mirror of
https://github.com/privatevoid-net/nix-super.git
synced 2024-11-22 14:06:16 +02:00
121 lines
No EOL
4 KiB
XML
121 lines
No EOL
4 KiB
XML
<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>
|
|
|
|
<example xml:id='ex-hello-composition'>
|
|
|
|
<title>Composing GNU Hello
|
|
(<filename>all-packages.nix</filename>)</title>
|
|
<programlisting>
|
|
...
|
|
|
|
rec { <co xml:id='ex-hello-composition-co-1' />
|
|
|
|
hello = import ../applications/misc/hello/ex-1 <co xml:id='ex-hello-composition-co-2' /> { <co xml:id='ex-hello-composition-co-3' />
|
|
inherit fetchurl stdenv perl;
|
|
};
|
|
|
|
perl = import ../development/interpreters/perl { <co xml:id='ex-hello-composition-co-4' />
|
|
inherit fetchurl stdenv;
|
|
};
|
|
|
|
fetchurl = import ../build-support/fetchurl {
|
|
inherit stdenv; ...
|
|
};
|
|
|
|
stdenv = ...;
|
|
|
|
}
|
|
</programlisting>
|
|
</example>
|
|
|
|
<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. <xref linkend='ex-hello-composition' /> shows
|
|
some fragments of
|
|
<filename>all-packages.nix</filename>.</para>
|
|
|
|
<calloutlist>
|
|
|
|
<callout arearefs='ex-hello-composition-co-1'>
|
|
|
|
<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>
|
|
|
|
</callout>
|
|
|
|
<callout arearefs='ex-hello-composition-co-2'>
|
|
|
|
<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>
|
|
|
|
</callout>
|
|
|
|
<callout arearefs='ex-hello-composition-co-3'>
|
|
|
|
<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>
|
|
|
|
</callout>
|
|
|
|
<callout arearefs='ex-hello-composition-co-4'>
|
|
|
|
<para>Likewise, we have to instantiate Perl,
|
|
<varname>fetchurl</varname>, and the standard environment.</para>
|
|
|
|
</callout>
|
|
|
|
</calloutlist>
|
|
|
|
</section> |