101 lines
3.5 KiB
XML
101 lines
3.5 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-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>
|