nixpkgs/nixos/doc/manual/from_md/configuration/abstractions.section.xml

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<section xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" xml:id="sec-module-abstractions">
<title>Abstractions</title>
<para>
If you find yourself repeating yourself over and over, its time to
abstract. Take, for instance, this Apache HTTP Server configuration:
</para>
<programlisting language="bash">
{
services.httpd.virtualHosts =
{ &quot;blog.example.org&quot; = {
documentRoot = &quot;/webroot/blog.example.org&quot;;
adminAddr = &quot;alice@example.org&quot;;
forceSSL = true;
enableACME = true;
enablePHP = true;
};
&quot;wiki.example.org&quot; = {
documentRoot = &quot;/webroot/wiki.example.org&quot;;
adminAddr = &quot;alice@example.org&quot;;
forceSSL = true;
enableACME = true;
enablePHP = true;
};
};
}
</programlisting>
<para>
It defines two virtual hosts with nearly identical configuration;
the only difference is the document root directories. To prevent
this duplication, we can use a <literal>let</literal>:
</para>
<programlisting language="bash">
let
commonConfig =
{ adminAddr = &quot;alice@example.org&quot;;
forceSSL = true;
enableACME = true;
};
in
{
services.httpd.virtualHosts =
{ &quot;blog.example.org&quot; = (commonConfig // { documentRoot = &quot;/webroot/blog.example.org&quot;; });
&quot;wiki.example.org&quot; = (commonConfig // { documentRoot = &quot;/webroot/wiki.example.com&quot;; });
};
}
</programlisting>
<para>
The <literal>let commonConfig = ...</literal> defines a variable
named <literal>commonConfig</literal>. The <literal>//</literal>
operator merges two attribute sets, so the configuration of the
second virtual host is the set <literal>commonConfig</literal>
extended with the document root option.
</para>
<para>
You can write a <literal>let</literal> wherever an expression is
allowed. Thus, you also could have written:
</para>
<programlisting language="bash">
{
services.httpd.virtualHosts =
let commonConfig = ...; in
{ &quot;blog.example.org&quot; = (commonConfig // { ... })
&quot;wiki.example.org&quot; = (commonConfig // { ... })
};
}
</programlisting>
<para>
but not <literal>{ let commonConfig = ...; in ...; }</literal> since
attributes (as opposed to attribute values) are not expressions.
</para>
<para>
<emphasis role="strong">Functions</emphasis> provide another method
of abstraction. For instance, suppose that we want to generate lots
of different virtual hosts, all with identical configuration except
for the document root. This can be done as follows:
</para>
<programlisting language="bash">
{
services.httpd.virtualHosts =
let
makeVirtualHost = webroot:
{ documentRoot = webroot;
adminAddr = &quot;alice@example.org&quot;;
forceSSL = true;
enableACME = true;
};
in
{ &quot;example.org&quot; = (makeVirtualHost &quot;/webroot/example.org&quot;);
&quot;example.com&quot; = (makeVirtualHost &quot;/webroot/example.com&quot;);
&quot;example.gov&quot; = (makeVirtualHost &quot;/webroot/example.gov&quot;);
&quot;example.nl&quot; = (makeVirtualHost &quot;/webroot/example.nl&quot;);
};
}
</programlisting>
<para>
Here, <literal>makeVirtualHost</literal> is a function that takes a
single argument <literal>webroot</literal> and returns the
configuration for a virtual host. That function is then called for
several names to produce the list of virtual host configurations.
</para>
</section>