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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-module-abstractions">
<title>Abstractions</title>
<para>
If you find yourself repeating yourself over and over, it’s time to
abstract. Take, for instance, this Apache HTTP Server configuration:
<programlisting>
{
<xref linkend="opt-services.httpd.virtualHosts"/> =
{ "blog.example.org" = {
documentRoot = "/webroot/blog.example.org";
adminAddr = "alice@example.org";
forceSSL = true;
enableACME = true;
enablePHP = true;
};
"wiki.example.org" = {
documentRoot = "/webroot/wiki.example.org";
adminAddr = "alice@example.org";
forceSSL = true;
enableACME = true;
enablePHP = true;
};
};
}
</programlisting>
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>:
<programlisting>
let
commonConfig =
{ adminAddr = "alice@example.org";
forceSSL = true;
enableACME = true;
};
in
{
<xref linkend="opt-services.httpd.virtualHosts"/> =
{ "blog.example.org" = (commonConfig // { documentRoot = "/webroot/blog.example.org"; });
"wiki.example.org" = (commonConfig // { documentRoot = "/webroot/wiki.example.com"; });
};
}
</programlisting>
The <literal>let commonConfig = <replaceable>...</replaceable></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:
<programlisting>
{
<xref linkend="opt-services.httpd.virtualHosts"/> =
let commonConfig = <replaceable>...</replaceable>; in
{ "blog.example.org" = (commonConfig // { <replaceable>...</replaceable> })
"wiki.example.org" = (commonConfig // { <replaceable>...</replaceable> })
};
}
</programlisting>
but not <literal>{ let commonConfig = <replaceable>...</replaceable>; in
<replaceable>...</replaceable>; }</literal> since attributes (as opposed to
attribute values) are not expressions.
</para>
<para>
<emphasis>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:
<programlisting>
{
<xref linkend="opt-services.httpd.virtualHosts"/> =
let
makeVirtualHost = webroot:
{ documentRoot = webroot;
adminAddr = "alice@example.org";
forceSSL = true;
enableACME = true;
};
in
{ "example.org" = (makeVirtualHost "/webroot/example.org");
"example.com" = (makeVirtualHost "/webroot/example.com");
"example.gov" = (makeVirtualHost "/webroot/example.gov");
"example.nl" = (makeVirtualHost "/webroot/example.nl");
};
}
</programlisting>
Here, <varname>makeVirtualHost</varname> 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>
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