nixpkgs/nixos/doc/manual/configuration/abstractions.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"/> =
    [ { hostName = "example.org";
        documentRoot = "/webroot";
        adminAddr = "alice@example.org";
        enableUserDir = true;
      }
      { hostName = "example.org";
        documentRoot = "/webroot";
        adminAddr = "alice@example.org";
        enableUserDir = true;
        enableSSL = true;
        sslServerCert = "/root/ssl-example-org.crt";
        sslServerKey = "/root/ssl-example-org.key";
      }
    ];
}
</programlisting>
  It defines two virtual hosts with nearly identical configuration; the only difference is that the second one has SSL enabled. To prevent this duplication, we can use a <literal>let</literal>:
<programlisting>
let
  exampleOrgCommon =
    { hostName = "example.org";
      documentRoot = "/webroot";
      adminAddr = "alice@example.org";
      enableUserDir = true;
    };
in
{
  <xref linkend="opt-services.httpd.virtualHosts"/> =
    [ exampleOrgCommon
      (exampleOrgCommon // {
        enableSSL = true;
        sslServerCert = "/root/ssl-example-org.crt";
        sslServerKey = "/root/ssl-example-org.key";
      })
    ];
}
</programlisting>
  The <literal>let exampleOrgCommon = <replaceable>...</replaceable></literal> defines a variable named <literal>exampleOrgCommon</literal>. The <literal>//</literal> operator merges two attribute sets, so the configuration of the second virtual host is the set <literal>exampleOrgCommon</literal> extended with the SSL options.
 </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 exampleOrgCommon = <replaceable>...</replaceable>; in
    [ exampleOrgCommon
      (exampleOrgCommon // { <replaceable>...</replaceable> })
    ];
}
</programlisting>
  but not <literal>{ let exampleOrgCommon = <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 host name. This can be done as follows:
<programlisting>
{
  <xref linkend="opt-services.httpd.virtualHosts"/> =
    let
      makeVirtualHost = name:
        { hostName = name;
          documentRoot = "/webroot";
          adminAddr = "alice@example.org";
        };
    in
      [ (makeVirtualHost "example.org")
        (makeVirtualHost "example.com")
        (makeVirtualHost "example.gov")
        (makeVirtualHost "example.nl")
      ];
}
</programlisting>
  Here, <varname>makeVirtualHost</varname> is a function that takes a single argument <literal>name</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>

 <para>
  We can further improve on this by using the function <varname>map</varname>, which applies another function to every element in a list:
<programlisting>
{
  <xref linkend="opt-services.httpd.virtualHosts"/> =
    let
      makeVirtualHost = <replaceable>...</replaceable>;
    in map makeVirtualHost
      [ "example.org" "example.com" "example.gov" "example.nl" ];
}
</programlisting>
  (The function <literal>map</literal> is called a <emphasis>higher-order function</emphasis> because it takes another function as an argument.)
 </para>

 <para>
  What if you need more than one argument, for instance, if we want to use a different <literal>documentRoot</literal> for each virtual host? Then we can make <varname>makeVirtualHost</varname> a function that takes a <emphasis>set</emphasis> as its argument, like this:
<programlisting>
{
  <xref linkend="opt-services.httpd.virtualHosts"/> =
    let
      makeVirtualHost = { name, root }:
        { hostName = name;
          documentRoot = root;
          adminAddr = "alice@example.org";
        };
    in map makeVirtualHost
      [ { name = "example.org"; root = "/sites/example.org"; }
        { name = "example.com"; root = "/sites/example.com"; }
        { name = "example.gov"; root = "/sites/example.gov"; }
        { name = "example.nl"; root = "/sites/example.nl"; }
      ];
}
</programlisting>
  But in this case (where every root is a subdirectory of <filename>/sites</filename> named after the virtual host), it would have been shorter to define <varname>makeVirtualHost</varname> as
<programlisting>
makeVirtualHost = name:
  { hostName = name;
    documentRoot = "/sites/${name}";
    adminAddr = "alice@example.org";
  };
</programlisting>
  Here, the construct <literal>${<replaceable>...</replaceable>}</literal> allows the result of an expression to be spliced into a string.
 </para>
</section>