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Flexible CGI output with HTML templates

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Consider our free search engine script. It is an excellent application, but it wouldn’t be very helpful for others if it could display its results only in the output page format that we use for our site. Somebody else’s site will probably need the output page to contain site-specific navigation links, different colors, or even a completely different layout concept.

It is not uncommon to see CGI programs that contain the HTML code that is returned to the browser, inside one or more print statements in the code of the script. Clearly this is not a good idea, since it makes it at least difficult, if not nearly impossible, for a non-programmer to alter the output format. At best, a webmaster with little understanding of perl could identify the HTML code inside the program and alter some obvious items, like colours. Still, as applications grow more complex, this approach becomes even more difficult to manage. If control structures (if, while etc) are being used to generate the output, or if there are multiple pages of output composed by chunks of html that are being generated in different places in the code, even someone who is comfortable with perl could find it hard to figure out how to alter the look of the output. And this is not the only problem. The code itself becomes difficult to manage and maintain. Program code is difficult to read an maintain on its own right. Cluttering it with HTML here and there does not help the situation at all.

I will describe a different approach to the problem here, the one that we use for our programs and which proves to be a very good solution for almost any type of application. The idea in this approach is to isolate the HTML code in a separate file, which we call a template. A template is just like any regular HTML file, but it contains some special markup dictating where to insert the various dynamic data that will be produced by the CGI program. The program need only be concerned with generating the actual dynamic content, then it can simply insert it in appropriate place in the template and thus come up with the final output.

What a template looks like

The only thing that sets a template apart from a regular HTML file is that it contains comments of the form <!--cgi:variable-->. Such comments will be identified by the CGI program and replaced with the appropriate value for variable. The template may contain references to as many such variables as the CGI program is made to define. For example, take our search engine program. On the top of the search results page there appears some text saying what search was made and how many results it came up with. Assuming the program defines variables search_string and total_results, our template could look like:

<HTML>
...
Your search for <b><!--cgi:search_string--></b> 
resulted in <b><!--cgi:total_results--></b> matching pages.
...
</HTML>

Filling in the template

A simple way of doing this is the following:

open(TEMPLATE, $template_file);
while(<TEMPLATE>)
{
  $buffer .= $_;
}
$buffer =~ s/<!--cgi:search_string-->/$search_string/g;
$buffer =~ s/<!--cgi:total_results-->/$total_results/g;
print $buffer;

This will do the job, but we can certainly do much better than that. A realistic CGI-based application might contain many scripts generating many pages each, and each of those pages might contain much more than just a handful of variable data. Clearly we could write something much more general, a little module, that can handle output generation in the same way, but without requiring the programmer to explicitly perform substitutions of the template directives in the CGI program.

Perlfect::Template

As I already mentioned, we use that technique in all our programs here at Perlfect, and to make our life easier we have made a simple module that encapsulates all the functionality relating to templates that could be of use for CGI programming. The module is freely available for anyone to download and use for their own programs.

To introduce you to the use of Perlfect::Template I will show a simple example:

my $template = new Perlfect::Template("template.html");
my %data = (
            name => 'Nick',
            email => 'nick@perlfect.com',
            homepage => 'http://www.perlfect.com',
           );
my $html = $template->cast(%data);

First of all we create a Perlfect::Template object corresponding to the template file we want to use. The filename (or fully qualified path to it) is passed as an argument to the object constructor that returns a reference to the newly created object.

Then we prepare the data that we are going to cast to the template. Perlfect::Template expects us to supply it with a reference to a hash table (associative array) mapping data keys (template variables) to the values that should be inserted for them in the template. Thus all the processing done by the CGI program should end up defining a set of such key-value pairs in a hash reflecting the dynamic content of the output.

Finally, we call the cast method of the template object passing a reference to our data hash. The cast method does not alter the template, but simply uses it to produce a filled-in copy of it that is returned by the call. That means, that you can reuse a template as much as you like in a program. This allows you to construct your output from ‘sub-templates’. For example, in our search engine script, there is one template that defines the general layout of the results page, but there is also a template that defines the format of a single result listing. The program uses the latter to generate result listings by substituting, title, description, url, score, etc and thenputs them all together and inserts them in the general layout template. To create the result listings we only create one template object and use it for all results, each time passing a different hash reference to it. This has the advantage that the template file is read only once and that the processing needed to generate an other instance of the html output is minimal.

Passing substitution handlers..

In most cases the functionality we discussed so far is more than enough to do the job. There are cases though where passing static values for substitution in a template is a too restrictive mechanism. Sometimes you want the template to be able to describe not only the location of the substitutions but also some dynamic behaviour. In the example above I might want to let the template author choose whether the homepage url should appear in linked or plain text form. Of course, the template variable could just store the url and the template could contain a directive: <a href="<!--cgi:homepage-->">&lt!--cgi:homepage-->&lt/a> to construct the linked form, but in other situations ther might not be such workarounds. I would like to let the user pass parameters in a template directive, and instead of substituting a static string for the directive, I would like the substitution string to be generated by a function that will be called with the parameters in the directive.

Perlfect::Template allows you to use references to subroutines instead of scalars as values of the substitution hash. When a directive for suvh a template variable (that really is a subroutine) is met, Perlfect::Template will arrange for the subroutine to be called and will use ts return value as a substitution string. Further if the template directive includes arguments in parentheses as in <!--cgi:homepage('linked')--> the subroutine will be passed those aruments using perl’s standard argument passing mechanism. Arguments in the parentheses follow standard perl syntax for subroutine calls.

So let’s review our example above, now using the subroutine calling mechanism…

my $template = new Perlfect::Template("template.html");
my %data = (
            name => 'Nick',
            email => 'nick@perlfect.com',
            homepage => sub { 
                              if($_[0] eq 'linked') 
                                { return ... } 
                              else 
                                { return ... } 
                            } 
           );
my $html = $template->cast(%data);

And who said it’s only for HTML?

A final note to be made is that, while this tool was initially developed to aid the writing of CGI scripts that produce HTML output, it is absolutely fine to use for any kind of program that needs to format text files into a configurable layout. For example, one great use we’ve found is to create personalized email from a standard text template of the message and a database of names and email addresses of the recipients. In general any text file who is not likely to clash syntactically with the directive format of Perlfect::Template is perfectly suitable for processing with it.

Have fun programming with Perlfect::Template, and don’t hesitate to contact us and tell us what you think about it, or if you would suggest any extensions/modifications to it.

First appeared at Perlfect.com

Perlfect Solutions is the maker of Perlfect Search, an open
source, integrated, general purpose, site indexer and search engine
which can be found at http://www.perlfect.com/freescripts/search/.

Flexible CGI output with HTML templates
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