in reply to HTML::Template Tutorial

I'd like to add to jeffa's excellent tutorial by mentioning HTML::Template's query() method, which allows Perl programmers to determine which parameters are used on a given template.

Using HTML::Template, Perl programmers set the parameters used by the templates but don't need to worry about the templates themselves, provided they've agreed with the template developers what the parameters are called and where the templates are stored.

This separation of back-end programming and front-end development creates a problem: Which back-end code needs to be called in order to set all the parameters required by the template?

One approach is to determine which features are required on each page beforehand. The Perl programmer might write a mod_perl handler as below:

# Derive the template's filename from all characters in the # requested URI # after the last /, making sure we only allow word characters $url =~ m!/(\w)$!; my $template_filename = $1 or return DECLINED; my $template = HTML::Template->new(filename => $template_filename) or return SERVER_ERROR; if ($template_filename eq 'results') { # Write some code to generate a list of results and set # corresponsing parameters on $template } elsif ($template_filename eq 'weather') { # Write some code to find out what the weather is like and set # template parameters } elsif ($template_filename eq 'users' or $template_filename eq 'p +eople') { # Write some code to generate a list of users on the site and # set some template parameters }

This approach is inflexible: if the HTML developer decides to include existing functionality on an additional page, the Perl logic must also be updated. One way to avoid this is to set all the possible parameters for every request. This will be very slow for a typical Web site where most of the code isn't used for each request.

The cleanest way to deal with this problem is to use HTML::Template's query() method. This method allows us to find out whether the template uses a given parameter or not. For example, the weather code in the example above might set parameters called temperature and cloud_cover. To ensure that we only run those parts of the Perl program that the template requires, we could do something like:

if ($template->query(name => 'temperature' or $template->query(name => + 'cloud_cover') { # The code to find out what the weather is like goes here }

This is much better: HTML developers can use the weather parameters on any of the site's pages and Perl programmers don't need to update the code. It's likely that there will be lots of calls to the query method, though, which we can replace with a single call that takes no arguments to retrieve all the parameters used on the template:

my %param= map($_ => 1) $template->query(); if (exists $param{'temperature'} or exists $param{'cloud_cover'}) { # The weather code goes here }