etc: Add 'indent-package.el' script.

* configure.ac: Check for 'emacs', substitute 'EMACS', and emit
'etc/indent-package.el'.
* etc/indent-package.el.in: New file.
* doc/contributing.texi (Formatting Code): Mention
'etc/indent-package.el'.
(Submitting Patches): Likewise, and link to the above node.

Co-authored-by: Alex Kost <alezost@gmail.com>

.gitignore

@@ -128,3 +128,4 @@ stamp-h[0-9]
 tmp
 /doc/os-config-lightweight-desktop.texi
 /nix/scripts/download
+/etc/indent-package.el

configure.ac

@@ -232,6 +232,10 @@ AM_MISSING_PROG([DOT], [dot])
 dnl Manual pages.
 AM_MISSING_PROG([HELP2MAN], [help2man])
 
+dnl Emacs (optional), for 'etc/indent-package.el'.
+AC_PATH_PROG([EMACS], [emacs], [/usr/bin/emacs])
+AC_SUBST([EMACS])
+
 AC_CONFIG_FILES([Makefile
                  po/guix/Makefile.in
                  po/packages/Makefile.in
@@ -241,5 +245,6 @@ AC_CONFIG_FILES([scripts/guix], [chmod +x scripts/guix])
 AC_CONFIG_FILES([test-env:build-aux/test-env.in], [chmod +x test-env])
 AC_CONFIG_FILES([pre-inst-env:build-aux/pre-inst-env.in],
   [chmod +x pre-inst-env])
+AC_CONFIG_FILES([etc/indent-package.el], [chmod +x etc/indent-package.el])
 
 AC_OUTPUT

doc/contributing.texi

@@ -237,6 +237,8 @@ especially when matching lists.
 @node Formatting Code
 @subsection Formatting Code
 
+@cindex formatting code
+@cindex coding style
 When writing Scheme code, we follow common wisdom among Scheme
 programmers.  In general, we follow the
 @url{http://mumble.net/~campbell/scheme/style.txt, Riastradh's Lisp
@@ -246,8 +248,20 @@ please do read it.
 
 Some special forms introduced in Guix, such as the @code{substitute*}
 macro, have special indentation rules.  These are defined in the
-@file{.dir-locals.el} file, which Emacs automatically uses.  If you do
+@file{.dir-locals.el} file, which Emacs automatically uses.
-not use Emacs, please make sure to let your editor know the rules.
+
+@cindex indentation, of code
+@cindex formatting, of code
+If you do not use Emacs, please make sure to let your editor knows these
+rules.  To automatically indent a package definition, you can also run:
+
+@example
+./etc/indent-package.el gnu/packages/@var{file}.scm @var{package}
+@end example
+
+@noindent
+This automatically indents the definition of @var{package} in
+@file{gnu/packages/@var{file}.scm} by running Emacs in batch mode.
 
 We require all top-level procedures to carry a docstring.  This
 requirement can be relaxed for simple private procedures in the
@@ -358,6 +372,11 @@ Bundling unrelated changes together makes reviewing harder and slower.
 Examples of unrelated changes include the addition of several packages,
 or a package update along with fixes to that package.
 
+@item
+Please follow our code formatting rules, possibly running the
+@command{etc/indent-package.el} script to do that automatically for you
+(@pxref{Formatting Code}).
+
 @end enumerate
 
 When posting a patch to the mailing list, use @samp{[PATCH] @dots{}} as

etc/indent-package.el.in

@@ -0,0 +1,53 @@
+#!@EMACS@ --script
+;;; indent-package.el --- Run Emacs to indent a package definition.
+
+;; Copyright © 2017 Alex Kost <alezost@gmail.com>
+
+;; This file is part of GNU Guix.
+
+;; GNU Guix is free software; you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; GNU Guix is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;; This scripts indents the given package definition in the specified file
+;; using Emacs.
+
+;;; Code:
+
+;; Load Scheme indentation rules from the current directory.
+(with-temp-buffer
+  (scheme-mode)
+  (let ((default-directory (file-name-as-directory "."))
+        (enable-local-variables :all))
+    (hack-dir-local-variables)
+    (hack-local-variables-apply)))
+
+(pcase command-line-args-left
+  (`(,file-name ,package-name)
+   (find-file file-name)
+   (goto-char (point-min))
+   (if (re-search-forward (concat "^(define\\(-public\\) +"
+                                  package-name)
+                          nil t)
+       (let ((indent-tabs-mode nil))
+         (beginning-of-defun)
+         (indent-sexp)
+         (save-buffer)
+         (message "Done!"))
+     (error "Package '%s' not found in '%s'"
+            package-name file-name)))
+  (x
+   (error "Usage: indent-package.el FILE PACKAGE")))
+
+;;; indent-package.el ends here