs mode, you can do so in the mode hook (set indent-tabs-mode to t).

Code highlighting

Highlighting code is still a work in progress. Feedback on how it should work is welcomed as issues to this github project.

It may come as a surprise to some that Scaladoc comments (comments that start with exactly /**) are highlighted in the same color as strings. This is because Scaladoc comments get the font-lock-doc-face, which is usually an alias for font-lock-string-face (a heritage from lisp, the native language of emacs, where document comments are strings). If this really bothers you, you may customize the face (use M-x customize-face).

Free emacs tip: if you are using emacs from a text terminal with dark background and you are having trouble with colors, try setting the customization variable frame-background-mode to dark (use M-x customize-variable).

Highlighting of variable definitions

The highlighting of variable definitions, such as

var test = "some mutable variable"

now result in the variable name ("test" above) to be highlighted using the variable scala-font-lock:var-face. Per default, the value of scala-font-lock:var-face is 'font-lock-warning-face. You can always change the highlighting of vars by changing scala-font-lock:var-face through the Emacs face customization (use M-x customize-face).

Other features

  • highlights only properly formatted string and character constants
  • indenting a code line removes trailing whitespace

Known issues

do/while is not always handled correctly. For example:

while (bar)

The indenter thinks the second occurrence of foo is the body of the while. To work around this, terminate the while with a semicolon, or put a blank line after it.

Future work

  • syntax-begin-function for reliably fontifying elements which span multiple lines
  • beginning-of-defun, end-of-defun
  • movement commands to move to previous or next definition (val, var, def, class, trait, object)
  • highlight headings and annotations inside Scaladoc specially (use underline for headings)
  • highlight variables in string interpolation (scala 2.10)

All suggestions and especially pull requests are welcomed in github


Mode development: Heikki Vesalainen

Contributors and valuable feedback:

  • Ray Racine
  • Eiríkr Åsheim (aka Erik Osheim)
  • Seth Tisue
  • Gary Pamparà
  • Evan Meagher
  • Andrew Jones
  • Vasya Novikov
  • Hugh Giddens
  • Nic Ferrier
  • Tillmann Rendel
  • Jim Powers
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