nixpkgs/doc/coding-conventions.xml

<chapter xmlns="http://docbook.org/ns/docbook"
         xmlns:xlink="http://www.w3.org/1999/xlink"
         xml:id="chap-conventions">
 <title>Coding conventions</title>
 <section xml:id="sec-syntax">
  <title>Syntax</title>

  <itemizedlist>
   <listitem>
    <para>
     Use 2 spaces of indentation per indentation level in Nix expressions, 4
     spaces in shell scripts.
    </para>
   </listitem>
   <listitem>
    <para>
     Do not use tab characters, i.e. configure your editor to use soft tabs.
     For instance, use <literal>(setq-default indent-tabs-mode nil)</literal>
     in Emacs. Everybody has different tab settings so it’s asking for
     trouble.
    </para>
   </listitem>
   <listitem>
    <para>
     Use <literal>lowerCamelCase</literal> for variable names, not
     <literal>UpperCamelCase</literal>. Note, this rule does not apply to
     package attribute names, which instead follow the rules in
     <xref linkend="sec-package-naming"/>.
    </para>
   </listitem>
   <listitem>
    <para>
     Function calls with attribute set arguments are written as
<programlisting>
foo {
  arg = ...;
}
</programlisting>
     not
<programlisting>
foo
{
  arg = ...;
}
</programlisting>
     Also fine is
<programlisting>
foo { arg = ...; }
</programlisting>
     if it's a short call.
    </para>
   </listitem>
   <listitem>
    <para>
     In attribute sets or lists that span multiple lines, the attribute names
     or list elements should be aligned:
<programlisting>
# A long list.
list =
  [ elem1
    elem2
    elem3
  ];

# A long attribute set.
attrs =
  { attr1 = short_expr;
    attr2 =
      if true then big_expr else big_expr;
  };

# Alternatively:
attrs = {
  attr1 = short_expr;
  attr2 =
    if true then big_expr else big_expr;
};
</programlisting>
    </para>
   </listitem>
   <listitem>
    <para>
     Short lists or attribute sets can be written on one line:
<programlisting>
# A short list.
list = [ elem1 elem2 elem3 ];

# A short set.
attrs = { x = 1280; y = 1024; };
</programlisting>
    </para>
   </listitem>
   <listitem>
    <para>
     Breaking in the middle of a function argument can give hard-to-read code,
     like
<programlisting>
someFunction { x = 1280;
  y = 1024; } otherArg
  yetAnotherArg
</programlisting>
     (especially if the argument is very large, spanning multiple lines).
    </para>
    <para>
     Better:
<programlisting>
someFunction
  { x = 1280; y = 1024; }
  otherArg
  yetAnotherArg
</programlisting>
     or
<programlisting>
let res = { x = 1280; y = 1024; };
in someFunction res otherArg yetAnotherArg
</programlisting>
    </para>
   </listitem>
   <listitem>
    <para>
     The bodies of functions, asserts, and withs are not indented to prevent a
     lot of superfluous indentation levels, i.e.
<programlisting>
{ arg1, arg2 }:
assert system == "i686-linux";
stdenv.mkDerivation { ...
</programlisting>
     not
<programlisting>
{ arg1, arg2 }:
  assert system == "i686-linux";
    stdenv.mkDerivation { ...
</programlisting>
    </para>
   </listitem>
   <listitem>
    <para>
     Function formal arguments are written as:
<programlisting>
{ arg1, arg2, arg3 }:
</programlisting>
     but if they don't fit on one line they're written as:
<programlisting>
{ arg1, arg2, arg3
, arg4, ...
, # Some comment...
  argN
}:
</programlisting>
    </para>
   </listitem>
   <listitem>
    <para>
     Functions should list their expected arguments as precisely as possible.
     That is, write
<programlisting>
{ stdenv, fetchurl, perl }: <replaceable>...</replaceable>
</programlisting>
     instead of
<programlisting>
args: with args; <replaceable>...</replaceable>
</programlisting>
     or
<programlisting>
{ stdenv, fetchurl, perl, ... }: <replaceable>...</replaceable>
</programlisting>
    </para>
    <para>
     For functions that are truly generic in the number of arguments (such as
     wrappers around <varname>mkDerivation</varname>) that have some required
     arguments, you should write them using an <literal>@</literal>-pattern:
<programlisting>
{ stdenv, doCoverageAnalysis ? false, ... } @ args:

stdenv.mkDerivation (args // {
  <replaceable>...</replaceable> if doCoverageAnalysis then "bla" else "" <replaceable>...</replaceable>
})
</programlisting>
     instead of
<programlisting>
args:

args.stdenv.mkDerivation (args // {
  <replaceable>...</replaceable> if args ? doCoverageAnalysis &amp;&amp; args.doCoverageAnalysis then "bla" else "" <replaceable>...</replaceable>
})
</programlisting>
    </para>
   </listitem>
  </itemizedlist>
 </section>
 <section xml:id="sec-package-naming">
  <title>Package naming</title>

  <para>
   In Nixpkgs, there are generally three different names associated with a
   package:
   <itemizedlist>
    <listitem>
     <para>
      The <varname>name</varname> attribute of the derivation (excluding the
      version part). This is what most users see, in particular when using
      <command>nix-env</command>.
     </para>
    </listitem>
    <listitem>
     <para>
      The variable name used for the instantiated package in
      <filename>all-packages.nix</filename>, and when passing it as a
      dependency to other functions. Typically this is called the
      <emphasis>package attribute name</emphasis>. This is what Nix expression
      authors see. It can also be used when installing using <command>nix-env
      -iA</command>.
     </para>
    </listitem>
    <listitem>
     <para>
      The filename for (the directory containing) the Nix expression.
     </para>
    </listitem>
   </itemizedlist>
   Most of the time, these are the same. For instance, the package
   <literal>e2fsprogs</literal> has a <varname>name</varname> attribute
   <literal>"e2fsprogs-<replaceable>version</replaceable>"</literal>, is bound
   to the variable name <varname>e2fsprogs</varname> in
   <filename>all-packages.nix</filename>, and the Nix expression is in
   <filename>pkgs/os-specific/linux/e2fsprogs/default.nix</filename>.
  </para>

  <para>
   There are a few naming guidelines:
   <itemizedlist>
    <listitem>
     <para>
      Generally, try to stick to the upstream package name.
     </para>
    </listitem>
    <listitem>
     <para>
      Don’t use uppercase letters in the <literal>name</literal> attribute
      — e.g., <literal>"mplayer-1.0rc2"</literal> instead of
      <literal>"MPlayer-1.0rc2"</literal>.
     </para>
    </listitem>
    <listitem>
     <para>
      The version part of the <literal>name</literal> attribute
      <emphasis>must</emphasis> start with a digit (following a dash) — e.g.,
      <literal>"hello-0.3.1rc2"</literal>.
     </para>
    </listitem>
    <listitem>
     <para>
      If a package is not a release but a commit from a repository, then the
      version part of the name <emphasis>must</emphasis> be the date of that
      (fetched) commit. The date must be in <literal>"YYYY-MM-DD"</literal>
      format. Also append <literal>"unstable"</literal> to the name - e.g.,
      <literal>"pkgname-unstable-2014-09-23"</literal>.
     </para>
    </listitem>
    <listitem>
     <para>
      Dashes in the package name should be preserved in new variable names,
      rather than converted to underscores or camel cased — e.g.,
      <varname>http-parser</varname> instead of <varname>http_parser</varname>
      or <varname>httpParser</varname>. The hyphenated style is preferred in
      all three package names.
     </para>
    </listitem>
    <listitem>
     <para>
      If there are multiple versions of a package, this should be reflected in
      the variable names in <filename>all-packages.nix</filename>, e.g.
      <varname>json-c-0-9</varname> and <varname>json-c-0-11</varname>. If
      there is an obvious “default” version, make an attribute like
      <literal>json-c = json-c-0-9;</literal>. See also
      <xref linkend="sec-versioning" />
     </para>
    </listitem>
   </itemizedlist>
  </para>
 </section>
 <section xml:id="sec-organisation">
  <title>File naming and organisation</title>

  <para>
   Names of files and directories should be in lowercase, with dashes between
   words — not in camel case. For instance, it should be
   <filename>all-packages.nix</filename>, not
   <filename>allPackages.nix</filename> or
   <filename>AllPackages.nix</filename>.
  </para>

  <section xml:id="sec-hierarchy">
   <title>Hierarchy</title>

   <para>
    Each package should be stored in its own directory somewhere in the
    <filename>pkgs/</filename> tree, i.e. in
    <filename>pkgs/<replaceable>category</replaceable>/<replaceable>subcategory</replaceable>/<replaceable>...</replaceable>/<replaceable>pkgname</replaceable></filename>.
    Below are some rules for picking the right category for a package. Many
    packages fall under several categories; what matters is the
    <emphasis>primary</emphasis> purpose of a package. For example, the
    <literal>libxml2</literal> package builds both a library and some tools;
    but it’s a library foremost, so it goes under
    <filename>pkgs/development/libraries</filename>.
   </para>

   <para>
    When in doubt, consider refactoring the <filename>pkgs/</filename> tree,
    e.g. creating new categories or splitting up an existing category.
   </para>

   <variablelist>
    <varlistentry>
     <term>
      If it’s used to support <emphasis>software development</emphasis>:
     </term>
     <listitem>
      <variablelist>
       <varlistentry>
        <term>
         If it’s a <emphasis>library</emphasis> used by other packages:
        </term>
        <listitem>
         <para>
          <filename>development/libraries</filename> (e.g.
          <filename>libxml2</filename>)
         </para>
        </listitem>
       </varlistentry>
       <varlistentry>
        <term>
         If it’s a <emphasis>compiler</emphasis>:
        </term>
        <listitem>
         <para>
          <filename>development/compilers</filename> (e.g.
          <filename>gcc</filename>)
         </para>
        </listitem>
       </varlistentry>
       <varlistentry>
        <term>
         If it’s an <emphasis>interpreter</emphasis>:
        </term>
        <listitem>
         <para>
          <filename>development/interpreters</filename> (e.g.
          <filename>guile</filename>)
         </para>
        </listitem>
       </varlistentry>
       <varlistentry>
        <term>
         If it’s a (set of) development <emphasis>tool(s)</emphasis>:
        </term>
        <listitem>
         <variablelist>
          <varlistentry>
           <term>
            If it’s a <emphasis>parser generator</emphasis> (including lexers):
           </term>
           <listitem>
            <para>
             <filename>development/tools/parsing</filename> (e.g.
             <filename>bison</filename>, <filename>flex</filename>)
            </para>
           </listitem>
          </varlistentry>
          <varlistentry>
           <term>
            If it’s a <emphasis>build manager</emphasis>:
           </term>
           <listitem>
            <para>
             <filename>development/tools/build-managers</filename> (e.g.
             <filename>gnumake</filename>)
            </para>
           </listitem>
          </varlistentry>
          <varlistentry>
           <term>
            Else:
           </term>
           <listitem>
            <para>
             <filename>development/tools/misc</filename> (e.g.
             <filename>binutils</filename>)
            </para>
           </listitem>
          </varlistentry>
         </variablelist>
        </listitem>
       </varlistentry>
       <varlistentry>
        <term>
         Else:
        </term>
        <listitem>
         <para>
          <filename>development/misc</filename>
         </para>
        </listitem>
       </varlistentry>
      </variablelist>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term>
      If it’s a (set of) <emphasis>tool(s)</emphasis>:
     </term>
     <listitem>
      <para>
       (A tool is a relatively small program, especially one intended to be
       used non-interactively.)
      </para>
      <variablelist>
       <varlistentry>
        <term>
         If it’s for <emphasis>networking</emphasis>:
        </term>
        <listitem>
         <para>
          <filename>tools/networking</filename> (e.g.
          <filename>wget</filename>)
         </para>
        </listitem>
       </varlistentry>
       <varlistentry>
        <term>
         If it’s for <emphasis>text processing</emphasis>:
        </term>
        <listitem>
         <para>
          <filename>tools/text</filename> (e.g. <filename>diffutils</filename>)
         </para>
        </listitem>
       </varlistentry>
       <varlistentry>
        <term>
         If it’s a <emphasis>system utility</emphasis>, i.e., something related or essential to the operation of a system:
        </term>
        <listitem>
         <para>
          <filename>tools/system</filename> (e.g. <filename>cron</filename>)
         </para>
        </listitem>
       </varlistentry>
       <varlistentry>
        <term>
         If it’s an <emphasis>archiver</emphasis> (which may include a compression function):
        </term>
        <listitem>
         <para>
          <filename>tools/archivers</filename> (e.g. <filename>zip</filename>,
          <filename>tar</filename>)
         </para>
        </listitem>
       </varlistentry>
       <varlistentry>
        <term>
         If it’s a <emphasis>compression</emphasis> program:
        </term>
        <listitem>
         <para>
          <filename>tools/compression</filename> (e.g.
          <filename>gzip</filename>, <filename>bzip2</filename>)
         </para>
        </listitem>
       </varlistentry>
       <varlistentry>
        <term>
         If it’s a <emphasis>security</emphasis>-related program:
        </term>
        <listitem>
         <para>
          <filename>tools/security</filename> (e.g. <filename>nmap</filename>,
          <filename>gnupg</filename>)
         </para>
        </listitem>
       </varlistentry>
       <varlistentry>
        <term>
         Else:
        </term>
        <listitem>
         <para>
          <filename>tools/misc</filename>
         </para>
        </listitem>
       </varlistentry>
      </variablelist>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term>
      If it’s a <emphasis>shell</emphasis>:
     </term>
     <listitem>
      <para>
       <filename>shells</filename> (e.g. <filename>bash</filename>)
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term>
      If it’s a <emphasis>server</emphasis>:
     </term>
     <listitem>
      <variablelist>
       <varlistentry>
        <term>
         If it’s a web server:
        </term>
        <listitem>
         <para>
          <filename>servers/http</filename> (e.g.
          <filename>apache-httpd</filename>)
         </para>
        </listitem>
       </varlistentry>
       <varlistentry>
        <term>
         If it’s an implementation of the X Windowing System:
        </term>
        <listitem>
         <para>
          <filename>servers/x11</filename> (e.g. <filename>xorg</filename> —
          this includes the client libraries and programs)
         </para>
        </listitem>
       </varlistentry>
       <varlistentry>
        <term>
         Else:
        </term>
        <listitem>
         <para>
          <filename>servers/misc</filename>
         </para>
        </listitem>
       </varlistentry>
      </variablelist>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term>
      If it’s a <emphasis>desktop environment</emphasis>:
     </term>
     <listitem>
      <para>
       <filename>desktops</filename> (e.g. <filename>kde</filename>,
       <filename>gnome</filename>, <filename>enlightenment</filename>)
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term>
      If it’s a <emphasis>window manager</emphasis>:
     </term>
     <listitem>
      <para>
       <filename>applications/window-managers</filename> (e.g.
       <filename>awesome</filename>, <filename>stumpwm</filename>)
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term>
      If it’s an <emphasis>application</emphasis>:
     </term>
     <listitem>
      <para>
       A (typically large) program with a distinct user interface, primarily
       used interactively.
      </para>
      <variablelist>
       <varlistentry>
        <term>
         If it’s a <emphasis>version management system</emphasis>:
        </term>
        <listitem>
         <para>
          <filename>applications/version-management</filename> (e.g.
          <filename>subversion</filename>)
         </para>
        </listitem>
       </varlistentry>
       <varlistentry>
        <term>
         If it’s for <emphasis>video playback / editing</emphasis>:
        </term>
        <listitem>
         <para>
          <filename>applications/video</filename> (e.g.
          <filename>vlc</filename>)
         </para>
        </listitem>
       </varlistentry>
       <varlistentry>
        <term>
         If it’s for <emphasis>graphics viewing / editing</emphasis>:
        </term>
        <listitem>
         <para>
          <filename>applications/graphics</filename> (e.g.
          <filename>gimp</filename>)
         </para>
        </listitem>
       </varlistentry>
       <varlistentry>
        <term>
         If it’s for <emphasis>networking</emphasis>:
        </term>
        <listitem>
         <variablelist>
          <varlistentry>
           <term>
            If it’s a <emphasis>mailreader</emphasis>:
           </term>
           <listitem>
            <para>
             <filename>applications/networking/mailreaders</filename> (e.g.
             <filename>thunderbird</filename>)
            </para>
           </listitem>
          </varlistentry>
          <varlistentry>
           <term>
            If it’s a <emphasis>newsreader</emphasis>:
           </term>
           <listitem>
            <para>
             <filename>applications/networking/newsreaders</filename> (e.g.
             <filename>pan</filename>)
            </para>
           </listitem>
          </varlistentry>
          <varlistentry>
           <term>
            If it’s a <emphasis>web browser</emphasis>:
           </term>
           <listitem>
            <para>
             <filename>applications/networking/browsers</filename> (e.g.
             <filename>firefox</filename>)
            </para>
           </listitem>
          </varlistentry>
          <varlistentry>
           <term>
            Else:
           </term>
           <listitem>
            <para>
             <filename>applications/networking/misc</filename>
            </para>
           </listitem>
          </varlistentry>
         </variablelist>
        </listitem>
       </varlistentry>
       <varlistentry>
        <term>
         Else:
        </term>
        <listitem>
         <para>
          <filename>applications/misc</filename>
         </para>
        </listitem>
       </varlistentry>
      </variablelist>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term>
      If it’s <emphasis>data</emphasis> (i.e., does not have a straight-forward executable semantics):
     </term>
     <listitem>
      <variablelist>
       <varlistentry>
        <term>
         If it’s a <emphasis>font</emphasis>:
        </term>
        <listitem>
         <para>
          <filename>data/fonts</filename>
         </para>
        </listitem>
       </varlistentry>
       <varlistentry>
        <term>
         If it’s related to <emphasis>SGML/XML processing</emphasis>:
        </term>
        <listitem>
         <variablelist>
          <varlistentry>
           <term>
            If it’s an <emphasis>XML DTD</emphasis>:
           </term>
           <listitem>
            <para>
             <filename>data/sgml+xml/schemas/xml-dtd</filename> (e.g.
             <filename>docbook</filename>)
            </para>
           </listitem>
          </varlistentry>
          <varlistentry>
           <term>
            If it’s an <emphasis>XSLT stylesheet</emphasis>:
           </term>
           <listitem>
            <para>
             (Okay, these are executable...)
            </para>
            <para>
             <filename>data/sgml+xml/stylesheets/xslt</filename> (e.g.
             <filename>docbook-xsl</filename>)
            </para>
           </listitem>
          </varlistentry>
         </variablelist>
        </listitem>
       </varlistentry>
      </variablelist>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term>
      If it’s a <emphasis>game</emphasis>:
     </term>
     <listitem>
      <para>
       <filename>games</filename>
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term>
      Else:
     </term>
     <listitem>
      <para>
       <filename>misc</filename>
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </section>

  <section xml:id="sec-versioning">
   <title>Versioning</title>

   <para>
    Because every version of a package in Nixpkgs creates a potential
    maintenance burden, old versions of a package should not be kept unless
    there is a good reason to do so. For instance, Nixpkgs contains several
    versions of GCC because other packages don’t build with the latest
    version of GCC. Other examples are having both the latest stable and latest
    pre-release version of a package, or to keep several major releases of an
    application that differ significantly in functionality.
   </para>

   <para>
    If there is only one version of a package, its Nix expression should be
    named <filename>e2fsprogs/default.nix</filename>. If there are multiple
    versions, this should be reflected in the filename, e.g.
    <filename>e2fsprogs/1.41.8.nix</filename> and
    <filename>e2fsprogs/1.41.9.nix</filename>. The version in the filename
    should leave out unnecessary detail. For instance, if we keep the latest
    Firefox 2.0.x and 3.5.x versions in Nixpkgs, they should be named
    <filename>firefox/2.0.nix</filename> and
    <filename>firefox/3.5.nix</filename>, respectively (which, at a given
    point, might contain versions <literal>2.0.0.20</literal> and
    <literal>3.5.4</literal>). If a version requires many auxiliary files, you
    can use a subdirectory for each version, e.g.
    <filename>firefox/2.0/default.nix</filename> and
    <filename>firefox/3.5/default.nix</filename>.
   </para>

   <para>
    All versions of a package <emphasis>must</emphasis> be included in
    <filename>all-packages.nix</filename> to make sure that they evaluate
    correctly.
   </para>
  </section>
 </section>
 <section xml:id="sec-sources">
  <title>Fetching Sources</title>

  <para>
   There are multiple ways to fetch a package source in nixpkgs. The general
   guideline is that you should package sources with a high degree of
   availability. Right now there is only one fetcher which has mirroring
   support and that is <literal>fetchurl</literal>. Note that you should also
   prefer protocols which have a corresponding proxy environment variable.
  </para>

  <para>
   You can find many source fetch helpers in
   <literal>pkgs/build-support/fetch*</literal>.
  </para>

  <para>
   In the file <literal>pkgs/top-level/all-packages.nix</literal> you can find
   fetch helpers, these have names on the form <literal>fetchFrom*</literal>.
   The intention of these are to provide snapshot fetches but using the same
   api as some of the version controlled fetchers from
   <literal>pkgs/build-support/</literal>. As an example going from bad to
   good:
   <itemizedlist>
    <listitem>
     <para>
      Bad: Uses <literal>git://</literal> which won't be proxied.
<programlisting>
src = fetchgit {
  url = "git://github.com/NixOS/nix.git";
  rev = "1f795f9f44607cc5bec70d1300150bfefcef2aae";
  sha256 = "1cw5fszffl5pkpa6s6wjnkiv6lm5k618s32sp60kvmvpy7a2v9kg";
}
</programlisting>
     </para>
    </listitem>
    <listitem>
     <para>
      Better: This is ok, but an archive fetch will still be faster.
<programlisting>
src = fetchgit {
  url = "https://github.com/NixOS/nix.git";
  rev = "1f795f9f44607cc5bec70d1300150bfefcef2aae";
  sha256 = "1cw5fszffl5pkpa6s6wjnkiv6lm5k618s32sp60kvmvpy7a2v9kg";
}
</programlisting>
     </para>
    </listitem>
    <listitem>
     <para>
      Best: Fetches a snapshot archive and you get the rev you want.
<programlisting>
src = fetchFromGitHub {
  owner = "NixOS";
  repo = "nix";
  rev = "1f795f9f44607cc5bec70d1300150bfefcef2aae";
  sha256 = "1i2yxndxb6yc9l6c99pypbd92lfq5aac4klq7y2v93c9qvx2cgpc";
}
</programlisting>
      Find the value to put as <literal>sha256</literal> by running
      <literal>nix run -f '&lt;nixpkgs&gt;' nix-prefetch-github -c nix-prefetch-github --rev 1f795f9f44607cc5bec70d1300150bfefcef2aae NixOS nix</literal>
      or <literal>nix-prefetch-url --unpack https://github.com/NixOS/nix/archive/1f795f9f44607cc5bec70d1300150bfefcef2aae.tar.gz</literal>.
     </para>
    </listitem>
   </itemizedlist>
  </para>
 </section>
 <section xml:id="sec-patches">
  <title>Patches</title>

  <para>
   Patches available online should be retrieved using
   <literal>fetchpatch</literal>.
  </para>

  <para>
<programlisting>
patches = [
  (fetchpatch {
    name = "fix-check-for-using-shared-freetype-lib.patch";
    url = "http://git.ghostscript.com/?p=ghostpdl.git;a=patch;h=8f5d285";
    sha256 = "1f0k043rng7f0rfl9hhb89qzvvksqmkrikmm38p61yfx51l325xr";
  })
];
</programlisting>
  </para>

  <para>
   Otherwise, you can add a <literal>.patch</literal> file to the
   <literal>nixpkgs</literal> repository. In the interest of keeping our
   maintenance burden to a minimum, only patches that are unique to
   <literal>nixpkgs</literal> should be added in this way.
  </para>

  <para>
<programlisting>
patches = [ ./0001-changes.patch ];
</programlisting>
  </para>

  <para>
   If you do need to do create this sort of patch file, one way to do so is
   with git:
   <orderedlist>
    <listitem>
     <para>
      Move to the root directory of the source code you're patching.
<screen>
$ cd the/program/source</screen>
     </para>
    </listitem>
    <listitem>
     <para>
      If a git repository is not already present, create one and stage all of
      the source files.
<screen>
$ git init
$ git add .</screen>
     </para>
    </listitem>
    <listitem>
     <para>
      Edit some files to make whatever changes need to be included in the
      patch.
     </para>
    </listitem>
    <listitem>
     <para>
      Use git to create a diff, and pipe the output to a patch file:
<screen>
$ git diff > nixpkgs/pkgs/the/package/0001-changes.patch</screen>
     </para>
    </listitem>
   </orderedlist>
  </para>
 </section>
</chapter>