nixpkgs/doc/coding-conventions.xml

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<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 its 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>
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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>Dont 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
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the version part of the name <emphasis>must</emphasis> be the date of that
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(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>
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<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 its 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 its used to support <emphasis>software development</emphasis>:</term>
<listitem>
<variablelist>
<varlistentry>
<term>If its 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 its a <emphasis>compiler</emphasis>:</term>
<listitem>
<para><filename>development/compilers</filename> (e.g. <filename>gcc</filename>)</para>
</listitem>
</varlistentry>
<varlistentry>
<term>If its an <emphasis>interpreter</emphasis>:</term>
<listitem>
<para><filename>development/interpreters</filename> (e.g. <filename>guile</filename>)</para>
</listitem>
</varlistentry>
<varlistentry>
<term>If its a (set of) development <emphasis>tool(s)</emphasis>:</term>
<listitem>
<variablelist>
<varlistentry>
<term>If its 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 its 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 its a (set of) <emphasis>tool(s)</emphasis>:</term>
<listitem>
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<para>(A tool is a relatively small program, especially one intended
to be used non-interactively.)</para>
<variablelist>
<varlistentry>
<term>If its for <emphasis>networking</emphasis>:</term>
<listitem>
<para><filename>tools/networking</filename> (e.g. <filename>wget</filename>)</para>
</listitem>
</varlistentry>
<varlistentry>
<term>If its for <emphasis>text processing</emphasis>:</term>
<listitem>
<para><filename>tools/text</filename> (e.g. <filename>diffutils</filename>)</para>
</listitem>
</varlistentry>
<varlistentry>
<term>If its 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 its 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 its 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 its 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 its a <emphasis>shell</emphasis>:</term>
<listitem>
<para><filename>shells</filename> (e.g. <filename>bash</filename>)</para>
</listitem>
</varlistentry>
<varlistentry>
<term>If its a <emphasis>server</emphasis>:</term>
<listitem>
<variablelist>
<varlistentry>
<term>If its a web server:</term>
<listitem>
<para><filename>servers/http</filename> (e.g. <filename>apache-httpd</filename>)</para>
</listitem>
</varlistentry>
<varlistentry>
<term>If its 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 its 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 its a <emphasis>window manager</emphasis>:</term>
<listitem>
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<para><filename>applications/window-managers</filename> (e.g. <filename>awesome</filename>, <filename>stumpwm</filename>)</para>
</listitem>
</varlistentry>
<varlistentry>
<term>If its an <emphasis>application</emphasis>:</term>
<listitem>
<para>A (typically large) program with a distinct user
interface, primarily used interactively.</para>
<variablelist>
<varlistentry>
<term>If its 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 its for <emphasis>video playback / editing</emphasis>:</term>
<listitem>
<para><filename>applications/video</filename> (e.g. <filename>vlc</filename>)</para>
</listitem>
</varlistentry>
<varlistentry>
<term>If its for <emphasis>graphics viewing / editing</emphasis>:</term>
<listitem>
<para><filename>applications/graphics</filename> (e.g. <filename>gimp</filename>)</para>
</listitem>
</varlistentry>
<varlistentry>
<term>If its for <emphasis>networking</emphasis>:</term>
<listitem>
<variablelist>
<varlistentry>
<term>If its a <emphasis>mailreader</emphasis>:</term>
<listitem>
<para><filename>applications/networking/mailreaders</filename> (e.g. <filename>thunderbird</filename>)</para>
</listitem>
</varlistentry>
<varlistentry>
<term>If its a <emphasis>newsreader</emphasis>:</term>
<listitem>
<para><filename>applications/networking/newsreaders</filename> (e.g. <filename>pan</filename>)</para>
</listitem>
</varlistentry>
<varlistentry>
<term>If its 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 its <emphasis>data</emphasis> (i.e., does not have a
straight-forward executable semantics):</term>
<listitem>
<variablelist>
<varlistentry>
<term>If its a <emphasis>font</emphasis>:</term>
<listitem>
<para><filename>data/fonts</filename></para>
</listitem>
</varlistentry>
<varlistentry>
<term>If its related to <emphasis>SGML/XML processing</emphasis>:</term>
<listitem>
<variablelist>
<varlistentry>
<term>If its 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 its 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 its 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 dont 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>
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<section xml:id="sec-sources"><title>Fetching Sources</title>
<para>There are multiple ways to fetch a package source in nixpkgs. The
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general guideline is that you should package sources with a high degree of
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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>
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<listitem>
<para>Bad: Uses <literal>git://</literal> which won't be proxied.
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<programlisting>
src = fetchgit {
url = "git://github.com/NixOS/nix.git";
rev = "1f795f9f44607cc5bec70d1300150bfefcef2aae";
sha256 = "1cw5fszffl5pkpa6s6wjnkiv6lm5k618s32sp60kvmvpy7a2v9kg";
}
</programlisting>
</para>
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</listitem>
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<listitem>
<para>Better: This is ok, but an archive fetch will still be faster.
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<programlisting>
src = fetchgit {
url = "https://github.com/NixOS/nix.git";
rev = "1f795f9f44607cc5bec70d1300150bfefcef2aae";
sha256 = "1cw5fszffl5pkpa6s6wjnkiv6lm5k618s32sp60kvmvpy7a2v9kg";
}
</programlisting>
</para>
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</listitem>
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<listitem>
<para>Best: Fetches a snapshot archive and you get the rev you want.
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<programlisting>
src = fetchFromGitHub {
owner = "NixOS";
repo = "nix";
rev = "1f795f9f44607cc5bec70d1300150bfefcef2aae";
sha256 = "04yri911rj9j19qqqn6m82266fl05pz98inasni0vxr1cf1gdgv9";
}
</programlisting>
</para>
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</listitem>
</itemizedlist>
</para>
</section>
<section xml:id="sec-patches"><title>Patches</title>
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<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>