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<section xmlns="http://docbook.org/ns/docbook"
xmlns:xlink="http://www.w3.org/1999/xlink"
xmlns:xi="http://www.w3.org/2001/XInclude"
version="5.0"
xml:id='sec-arguments'>
<title>Arguments and Variables</title>
<para>The Nix expression in <xref linkend='ex-hello-nix' /> is a
function; it is missing some arguments that have to be filled in
somewhere. In the Nix Packages collection this is done in the file
<filename>pkgs/top-level/all-packages.nix</filename>, where all Nix
expressions for packages are imported and called with the appropriate
arguments. Here are some fragments of
<filename>all-packages.nix</filename>, with annotations of what they
mean:</para>
<programlisting>
...
rec { ①
hello = import ../applications/misc/hello/ex-1 ② { ③
inherit fetchurl stdenv perl;
};
perl = import ../development/interpreters/perl { ④
inherit fetchurl stdenv;
};
fetchurl = import ../build-support/fetchurl {
inherit stdenv; ...
};
stdenv = ...;
}
</programlisting>
<orderedlist>
<listitem>
<para>This file defines a set of attributes, all of which are
concrete derivations (i.e., not functions). In fact, we define a
<emphasis>mutually recursive</emphasis> set of attributes. That
is, the attributes can refer to each other. This is precisely
what we want since we want to <quote>plug</quote> the
various packages into each other.</para>
</listitem>
<listitem>
<para>Here we <emphasis>import</emphasis> the Nix expression for
GNU Hello. The import operation just loads and returns the
specified Nix expression. In fact, we could just have put the
contents of <xref linkend='ex-hello-nix' /> in
<filename>all-packages.nix</filename> at this point. That
would be completely equivalent, but it would make the file rather
bulky.</para>
<para>Note that we refer to
<filename>../applications/misc/hello/ex-1</filename>, not
<filename>../applications/misc/hello/ex-1/default.nix</filename>.
When you try to import a directory, Nix automatically appends
<filename>/default.nix</filename> to the file name.</para>
</listitem>
<listitem>
<para>This is where the actual composition takes place. Here we
<emphasis>call</emphasis> the function imported from
<filename>../applications/misc/hello/ex-1</filename> with a set
containing the things that the function expects, namely
<varname>fetchurl</varname>, <varname>stdenv</varname>, and
<varname>perl</varname>. We use inherit again to use the
attributes defined in the surrounding scope (we could also have
written <literal>fetchurl = fetchurl;</literal>, etc.).</para>
<para>The result of this function call is an actual derivation
that can be built by Nix (since when we fill in the arguments of
the function, what we get is its body, which is the call to
<varname>stdenv.mkDerivation</varname> in <xref
linkend='ex-hello-nix' />).</para>
<note><para>Nixpkgs has a convenience function
<function>callPackage</function> that imports and calls a
function, filling in any missing arguments by passing the
corresponding attribute from the Nixpkgs set, like this:
<programlisting>
hello = callPackage ../applications/misc/hello/ex-1 { };
</programlisting>
If necessary, you can set or override arguments:
<programlisting>
hello = callPackage ../applications/misc/hello/ex-1 { stdenv = myStdenv; };
</programlisting>
</para></note>
</listitem>
<listitem>
<para>Likewise, we have to instantiate Perl,
<varname>fetchurl</varname>, and the standard environment.</para>
</listitem>
</orderedlist>
</section>
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