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102 lines
3.5 KiB
XML
102 lines
3.5 KiB
XML
<section xmlns="http://docbook.org/ns/docbook"
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xmlns:xlink="http://www.w3.org/1999/xlink"
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xmlns:xi="http://www.w3.org/2001/XInclude"
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version="5.0"
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xml:id="sec-module-abstractions">
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<title>Abstractions</title>
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<para>
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If you find yourself repeating yourself over and over, it’s time to
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abstract. Take, for instance, this Apache HTTP Server configuration:
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<programlisting>
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{
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<xref linkend="opt-services.httpd.virtualHosts"/> =
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{ "blog.example.org" = {
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documentRoot = "/webroot/blog.example.org";
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adminAddr = "alice@example.org";
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forceSSL = true;
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enableACME = true;
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enablePHP = true;
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};
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"wiki.example.org" = {
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documentRoot = "/webroot/wiki.example.org";
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adminAddr = "alice@example.org";
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forceSSL = true;
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enableACME = true;
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enablePHP = true;
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};
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};
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}
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</programlisting>
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It defines two virtual hosts with nearly identical configuration; the only
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difference is the document root directories. To prevent this
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duplication, we can use a <literal>let</literal>:
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<programlisting>
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let
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commonConfig =
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{ adminAddr = "alice@example.org";
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forceSSL = true;
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enableACME = true;
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};
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in
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{
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<xref linkend="opt-services.httpd.virtualHosts"/> =
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{ "blog.example.org" = (commonConfig // { documentRoot = "/webroot/blog.example.org"; });
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"wiki.example.org" = (commonConfig // { documentRoot = "/webroot/wiki.example.com"; });
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};
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}
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</programlisting>
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The <literal>let commonConfig = <replaceable>...</replaceable></literal>
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defines a variable named <literal>commonConfig</literal>. The
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<literal>//</literal> operator merges two attribute sets, so the
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configuration of the second virtual host is the set
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<literal>commonConfig</literal> extended with the document root option.
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</para>
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<para>
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You can write a <literal>let</literal> wherever an expression is allowed.
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Thus, you also could have written:
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<programlisting>
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{
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<xref linkend="opt-services.httpd.virtualHosts"/> =
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let commonConfig = <replaceable>...</replaceable>; in
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{ "blog.example.org" = (commonConfig // { <replaceable>...</replaceable> })
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"wiki.example.org" = (commonConfig // { <replaceable>...</replaceable> })
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};
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}
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</programlisting>
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but not <literal>{ let commonConfig = <replaceable>...</replaceable>; in
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<replaceable>...</replaceable>; }</literal> since attributes (as opposed to
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attribute values) are not expressions.
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</para>
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<para>
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<emphasis>Functions</emphasis> provide another method of abstraction. For
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instance, suppose that we want to generate lots of different virtual hosts,
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all with identical configuration except for the document root. This can be done
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as follows:
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<programlisting>
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{
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<xref linkend="opt-services.httpd.virtualHosts"/> =
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let
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makeVirtualHost = webroot:
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{ documentRoot = webroot;
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adminAddr = "alice@example.org";
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forceSSL = true;
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enableACME = true;
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};
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in
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{ "example.org" = (makeVirtualHost "/webroot/example.org");
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"example.com" = (makeVirtualHost "/webroot/example.com");
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"example.gov" = (makeVirtualHost "/webroot/example.gov");
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"example.nl" = (makeVirtualHost "/webroot/example.nl");
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};
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}
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</programlisting>
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Here, <varname>makeVirtualHost</varname> is a function that takes a single
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argument <literal>webroot</literal> and returns the configuration for a virtual
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host. That function is then called for several names to produce the list of
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virtual host configurations.
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</para>
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</section>
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