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Jonathan Swartz


Mason::Manual::Filters - Content filters in Mason


version 2.02


Filters can be used to process portions of content in a component.

A set of filters comes built-in with Mason - see Mason::Filters::Standard. Others will be available on CPAN, and it is easy to create your own.


Block invocation

Here's the standard way of invoking a filter:

   <% $.Trim { %>
   This string will be trimmed

An open brace at the end of a <% %> tag denotes a filter call. The filtered content begins just afterwards and ends at the </%> tag.

The expression $.Trim, aka $self->Trim, is a method call on the component object which returns a filter. In general everything before the brace is evaluated and is expected to return a filter or list of filters.

By convention, and to avoid name clashes with other component methods, filters use CamelCase rather than traditional underscore names.

Filters can take arguments:

   <% $.Repeat(3) { %>
     There's no place like home.

     ==>  There's no place like home.
          There's no place like home.
          There's no place like home.

Again, the expression $.Repeat(3) returns a filter, meaning that it can be curried:

   % my $repeat_three = $.Repeat(3);
   <% $repeat_three { %>
     There's no place like home.

A simple filter is just a subroutine that takes text as input and return the new text. Thus you can create one-off filters with anonymous subroutines:

   <% sub { reverse($_[0]) } %>Hello</%>

     ==> olleH

Filters can be nested, with separate tags:

   <% $.Trim { %>
     <% sub { uc($_[0]) } { %>
       This string will be trimmed and uppercased

or within a single tag:

   <% $.Trim, sub { uc($_[0]) } { %>
     This will be trimmed and uppercased

Multiple filters within the same tag are applied, intuitively, in reverse order with the last one being innermost. e.g. in this block

   % my $i = 1;
   <% $.Repeat(3), $.Cache($key, '1 hour') { %> <% $i++ %> </%>

      => 1 1 1

the output of <% $i++ %> is cached, and then repeated three times, whereas in this block

   % my $i = 1;
   <% $.Cache($key, '1 hour'), $.Repeat(3) { %> <% $i++ %> </%>

      => 1 2 3

<% $i++ %> is executed and output three times, and then the whole thing cached.

Pipe invocation

Filters can also appear in a limited way inside a regular <% %> tag:

    <% $content | NoBlankLines,Trim %>

The filter list appears after a << | >> character and must contain one or more comma-separated names. The names are treated as methods on the current component class. With this syntax you cannot use anonymous subroutines or variables as filters, or pass arguments to filters. However in a pinch you can define local filter methods to get around this, e.g.

    %% method Repeat3 { $.Repeat(3); }
    <% $message_body | Repeat3 %>

One common use of this form is to escape HTML strings in web content, using the HTML filter in Mason::Plugin::HTMLFilters:

    <% $message_body | HTML %>

Manual invocation

apply_filter can be used to manually apply a filter to a string. It returns the filtered output.


Package and naming

By convention, filters are placed in roles so that they can be composed into Mason::Component or a subclass thereof. Take a look at Mason::Filters::Standard for an example.

Also by convention, filters use CamelCase rather than traditional underscore_separated naming. Filter methods have to coexist with other methods in the Mason::Component namespace, so have to be distinguishable somehow, and we thought this was preferable to a "filter_" prefix or suffix. Of course, you are free to choose your own convention, but you should expect this naming in the standard filters at least.

Here's a filter package that implements two filters, Upper and Lower:

    package MyApp::Filters;
    use Mason::PluginRole;
    method Upper () {
        return sub { uc($_[0]) }
    method Lower () {
        return sub { lc($_[0]) }

To use these in a component:

    with 'MyApp::Filters';

    <% $.Upper { %>

Or if you want them available to all components, put them in Base.pm at the top of your component hierarchy, or in your application's Mason::Component subclass.

Simple vs. dynamic filters

A simple filter is a code ref which takes a string and returns the output. Your filter method should return this code ref. e.g.

    method Rot13 () {
        return sub {
            my $text = $_[0];
            $text =~ tr/a-zA-Z/n-za-mN-ZA-M/;
            return $text;

A dynamic filter is an object of class Mason::DynamicFilter. It contains a code ref which takes a yield block and returns the output. A yield block is a zero-argument code ref that returns a content string. e.g. this is functionally identical to the above:

    method Rot13 () {
        return Mason::DynamicFilter->new(
            filter => sub {
                my $yield = $_[0];
                my $text = $yield->();
                $text =~ tr/a-zA-Z/n-za-mN-ZA-M/;
                return $text;

The dynamic filter obviously doesn't buy you anything in this case, and for the majority of filters they are unneeded. The real power of dynamic filters is that they can choose if and when to execute the yield block. For example, here is an implementation (slightly expanded for explanatory purposes) of the Cache filter in Mason::Plugin::Cache:

    method Cache ( $key, $set_options ) {
        return Mason::DynamicFilter->new(
            filter => sub {
                my $yield = $_[0];
                my $cache = $self->cache;
                my $output = $cache->get( $key );
                if (!$output) {
                    $output = $yield->();
                    $cache->set( $key, $output, $set_options );
                return $output;

Notice that we call $cache->get first, and return the output immediately if it is in the cache. Only on a cache miss do we actually execute the (presumably expensive) yield block.

Defer and Repeat are two other examples of dynamic filters. See Mason::Filters::Standard for their implementations.

<%filter> block

You can use the <%filter> block to define filters that output content. It works just like a <%method> block, except that you can call $yield->() to generate the original content. e.g.

    <%filter Item ($class)>
    <li class="<% $class %>"><% $yield->() %></li>

    <% $.Item('std') %> First </%>
    <% $.Item('std') %> Second </%>


    <li class="std"> First </li>
    <li class="std"> Second </li>




Jonathan Swartz <swartz@pobox.com>


This software is copyright (c) 2011 by Jonathan Swartz.

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.