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;
};

# Combined
listOfAttrs = [
  {
    attr1 = 3;
    attr2 = "fff";
  }
  {
    attr1 = 5;
    attr2 = "ggg";
  }
];
</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>
   The key words <emphasis>must</emphasis>, <emphasis>must not</emphasis>,
   <emphasis>required</emphasis>, <emphasis>shall</emphasis>, <emphasis>shall
   not</emphasis>, <emphasis>should</emphasis>, <emphasis>should
   not</emphasis>, <emphasis>recommended</emphasis>, <emphasis>may</emphasis>,
   and <emphasis>optional</emphasis> in this section are to be interpreted as
   described in <link xlink:href="https://tools.ietf.org/html/rfc2119">RFC
   2119</link>. Only <emphasis>emphasized</emphasis> words are to be
   interpreted in this way.
  </para>

  <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>
      The <literal>name</literal> attribute <emphasis>should</emphasis> be
      identical to the upstream package name.
     </para>
    </listitem>
    <listitem>
     <para>
      The <literal>name</literal> attribute <emphasis>must not</emphasis>
      contain uppercase letters — 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 <emphasis>must</emphasis> 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 <emphasis>should</emphasis> 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
      <emphasis>should</emphasis> 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 reproducible 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-source-hashes">
  <title>Obtaining source hash</title>

  <para>
   Preferred source hash type is sha256. There are several ways to get it.
  </para>

  <orderedlist>
   <listitem>
    <para>
     Prefetch URL (with <literal>nix-prefetch-<replaceable>XXX</replaceable>
     <replaceable>URL</replaceable></literal>, where
     <replaceable>XXX</replaceable> is one of <literal>url</literal>,
     <literal>git</literal>, <literal>hg</literal>, <literal>cvs</literal>,
     <literal>bzr</literal>, <literal>svn</literal>). Hash is printed to
     stdout.
    </para>
   </listitem>
   <listitem>
    <para>
     Prefetch by package source (with <literal>nix-prefetch-url
     '&lt;nixpkgs&gt;' -A <replaceable>PACKAGE</replaceable>.src</literal>,
     where <replaceable>PACKAGE</replaceable> is package attribute name). Hash
     is printed to stdout.
    </para>
    <para>
     This works well when you've upgraded existing package version and want to
     find out new hash, but is useless if package can't be accessed by
     attribute or package has multiple sources (<literal>.srcs</literal>,
     architecture-dependent sources, etc).
    </para>
   </listitem>
   <listitem>
    <para>
     Upstream provided hash: use it when upstream provides
     <literal>sha256</literal> or <literal>sha512</literal> (when upstream
     provides <literal>md5</literal>, don't use it, compute
     <literal>sha256</literal> instead).
    </para>
    <para>
     A little nuance is that <literal>nix-prefetch-*</literal> tools produce
     hash encoded with <literal>base32</literal>, but upstream usually provides
     hexadecimal (<literal>base16</literal>) encoding. Fetchers understand both
     formats. Nixpkgs does not standardize on any one format.
    </para>
    <para>
     You can convert between formats with nix-hash, for example:
<screen>
<prompt>$ </prompt>nix-hash --type sha256 --to-base32 <replaceable>HASH</replaceable>
</screen>
    </para>
   </listitem>
   <listitem>
    <para>
     Extracting hash from local source tarball can be done with
     <literal>sha256sum</literal>. Use <literal>nix-prefetch-url
     file:///path/to/tarball </literal> if you want base32 hash.
    </para>
   </listitem>
   <listitem>
    <para>
     Fake hash: set fake hash in package expression, perform build and extract
     correct hash from error Nix prints.
    </para>
    <para>
     For package updates it is enough to change one symbol to make hash fake.
     For new packages, you can use <literal>lib.fakeSha256</literal>,
     <literal>lib.fakeSha512</literal> or any other fake hash.
    </para>
    <para>
     This is last resort method when reconstructing source URL is non-trivial
     and <literal>nix-prefetch-url -A</literal> isn't applicable (for example,
     <link xlink:href="https://github.com/NixOS/nixpkgs/blob/d2ab091dd308b99e4912b805a5eb088dd536adb9/pkgs/applications/video/kodi/default.nix#L73">
     one of <literal>kodi</literal> dependencies</link>). The easiest way then
     would be replace hash with a fake one and rebuild. Nix build will fail and
     error message will contain desired hash.
    </para>
    <warning>
     <para>
      This method has security problems. Check below for details.
     </para>
    </warning>
   </listitem>
  </orderedlist>

  <section xml:id="sec-source-hashes-security">
   <title>Obtaining hashes securely</title>

   <para>
    Let's say Man-in-the-Middle (MITM) sits close to your network. Then instead
    of fetching source you can fetch malware, and instead of source hash you
    get hash of malware. Here are security considerations for this scenario:
   </para>

   <itemizedlist>
    <listitem>
     <para>
      <literal>http://</literal> URLs are not secure to prefetch hash from;
     </para>
    </listitem>
    <listitem>
     <para>
      hashes from upstream (in method 3) should be obtained via secure
      protocol;
     </para>
    </listitem>
    <listitem>
     <para>
      <literal>https://</literal> URLs are secure in methods 1, 2, 3;
     </para>
    </listitem>
    <listitem>
     <para>
      <literal>https://</literal> URLs are not secure in method 5. When
      obtaining hashes with fake hash method, TLS checks are disabled. So
      refetch source hash from several different networks to exclude MITM
      scenario. Alternatively, use fake hash method to make Nix error, but
      instead of extracting hash from error, extract
      <literal>https://</literal> URL and prefetch it with method 1.
     </para>
    </listitem>
   </itemizedlist>
  </section>
 </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>
<prompt>$ </prompt>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>
<prompt>$ </prompt>git init
<prompt>$ </prompt>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>
<prompt>$ </prompt>git diff > nixpkgs/pkgs/the/package/0001-changes.patch</screen>
     </para>
    </listitem>
   </orderedlist>
  </para>
 </section>
</chapter>