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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 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 && 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>/
|