Block invocation

   % $.Trim {{
     This string will be trimmed
   % }}  # end Trim

A double open brace ("{{") at the end of a "%-line" denotes a filter call. The filtered content begins just afterwards and ends at the "}}". Both "{{" and "}}" may be followed by a comment.

The expression "$.Trim", aka "$self->Trim", is a method call on the component object which returns a filter. In general everything before the "{{" 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.

Since the expression "$.Repeat(3)" returns a filter, it can be curried:

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

You can create one-off filters with anonymous subroutines. The subroutine receives the content in both $_[0] and $_, and should return the filtered content.

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

     ==> olleH


   % sub { s/ //g; $_[0] } {{
     A bunch of words
   % }}

     ==> Abunchofwords

Filters can be nested, with separate lines:

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

or on a single line:

   % $.Trim, sub { uc($_[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

    <% $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.

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

For consistency with other syntax, multiple names are applied in reverse order with the rightmost applied first.

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

    <% $message_body | H %>

Default filters

Manual invocation

    <%init>
    ...
    my $filtered_string = $m->filter($.Trim, $.NoBlankLines, $string);
    </%init>

Package and naming

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]) }
    }
    
    1;

To use these in a component:

    <%class>
    with 'MyApp::Filters';
    </%class> 

    % $.Upper {{
    ...
    % }}

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

Simple vs. dynamic filters

    # Uses $_[0]
    method Upper () {
        return sub { uc($_[0]) };
    }

    # Uses $_
    method Rot13 () {
        return sub { tr/a-zA-Z/n-za-mN-ZA-M/; $_ };
    }

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

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

    % $.Item('std') {{
      First
    % }}
    % $.Item('std') {{
      Second
    % }}

generates

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