note
tomhukins
<p>I'd like to add to [jeffa]'s excellent tutorial by mentioning [cpan://HTML::Template]'s <code>query()</code> method, which allows Perl programmers to determine which parameters are used on a given template.</p>
<p>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.</p>
<p>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?</p>
<p>One approach is to determine which features are required on each page
beforehand. The Perl programmer might write a mod_perl handler as
below:</p>
<code>
# 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 $template_filename eq 'people') {
# Write some code to generate a list of users on the site and
# set some template parameters
}
</code>
<p>This approach is inflexible: if the HTML developer decides to
include existing functionality on an additional page, the Perl logic
must also be updated. One way to avoid this is to set all the possible
parameters for every request. This will be very slow for a typical Web site where most of the code isn't used for each request.</p>
<p>The cleanest way to deal with this problem is to use HTML::Template's
<code>query()</code> method. This method allows us to find out whether
the template uses a given parameter or not. For example, the weather
code in the example above might set parameters called
<code>temperature</code> and <code>cloud_cover</code>. To ensure that
we only run those parts of the Perl program that the template requires,
we could do something like:</p>
<code>
if ($template->query(name => 'temperature' or $template->query(name => 'cloud_cover') {
# The code to find out what the weather is like goes here
}
</code>
<p>This is much better: HTML developers can use the weather parameters
on any of the site's pages and Perl programmers don't need to update the
code. It's likely that there will be lots of calls to the query method,
though, which we can replace with a single call that takes no arguments
to retrieve all the parameters used on the template: </p>
<code>
my %param= map($_ => 1) $template->query();
if (exists $param{'temperature'} or exists $param{'cloud_cover'}) {
# The weather code goes here
}
</code>
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