My office at work si located at the 6^th floor. There are lifts, of course, but we decided not to use them and we run the stairs instead. We originally kept the number of runs per person and month on a whiteboard, but I created a Dancer application for that. It displayed the numbers in a specific way, similarly how we did it on the whiteboard:

choroba

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In the first versions, I used ugly Template code to turn a number into the “fence”. The, I realised it's possible to create filters. I googled for the details and found the following email. It shows a way how a custom filter can be created:

# config file:

engines:
   template_toolkit:
     PLUGIN_BASE: 'Template::Plugin'


# The filter code:

package Template::Plugin::FilterFence;

use Template::Plugin::Filter;
use base qw( Template::Plugin::Filter );

sub filter {
     my ($self, $text) = @_;
     return join ' ' # etc
}

1;

# The template that uses the filter:

<% USE FilterFence %>
<div id="main">
     <table>
       <% FOREACH row IN rows %>
       <tr><td><% row.0 %><td><% row.1 | $FilterFence %>
       <% END %>
     </table>
</div>
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It worked for me, too. I even added a test file for the filter:

use Test::More;
use Template::Plugin::FilterFence;

my $t = 'Template::Plugin::FilterFence'->new;
is($t->filter(0), q(), 'zero');
is($t->filter(1), '|', 'one');
# etc.
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But I didn't like the dollar sign needed in the template file in front of the filter name. I asked on Dancer's IRC channel and was pointed to the “Example” section of Template::Plugin::Filter. So, I added the init sub to the filter:

sub init {
    my $self = shift;
    $self->install_filter('fence');
    return $self
}
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... and changed the template code to simpler

<% row.1 | fence %>
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But, my test started to fail with the following error:

Can't call method "define_filter" on an undefined value at .../Templat
+e/Plugin/Filter.pm line 117.
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It seemed like the filter tried to register itself to a template, which didn't exist. The first workaround I discovered was to fake the creation of the filter object:

my $t = bless {}, 'Template::Plugin::FilterFence';
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It fixed the test, but I didn't like it. What if an important thing is missing in such an object? So I experimented more and discovered deleting the init sub also solved the problem:

undef *Template::Plugin::FilterFence::init;
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I don't like this solution much, either. Is there a clean way how to run a custom filter in isolation to test it?

Update: Removed the original example, used a variant of the existing code to avoid confusion.