Layered Configuration Templates - Perl

As your site data structure becomes more complicated, you might find it easier to build it in layers using several templates. Example 2-30 shows a preprocessed configuration template that loads the site, col, and url templates using PROCESS .

Example 2-30. lib/configs

[% PROCESS site
         + col
         + url
-%]

We have already seen the site template in Example 2-29. Example 2-31 shows the col and url configuration templates.

Example 2-31. lib/col

[% site.rgb = {
      white = '#FFFFFF'
      black = '#000000'
     orange = '#FF6600'
  }

  site.col = {
      back = site.rgb.orange
      text = site.rgb.white
}
-%]

Example 2-31 shows the definition of a site.rgb hash and then another, site.col , which references values in the first. Template authors can use explicit colors, by referencing site.rgb.orange , for example, to fetch the correct RGB value, #FF6600 .Or they can code their templates to use colors defined in the site.col structure—for example, referencing site.col.back in the html template to set the bgcolor attribute of the HTML element. Either way, the colors are defined in one place, and the symbolic names allow us to see at a glance that the background color for the pages in the site is currently orange.

The url template is a little simpler, but also illustrates how variables can be built in stages (see Example 2-32).

Example 2-32. lib/url

[% url = 'http://tt2.org/ttbook'

   site.url = {
       root = url
       home = "$url/index.html"
       help = "$url/help.html"
     images = "$url/images"
   }

-%] 

The benefits of this approach are twofold. The first is that you can save yourself a great deal of typing by replacing a long-winded URL with a shorter variable name. The second benefit is that you can easily change all the URL values in a single stroke by changing the root url from which they are constructed.

One advantage of building a complex data structure from several templates is that you can easily replace one of the templates without affecting the others. For exam ple, you might want to use a different set of URL values at some point. Rather than edit the url template, you can copy the contents to a new file (e.g., url2), make the changes there, and then update the configs template accordingly:

[% PROCESS site
         + col
         + url2
-% ]

If you must revert to the old URLs at a later date, you need to change only the configs template to load url instead of url2. You can also use this approach to load different configuration templates based on a conditional expression. For example:

[% PROCESS site
         + col;

  IF developing;
    PROCESS url2;
  ELSE;
    PROCESS url;
  END
-%]