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>.  TODO: naming of
  attributes in
  <filename>all-packages.nix</filename>?</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.  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-pre-r3910"</literal> instead of
  <literal>"hello-svn-r3910"</literal>, as the latter would be seen as
  a package named <literal>hello-svn</literal> by
  <command>nix-env</command>.</para></listitem>

  <listitem><para>If package is fetched from git's commit 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 add <literal>"git"</literal> to the name - e.g.,
  <literal>"pkgname-git-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
  (which was convention up to around 2013 and most names
   still have underscores instead of dashes) — e.g.,
  <varname>http-parser</varname> instead of
  <varname>http_parser</varname>.</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 intented
      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>
    (including <emphasis>window managers</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 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 guidline 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>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>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>Fetches a snapshot archive and you get the rev you want.
<programlisting>
src = fetchFromGitHub {
  owner = "NixOS";
  repo = "nix";
  rev = "1f795f9f44607cc5bec70d1300150bfefcef2aae";
  sha256 = "04yri911rj9j19qqqn6m82266fl05pz98inasni0vxr1cf1gdgv9";
}
</programlisting>
        </para>
      </listitem>
    </itemizedlist>
  </para>
</section>
</chapter>