Examples and Recommended Usage

Most useful for conditional and loop structures - if, while, foreach, , etc. - as well as side-effect commands like assignments. To improve readability, always put a space after the '%'. Examples:

o Conditional code

    % my $ua = $r->header_in('User-Agent');
    % if ($ua =~ /msie/i) {
    Welcome, Internet Explorer users
    ...
    % } elsif ($ua =~ /mozilla/i) {
    Welcome, Netscape users
    ...
    % }

o HTML list formed from array

    <ul>
    % foreach $item (@list) {
    <li><% $item %></li>
    % }
    </ul>

o HTML list formed from hash

    <ul>
    % while (my ($key,$value) = each(%ENV)) {
    <li>
    <b><% $key %></b>: <% $value %>
    </li>
    % }
    </ul>

o HTML table formed from list of hashes

    <table>
    % foreach my $h (@loh) {
    <tr>
    <td><% $h->{foo} %></td>
    <td bgcolor=#ee0000><% $h->{bar} %></td>
    <td><% $h->{baz} %></td>
    </tr>
    % }
    </table>

<% xxx %>

Most useful for printing out variables, as well as more complex expressions. To improve readability, always separate the tag and expression with spaces. Examples:

  Dear <% $name %>: We will come to your house at <% $address %> in the
  fair city of <% $city %> to deliver your $<% $amount %> dollar prize!

  The answer is <% ($y+8) % 2 %>.

  You are <% $age < 18 ? 'not' : '' %> permitted to enter this site.

<%perl> xxx </%perl>

Useful for Perl blocks of more than a few lines.

Request Objects

$r, the mod_perl request object, provides a Perl API to the current Apache request. It is fully described in Apache.pod. Here is a sampling of methods useful to component developers:

    $r->uri             # the HTTP request URI
    $r->header_in(..)   # get the named HTTP header line
    $r->content_type    # set or retrieve content-type
    $r->header_out(..)  # set or retrieve an outgoing header

    $r->content         # don't use this one! (see Tips and Traps)

$m, the Mason request object, provides an analogous API for Mason. Almost all Mason features not activated by syntactic tags are accessed via $m methods. You'll be introduced to these methods throughout this document as they are needed. For a description of all methods see HTML::Mason::Request.

Because these are always set inside components, you should not ever define other variables with the same name, or else your code may fail in strange and mysterious ways.

Component Objects

Note that for many basic applications all you'll want to do with components is call them, for which no object method is needed. See next section.

System Objects

Components that output HTML

    <& comp_path, [name=>value, ...] &>

comp_path:

The component path. With a leading '/', the path is relative to the component root (comp_root). Otherwise, it is relative to the location of the calling component.

name => value pairs:

Parameters are passed as one or more "name => value" pairs, e.g. "player => 'M. Jordan'".

comp_path may be a literal string (quotes optional) or a Perl expression that evaluates to a string. To eliminate the need for quotes in most cases, Mason employs some magic parsing: If the first character is one of "[\w/_.]", comp_path is assumed to be a literal string running up to the first comma or &>. Otherwise, comp_path is evaluated as an expression.

Here are some examples:

    # relative component paths
    <& topimage &>
    <& tools/searchbox &>

    # absolute component path
    <& /shared/masthead, color=>'salmon' &>

    # this component path MUST have quotes because it contains a comma
    <& "sugar,eggs", mix=>1 &>

    # variable component path
    <& $comp &>

    # variable component and arguments
    <& $comp, %args &>

    # you can use arbitrary expression for component path, but it cannot
    # begin with a letter or number; delimit with () to remedy this
    <& (int(rand(2)) ? 'thiscomp' : 'thatcomp'), id=>123 &>

Several request methods also exist for calling components. "$m->comp" performs the equivalent action to <& &>:

    $m->comp('/shared/masthead', color=>'salmon');

"$m->scomp" is like the sprintf version of "$m->comp": it returns the component output, allowing the caller to examine and modify it before printing:

    my $masthead = $m->scomp('/shared/masthead', color=>'salmon');
    $masthead =~ ...;
    $m->print($masthead);

Component Calls with Content

    <&| /path/to/comp &> this is the content </&>
    <&| comp, arg1 => 'hi' &> filters can take arguments </&>
    <&| comp &> content can include <% "tags" %> of all kinds </&>
    <&| comp1 &> nesting is also <&| comp2 &> OK </&> </&>
    <&| SELF:method1 &> subcomponents can be filters </&>

The filtering component can be called in all the same ways a normal component is called, with arguments and so forth. The only difference between a filtering component and a normal component is that a filtering component is expected to fetch the content by calling $m->content and do something with it.

The ending tag may optionally contain the name of the component, and Mason will verify that it matches the name in the starting tag. This may be helpful when the tags are far apart or nested. To avoid ambiguous situations, this is only allowed when the component name is an unquoted literal (starting with "[\w/_.]"). For anything more complicated, such as "<|& $var &>" or "<&| 'name' &>", the simple "</&>" form must be used.

   <&| "outer" &>
     <&| /inner/comp, arg=>'this' &>
       <&| .mycomp &>
          Yada yada yada
       </& .mycomp >
     </& /inner/comp >
   </&>

Here is an example of a component used for localization. Its content is a series of strings in different languages, and it selects the correct one based on a global $lang variable, which could be setup in a site-level autohandler.

   <&| /i18n/itext &>
      <en>Hello, <% $name %> This is a string in English</en>
      <de>Schoene Gruesse, <% $name %>, diese Worte sind auf Deutsch</de>
      <pig>ellohay <% substr($name,2).substr($name,1,1).'ay' %>,
      isthay isay igpay atinlay</pig>
   </&>

Here is the /i18n/itext component:

   <% $text %>

   <%init>
   # this assumes $lang is a global variable which has been set up earlier.
   local $_ = $m->content;
   my ($text) = m{<$lang>(.*?)</$lang>};
   </%init>

You can explicitly check whether a component has passed content by checking the boolean "$m->has_content". This allows you to write a component that will do different things depending on whether it was passed content. However, before overloading a component in this way, consider whether splitting the behavior into two distinct components would work as well.

If a normal component which does not call "$m->content" is called with content, the content will not be output.

If you wrap a filtering component call around the entire component, the result will be functionally similar to a "<%filter>" section. See also Filtering.

Advanced Components Calls with Content

The tricky part of using filter components as control structures is setting up variables which can be accessed from both the filter component and the content, which is in the component which calls the filter component. The content has access to all variables in the surrounding component, but the filtering component does not. There are two ways to do this: use global variables, or pass a reference to a lexical variable to the filter component.

Here is a simple example using the second method:

    % my $var;
    <ol>
    <&| list_items , list => \@items, var => \$var &>
    <li> <% $var %></li>
    </&>
    </ol>

list_items component:

    <%args>
    @list
    $var
    </%args>
    % foreach (@list) {
    % $$var = $_;  # $var is a reference
    <% $m->content %>
    % }

Using global variables can be somewhat simpler. Below is the same example, with $var defined as a global variable. The site administrator must make sure that $var is included in Mason's allow_globals parameter. Local-izing $var within the filter component will allow the list_items component to be nested.

    <ol>
    <&| list_items, list => \@items &>
    <li> <% $var %></li>
    </&>
    </ol>

list_items component:

    <%args>
    @list
    </%args>
    % foreach (@list) {
    % local $var = $_;
    <% $m->content %>
    % }

Besides remembering to include $var in allow_globals, the developers should take care not to use that variable is other places where it might conflict with usage by the filter component. Local-izing $var will also provide some protection against using it in other places.

An even simpler method is to use the $_ variable. It is already global, and is automatically local-ized by the foreach statement:

    <ol>
    <&| list_items, list => \@items &>
    <li> <% $_ %> </li>
    </&>
    </ol>

list_items component:

    <%args>
    @list
    </%args>
    % foreach (@list) {
    <% $m->content %>
    % }

Components that Return Values

While we will demonstrate how this is done, we strongly encourage you to put code like this in modules instead. There are several reasons why this is a good idea:

• You can re-use this code outside of Mason.
• It is easy to preload module code when running under mod_perl, which can lower memory usage.
• Using Mason components as subroutines is slower than just using modules to do the same thing.
• It's easier to regression test module code.

With that being said, there are times when you may want to write a component which returns a value.

As an example, you might have a component "is_netscape" that analyzes the user agent to determine whether it is a Netscape browser:

    <%init>
    my $ua = $r->header_in('User-Agent');
    return ($ua =~ /Mozilla/i && $ua !~ /MSIE/i) ? 1 : 0;
    </%init>

Because components are implemented underneath with Perl subroutines, they can return values and even understand scalar/list context. e.g. The result of wantarray() inside a component will reflect whether the component was called in scalar or list context.

The <& &> notation only calls a component for its side-effect, and discards its return value, if any. To get at the return value of a component, use the "$m->comp" command:

    % if ($m->comp('is_netscape')) {
    Welcome, Netscape user!
    % }

Mason adds a "return undef" to the bottom of each component to provide an empty default return value. To return your own value from a component, you must use an explicit "return" statement. You cannot rely on the usual Perl trick of letting return values "fall through".

While it is possible for a component to generate output and return values, there is very little reason for a component to do both. For example, it would not be very friendly for "is_netscape" to output "hi Mom" while it was computing its value, thereby surprising the "if" statement! Conversely, any value returned by an output generating component would typically be discarded by the <& &> tag that invoked it.

Subrequests

A subrequest is simply a Mason Request object and has all of the methods normally associated with one.

To create a subrequest you simply use the "$m->make_subrequest" method. This method can take any parameters belonging to HTML::Mason::Request, such as autoflush or out_method. Once you have a new request object you simply call its "exec" method to execute it, which takes exactly the same parameters as the "comp" method.

Since subrequests inherit their parent request's parameters, output from a component called via a subrequest goes to the same destination as output from components called during the parent request. Of course, you can change this.

Here are some examples:

  <%perl>
   my $req = $m->make_subrequest( comp => '/some/comp', args => [ id => 172 ] );
   $req->exec;
  </%perl>

If you want to capture the subrequest's output in a scalar, you can simply pass an out_method parameter to "$m->make_subrequest":

  <%perl>
   my $buffer;
   my $req =
       $m->make_subrequest
           ( comp => '/some/comp', args => [ id => 172 ], out_method => \$buffer );
   $req->exec;
  </%perl>

Now $buffer contains all the output from that call to /some/comp.

For convenience, Mason also provides an "$m->subexec" method. This method takes the same arguments as "$m->comp" and internally calls "$m->make_subrequest" and then "exec" on the created request, all in one fell swoop. This is useful in cases where you have no need to override any of the parent request object's attributes.

By default, output from a subrequest appears inline in the calling component, at the point where it is executed. If you wish to do something else, you will need to explicitly override the subrequest's out_method parameter.

Mason Request objects are only designed to handle a single call to "exec". If you wish to make multiple subrequests, you must create a new subrequest object for each one.

dhandlers

Consider the following URL, in which newsfeeds/ exists but not the subdirectory LocalNews nor the component Story1:

    http://myserver/newsfeeds/LocalNews/Story1

In this case Mason constructs the following search path:

    /newsfeeds/LocalNews/Story1         => no such thing
    /newsfeeds/LocalNews/dhandler       => no such thing
    /newsfeeds/dhandler                 => found! (search ends)
    /dhandler

The found dhandler would read "LocalNews/Story1" from "$m->dhandler_arg" and use it as a retrieval key into a database of stories.

Here's how a simple /newsfeeds/dhandler might look:

    <& header &>
    <b><% $headline %></b><p>
    <% $body %>
    <& footer &>

    <%init>
    my $arg = $m->dhandler_arg;                # get rest of path
    my ($section, $story) = split("/", $arg);  # split out pieces
    my $sth = $DBH->prepare
        (qq{SELECT headline,body FROM news
            WHERE section = ? AND story = ?);
    $sth->execute($section, $story);
    my ($headline, $body) = $sth->fetchrow_array;
    return 404 if !$headline;                  # return "not found" if no such story
    </%init>

By default dhandlers do not get a chance to handle requests to a directory itself (e.g. /newsfeeds). These are automatically deferred to Apache, which generates an index page or a FORBIDDEN error. Often this is desirable, but if necessary the administrator can let in directory requests as well; see the allowing directory requests section of the administrator's manual.

A component or dhandler that does not want to handle a particular request may defer control to the next dhandler by calling "$m->decline".

When using dhandlers under mod_perl, you may find that sometimes Apache will not set a content type for a response. This usually happens when a dhandler handles a request for a non-existent file or directory. You can add a "<Location>" or "<LocationMatch>" block containing a "SetType" directive to your Apache config file, or you can just set the content type dynamically by calling "$r->content_type".

The administrator can customize the file name used for dhandlers with the dhandler_name parameter.

autohandlers

Autohandlers are directory based. When Mason determines the top-level component, it checks that directory and all parent directories for a component called autohandler. If found, the autohandler is called first. After performing its actions, the autohandler typically calls "$m->call_next" to transfer control to the original intended component.

"$m->call_next" works just like "$m->comp" except that the component path and arguments are implicit. You can pass additional arguments to "$m->call_next"; these are merged with the original arguments, taking precedence in case of conflict. This allows you, for example, to override arguments passed in the URL.

Here is an autohandler that adds a common header and footer to each page underneath its directory:

    <html>
    <head><title>McHuffy Incorporated</title></head>
    <body style="background-color: pink">

    % $m->call_next;

    <hr />
    Copyright 1999 McHuffy Inc.
    </body>
    </html>

Same idea, using components for the header/footer:

    <& /shared/header &>
    % $m->call_next;
    <& /shared/footer &>

The next autohandler applies a filter to its pages, adding an absolute hostname to relative image URLs:

    % $m->call_next;

    <%filter>
    s{(<img[^>]+src=\")/} {$1http://images.mysite.com/}ig;
    </%filter>

Most of the time autohandler can simply call "$m->call_next" without needing to know what the next component is. However, should you need it, the component object is available from "$m->fetch_next". This is useful for calling the component manually, e.g. if you want to suppress some original arguments or if you want to use "$m->scomp" to store and process the output.

If more than one autohandler applies to a page, each autohandler gets a chance to run. The top-most autohandler runs first; each "$m->call_next" transfers control to the next autohandler and finally to the originally called component. This allows you, for example, to combine general site-wide templates and more specific section-based templates.

Autohandlers can be made even more powerful in conjunction with Mason's object-oriented style features: methods, attributes, and inheritance. In the interest of space these are discussed in a separate section, Object-Oriented Techniques.

The administrator can customize the file name used for autohandlers with the autohandler_name parameter.

dhandlers vs. autohandlers

As a rule of thumb: use an autohandler when you have a set of components to handle your pages and you want to augment them with a template/filter. Use a dhandler when you want to create a set of "virtual URLs" that don't correspond to any actual components, or to provide default behavior for a directory.

dhandlers and autohandlers can even be used in the same directory. For example, you might have a mix of real URLs and virtual URLs to which you would like to apply a common template/filter.

In Component Calls

    <& /sales/header, s => 'dog', l => [2, 3, 4], h => {a => 7, b => 8} &>

This command passes a scalar ($s), a list (@l), and a hash (%h). The list and hash must be passed as references, but they will be automatically dereferenced in the called component.

In HTTP requests

    http://www.foo.com/mktg/prods.html?str=dog&lst=2&lst=3&lst=4

or an HTTP request with some POST content. Mason automatically parses the GET/POST values and makes them available to the component as parameters.

Accessing Parameters

1. Declared named arguments: Components can define an "<%args>" section listing argument names, types, and default values. For example:

    <%args>
    $a
    @b       # a comment
    %c

    # another comment
    $d => 5
    $e => $d*2
    @f => ('foo', 'baz')
    %g => (joe => 1, bob => 2)
    </%args>

Here, $a, @b, and %c are required arguments; the component generates an error if the caller leaves them unspecified. $d, $e, @f and %g are optional arguments; they are assigned the specified default values if unspecified. All the arguments are available as lexically scoped ("my") variables in the rest of the component.

Arguments are separated by one or more newlines. Comments may be used at the end of a line or on their own line.

Default expressions are evaluated in top-to-bottom order, and one expression may reference an earlier one (as $e references $d above).

Only valid Perl variable names may be used in "<%args>" sections. Parameters with non-valid variable names cannot be pre-declared and must be fetched manually out of the %ARGS hash (see below). One common example of undeclarable parameters are the "button.x/button.y" parameters sent for a form submit.

2. %ARGS hash: This variable, always available, contains all of the parameters passed to the component (whether or not they were declared). It is especially handy for dealing with large numbers of parameters, dynamically named parameters, or parameters with non-valid variable names. %ARGS can be used with or without an "<%args>" section, and its contents are unrelated to what you have declared in "<%args>".

Here's how to pass all of a component's parameters to another component:

    <& template, %ARGS &>

Parameter Passing Examples

1. Passing a scalar id with value 5.

  In a URL: /my/URL?id=5
  In a component call: <& /my/comp, id => 5 &>
  In the called component, if there is a declared argument named...
    $id, then $id will equal 5
    @id, then @id will equal (5)
    %id, then an error occurs
  In addition, $ARGS{id} will equal 5.

2. Passing a list colors with values red, blue, and green.

  In a URL: /my/URL?colors=red&colors=blue&colors=green
  In an component call: <& /my/comp, colors => ['red', 'blue', 'green'] &>
  In the called component, if there is a declared argument named...
    $colors, then $colors will equal ['red', 'blue', 'green']
    @colors, then @colors will equal ('red', 'blue', 'green')
    %colors, then an error occurs
  In addition, $ARGS{colors} will equal ['red', 'blue', 'green'].

3. Passing a hash grades with pairs Alice => 92 and Bob => 87.

  In a URL: /my/URL?grades=Alice&grades=92&grades=Bob&grades=87
  In an component call: <& /my/comp, grades => {Alice => 92, Bob => 87} &>
  In the called component, if there is a declared argument named...
    @grades, then @grades will equal ('Alice', 92, 'Bob', 87)
    %grades, then %grades will equal (Alice => 92, Bob => 87)
  In addition, $grade and $ARGS{grades} will equal
    ['Alice',92,'Bob',87] in the URL case, or {Alice => 92, Bob => 87}
    in the component call case.  (The discrepancy exists because, in a
    query string, there is no detectable difference between a list or
    hash.)

Using @_ instead

    <& /mktg/prods.html', 'dog', [2, 3, 4], {a => 7, b => 8} &>

and access them as usual through Perl's @_ array:

    my ($scalar, $listref, $hashref) = @_;

In this case no "<%args>" section is necessary.

We generally recommend named parameters for the benefits of readability, syntax checking, and default value automation. However using @_ may be convenient for very small components, especially subcomponents created with "<%def>".

Before Mason 1.21, @_ contained copies of the caller's arguments. In Mason 1.21 and beyond, this unnecessary copying was eliminated and @_ now contains aliases to the caller's arguments, just as with regular Perl subroutines. For example, if a component updates $_[0], the corresponding argument is updated (or an error occurs if it is not updateable).

Most users won't notice this change because "<%args>" variables and the %ARGS hash always contain copies of arguments.

See perlsub for more information on @_ aliasing.

<%init>

Technically an "<%init>" block is equivalent to a "<%perl>" block at the beginning of the component. However, there is an aesthetic advantage of placing this block at the end of the component rather than the beginning.

We've found that the most readable components (especially for non-programmers) contain HTML in one continuous block at the top, with simple substitutions for dynamic elements but no distracting blocks of Perl code. At the bottom an "<%init>" block sets up the substitution variables. This organization allows non-programmers to work with the HTML without getting distracted or discouraged by Perl code. For example:

    <html>
    <head><title><% $headline %></title></head>
    <body>
    <h2><% $headline %></h2>
    <p>By <% $author %>, <% $date %></p>

    <% $body %>

    </body>
    </html>

    <%init>
    # Fetch article from database
    my $dbh = DBI::connect ...;
    my $sth = $dbh->prepare("select * from articles where id = ?");
    $sth->execute($article_id);
    my ($headline, $date, $author, $body) = $sth->fetchrow_array;
    # Massage the fields
    $headline = uc($headline);
    my ($year, $month, $day) = split('-', $date);
    $date = "$month/$day";
    </%init>

    <%args>
    $article_id
    </%args>

<%cleanup>

A "<%cleanup>" block is equivalent to a "<%perl>" block at the end of the component. This means it will NOT execute if the component explicitly returns, or if an abort or error occurs in that component or one of its children. Because of this limitation, and because Perl is usually so good about cleaning up at the end of a lexical scope (e.g. component), "<%cleanup>" sections are rarely needed.

If you need code that is guaranteed to run when the component or request exits, consider using a mod_perl cleanup handler, or creating a custom class with a DESTROY method.

<%once>

This section is useful for declaring persistent component-scoped lexical variables (especially objects that are expensive to create), declaring subroutines (both named and anonymous), and initializing state.

This code does not run inside a request context. You cannot call components or access $m or $r from this section. Also, do not attempt to "return()" from a "<%once>" section; the current compiler cannot properly handle it.

Normally this code will execute individually from every HTTP child that uses the component. However, if the component is preloaded, this code will only execute once in the parent. Unless you have total control over what components will be preloaded, it is safest to avoid initializing variables that can't survive a fork(), e.g. DBI handles. Use code like this to initialize such variables in the "<%init>" section:

    <%once>
    my $dbh;      # declare but don't assign
    ...
    </%once>

    <%init>
    $dbh ||= DBI::connect ...
    ...
    </%init>

In addition, using $m or $r in this section will not work in a preloaded component, because neither of those variable exist when a component is preloaded.

<%shared>

A "<%shared>" section is useful for initializing variables needed in, say, the main body and one more subcomponents or methods. See Object-Oriented Techniques for an example of usage.

It's important to realize that you do not have access to the %ARGS hash or variables created via an "<%args>" block inside a shared section. However, you can access arguments via $m->request_args.

Additionally, you cannot call a components' own methods or subcomponents from inside a "<%shared>", though you can call other components.

Avoid using "<%shared>" for side-effect code that needs to run at a predictable time during page generation. You may assume only that "<%shared>" runs just before the first code that needs it and runs at most once per request.

In the current implementation, the scope sharing is done with closures, so variables will only be shared if they are visible at compile-time in the other parts of the component. In addition, you can't rely on the specific destruction time of the shared variables, because they may not be destroyed until the first time the "<%shared>" section executes in a future request. "<%init>" offers a more predictable execution and destruction time.

Currently any component with a "<%shared>" section incurs an extra performance penalty, because Mason must recreate its anonymous subroutines the first time each new request uses the component. The exact penalty varies between systems and for most applications will be unnoticeable. However, one should avoid using "<%shared>" when patently unnecessary, e.g. when an "<%init>" would work just as well.

Do not attempt to "return()" from a "<%shared>" section; the current compiler cannot properly handle it.

<%def name>

The name consists of characters in the set "[\w._-]". To call a subcomponent simply use its name in <& &> or "$m->comp". A subcomponent can only be seen from the surrounding component.

If you define a subcomponent with the same name as a file-based component in the current directory, the subcomponent takes precedence. You would need to use an absolute path to call the file-based component. To avoid this situation and for general clarity, we recommend that you pick a unique way to name all of your subcomponents that is unlikely to interfere with file-based components. A commonly accepted practice is to start subcomponent names with ".".

While inside a subcomponent, you may use absolute or relative paths to call file-based components and also call any of your "sibling" subcomponents.

The lexical scope of a subcomponent is separate from the main component. However a subcomponent can declare its own "<%args>" section and have relevant values passed in. You can also use a "<%shared>" section to declare variables visible from both scopes.

In the following example, we create a ".link" subcomponent to produce a standardized hyperlink:

    <%def .link>
    <a href="http://www.<% $site %>.com"><% $label %></a>

    <%args>
    $site
    $label=>ucfirst($site)
    </%args>
    </%def>

    Visit these sites:
    <ul>
     <li><& .link, site=>'yahoo' &></li>
     <li><& .link, site=>'cmp', label=>'CMP Media' &></li>
     <li><& .link, site=>'excite' &></li>
    </ul>

<%method name>

There are two ways to call a method. First, via a path of the form "comp:method":

    <& /foo/bar:method1 &>

    $m->comp('/foo/bar:method1');

Second, via the call_method component method:

    my $comp = $m->fetch_comp('/foo/bar');
    ...
    $comp->call_method('method1');

Methods are commonly used in conjunction with autohandlers to make templates more flexible. See Object-Oriented Techniques for more information.

You cannot create a subcomponent and method with the same name. This is mostly to prevent obfuscation and accidental errors.

<%flags>

Currently there is only one flag, "inherit", which specifies the component's parent in the form of a relative or absolute component path. A component inherits methods and attributes from its parent; see Object-Oriented Techniques for examples.

    <%flags>
    inherit=>'/site_handler'
    </%flags>

<%attr>

    <%attr>
    color => 'blue'
    fonts => [qw(arial geneva helvetica)]
    </%attr>

To query an attribute of a component, use the "attr" method:

    my $color = $comp->attr('color')

where $comp is a component object.

Mason evaluates attribute values once when loading the component. This makes them faster but less flexible than methods.

<%filter> section

This simple filter converts the component output to UPPERCASE:

    <%filter>
    tr/a-z/A-Z/
    </%filter>

The following navigation bar uses a filter to "unlink" and highlight the item corresponding to the current page:

    <a href="/">Home</a> | <a href="/products/">Products</a> |
    <a href="/bg.html">Background</a> | <a href="/finance/">Financials</a> |
    <a href="/support/">Tech Support</a> | <a href="/contact.html">Contact Us</a>

    <%filter>
    my $uri = $r->uri;
    s{<a href="$uri/?">(.*?)</a>} {<b>$1</b>}i;
    </%filter>

This allows a designer to code such a navigation bar intuitively without "if" statements surrounding each link! Note that the regular expression need not be very robust as long as you have control over what will appear in the body.

A filter block does not have access to variables declared in a component's "<%init>" section, though variables declared in the "<%args>", "<%once>" or "<%shared>" blocks are usable in a filter.

It should be noted that a filter cannot rely on receiving all of a component's output at once, and so may be called multiple times with different chunks of output. This can happen if autoflush is on, or if a filter-containing component, or the components it calls, call the "$m->flush_buffer()" method.

You should never call Perl's "return()" function inside a filter section, or you will not see any output at all.

You can use Component Calls with Content if you want to filter specific parts of a component rather than the entire component.

<%doc>

<% # comment... %>

    <% # This is a single-line comment %>

    <%
       # This is a
       # multi-line comment
    %>

%# comment

% if (0) { }

   % if (0) {
   ...
   % }

will be skipped by Mason, including component calls. While we don't recommend this for comments per se, it is a useful notation for "commenting out" code that you don't want to run.

HTML/XML/... comments

• They will be sent to the client and appear in the source of the page.
• They do not block component calls and other code from running, so don't try to use them to comment out code!

     <!-- Oops, the code below will still run
        <& /shared/expensive.mhtml &>
     -->
      

<%text>

    <%text>
    % This is an example of a Perl line.
    <% This is an example of an expression block. %>
    </%text>

This works for almost everything, but doesn't let you output "</%text>" itself! When all else fails, use "$m->print":

    % $m->print('The tags are <%text> and </%text>.');

Escaping expressions

Any "<% %>" expression may be terminated by a '|' and one or more escape flags (plus arbitrary whitespace), separated by commas:

    <% $file_data |h %>

The current valid flags are:

• h

  Escape HTML ('<' => '&lt;', etc.) using "HTML::Entities::encode()". Before Perl 5.8.0 this module assumes that text is in the ISO-8859-1 character set; see the next section for how to override this escaping. After 5.8.0, the encoding assumes that text is in Unicode.

• u

  Escape a URL query string (':' => '%3A', etc.) - all but [a-zA-Z0-9_.-]

• n

  This is a special flag indicating that the default escape flags should not be used for this substitution.

The administrator may specify a set of default escape flags via the default_escape_flags parameter. For example, if the administrator sets default_escape_flags to "['h']", then all <% %> expressions will automatically be HTML-escaped. In this case you would use the "n" flag to turn off HTML-escaping for a specific expression:

    <% $html_block |n %>

Multiple escapes can be specified as a comma-separated list:

    <% $uri | u, n %>

The old pre-defined escapes, 'h', 'u', and 'n', can be used without commas, so that this is legal:

    <% $uri | un %>

However, this only works for these three escapes, and no others. If you are using user-defined escapes as well, you must use a comma:

    <% $uri | u, add_session %>

User-defined Escapes

Besides the default escapes mentioned above, it is possible for the user to define their own escapes or to override the built-in 'h' and 'u' escapes.

This is done via the Interp object's escape_flags parameter or set_escape() method. Escape names may be any number of characters as long as it matches the regex "/^[\w-]+$/". The one exception is that you cannot override the 'n' flag.

Each escape flag is associated with a subroutine reference. The subroutine should expect to receive a scalar reference, which should be manipulated in place. Any return value from this subroutine is ignored.

Escapes can be defined at any time but using an escape that is not defined will cause an error when executing that component.

A common use for this feature is to override the built-in HTML escaping, which will not work with non-ISO-8559-1 encodings. If you are using such an encoding and want to switch the 'h' flag to do escape just the minimal set of characters ("<", ">", "&", """), put this in your Apache configuration:

   PerlSetVar  MasonEscapeFlags  "h => \&HTML::Mason::Escapes::basic_html_escape"

Or, in a top-level autohandler:

    $m->interp->set_escape( h => \&HTML::Mason::Escapes::basic_html_escape );

Or you could write your own escape function for a particular encoding:

    $ah->interp->set_escape( h => \&my_html_escape );

And of course this can be used for all sorts of other things, like a naughty words filter for the easily offended:

    $interp->set_escape( 'no-naughty' => \&remove_naughty_words );

Manually applying escapes

You can manually apply one or more escapes to text using the Interp object's "apply_escapes()" method. e.g.

    $m->interp->apply_escapes( 'some html content', 'h' );

Backslash at end of line

    <pre>
    foo
    % if (1) {
    bar
    % }
    baz
    </pre>

outputs

    foo
    bar
    baz

because of the newlines on lines 2 and 4. (Lines 3 and 5 do not generate a newline because the entire line is taken by Perl.) To suppress the newlines:

    <pre>
    foo\
    % if (1) {
    bar\
    % }
    baz
    </pre>

which prints

    foobarbaz

Basic Usage

    my $result = $m->cache->get('key');
    if (!defined($result)) {
        ... compute $result ...
        $m->cache->set('key', $result);
    }

"$m->cache->get" attempts to retrieve this component's cache value. If the value is available it is placed in $result. If the value is not available, $result is computed and stored in the cache by "$m->cache->set".

Multiple Keys/Values

    $m->cache->set(name => $string);
    $m->cache->set(friends => \@list);
    $m->cache->set(map => \%hash);

You can fetch all the keys in a cache with

    my @idents = $m->cache->get_keys;

It should be noted that Mason reserves all keys beginning with "__mason" for its own use.

Expiration

    $m->cache->set('name1', $string1, '5 min');  # Expire in 5 minutes
    $m->cache->set('name2', $string2, '3h');     # Expire in 3 hours

To change the expiration time for a piece of data, call "set" again with the new expiration. To expire an item immediately, use "$m->cache->remove".

You can also specify an expiration condition when you fetch the item, using the expire_if option:

    my $result = $m->cache->get('key',
        expire_if=>sub { $_[0]->get_created_at < (stat($file))[9] });

expire_if takes an anonymous subroutine, which is called with the cache object as its only parameter. If the subroutine returns a true value, the item is expired. In the example above, we expire the item whenever a certain file changes.

Finally, you can expire a cache item from an external script; see Accessing a Cache Externally below.

Avoiding Concurrent Recomputation

   my $result = $m->cache->get('key');
   if (!defined($result)) {
       ... compute $result ...
       $m->cache->set('key', $result);
   }

can suffer from a kind of race condition for caches that are accessed frequently and take a long time to recompute.

Suppose that a particular cache value is accessed five times a second and takes three seconds to recompute. When the cache expires, the first process comes in, sees that it is expired, and starts to recompute the value. The second process comes in and does the same thing. This sequence continues until the first process finishes and stores the new value. On average, the value will be recomputed and written to the cache 15 times!

One solution is the busy_lock flag:

   my $result = $m->cache->get('key', busy_lock=>'30 sec');

In this case, when the value cannot be retrieved, "get()" sets the expiration time of the value 30 seconds in the future before returning "undef". This tells the first process to compute the new value while causing subsequent processes to use the old value for 30 seconds.

Should the 30 seconds expire before the first process is done, a second process will start computing the new value while setting the expiration time yet another 30 seconds in the future, and so on.

The disadvantage of this solution is that multiple writes to the cache will be performed for each "set()".

Another solution, available only if you are using "CHI", is "expires_variance" which will create a variable time window during which expiration may occur. See the "CHI" documentation for details.

Caching All Output

It is typically used right at the top of an "<%init>" section:

    <%init>
    return if $m->cache_self(key => 'fookey', expires_in => '3 hours',
                             ... <other cache options> ...);
     ... <rest of init> ...
    </%init>

A full list of parameters and examples are available in the cache_self section of the Request manual.

Cache Object

    my $co = $m->cache->get_object('name1');
    $co->get_created_at();    # when was object stored in cache
    $co->get_expires_at();    # when does object expire

Choosing a Cache Subclass - with Cache::Cache

By default "$m->cache" uses "FileCache", but you can override this with the cache_class keyword. The value must be the name of a "Cache::Cache" subclass; the prefix "Cache::" need not be included. For example:

    my $result = $m->cache(cache_class => 'MemoryCache')->get('key');
    $m->cache(cache_class => 'MemoryCache')->set(key => $result);

You can even specify different subclasses for different keys in the same component. Just make sure the correct value is passed to all calls to "$m->cache"; Mason does not remember which subclass you have used for a given component or key.

The administrator can set the default cache subclass used by all components with the data_cache_defaults parameter.

Choosing a Cache Subclass - with CHI

"CHI::Driver::File" is the default, but you can override this with the driver keyword. The value must be the name of a "CHI::Driver" subclass; the prefix "CHI::Driver::" need not be included. For example:

    my $cache = $m->cache(driver => 'Memcached', servers => [ ... ]);
    my $result = $cache->get('key');
    $cache->set(key => $result);

You can even specify different subclasses for different keys in the same component. Just make sure the correct value is passed to all calls to "$m->cache"; Mason does not remember which subclass you have used for a given component or key.

The administrator can set the default cache subclass used by all components with the data_cache_defaults parameter.

Accessing a Cache Externally

• the namespace associated with the component. For "Cache::Cache", the function "HTML::Mason::Utils::data_cache_namespace", given a component id (usually just the component path), returns the namespace. For "CHI", the component id/path itself is the namespace.
• the cache_root, for file-based caches only. Defaults to the "cache" subdirectory under the Mason data directory.

Given this information you can get a handle on the component's cache. For example, the following code removes a cache item for component /foo/bar, assuming the data directory is /usr/local/www/mason and you are using the default file backend:

    use HTML::Mason::Utils qw(data_cache_namespace);

    # With Cache::Cache
    my $cache = new Cache::FileCache
        ( { namespace => data_cache_namespace("/foo/bar"),
            cache_root => "/usr/local/www/mason/cache" } );

    # With CHI
    my $cache = CHI->new
        ( driver => 'File',
          namespace => "/foo/bar",
          cache_root => "/usr/local/www/mason/cache" );

    # Remove one key
    $cache->remove('key1');

    # Remove all keys
    $cache->clear;

Mason 1.0x Cache API

The "access_data_cache" function is no longer available; this will need to be converted to use "Cache::Cache" directly, as described in the previous section.

Sending HTTP Headers

To determine the exact header behavior on your system, you need to know whether your server's default is to have autoflush on or off. Your administrator should have this information. If your administrator doesn't know then it is probably off, the default.

With autoflush off the header situation is extremely simple: Mason waits until the very end of the request to send headers. Any component can modify or augment the headers.

With autoflush on the header situation is more complex. Mason will send headers just before sending the first output. This means that if you want to affect the headers with autoflush on, you must do so before any component sends any output. Generally this takes place in an "<%init>" section.

For example, the following top-level component calls another component to see whether the user has a cookie; if not, it inserts a new cookie into the header.

    <%init>
    my $cookie = $m->comp('/shared/get_user_cookie');
    if (!$cookie) {
        $cookie = new CGI::Cookie (...);
        $r->header_out('Set-cookie' => $cookie);
    }
    ...
    </%init>

With autoflush off this code will always work. Turn autoflush on and this code will only work as long as /shared/get_user_cookie doesn't output anything (given its functional nature, it shouldn't).

The administrator can turn off automatic header sending via the auto_send_headers parameter. You can also turn it off on individual pages with

    $m->auto_send_headers(0);

Returning HTTP Status

Simply returning an error status (such as 404) from the top-most component has two problems in practice. First, the decision to return an error status often resides further down in the component stack. Second, you may have generated some content by the time this decision is made. (Both of these are more likely to be true when using autohandlers.)

Thus the safer way to generate an error status is

   $m->clear_buffer;
   $m->abort($status);

"$m->abort" bypasses the component stack and ensures that $status is returned from the top-most component. It works by throwing an exception. If you wrapped this code (directly or indirectly) in an eval, you must take care to rethrow the exception, or the status will not make it out:

   eval { $m->comp('...') };
   if (my $err = $@) {
      if ($m->aborted) {
          die $err;
      } else {
          # deal with non-abort exceptions
      }
   }

Filters and $m->abort

A filter section will still be called after a component aborts with "$m->abort". You can always check "$m->aborted" in your "<%filter>" block if you don't want to run the filter after an abort.

  <%filter>
  unless ( $m->aborted ) {
      $_ .= ' filter stuff';
  }
  </%filter>

External Redirects

    $m->redirect($url);    # Redirects with 302 status

This method uses the clear_buffer/abort technique mentioned above, so the same warnings apply regarding evals.

Also, if you generate any output after calling "$m->redirect", then this output will be sent, and will break the redirect. For example:

  % eval { $m->comp('redirect', ...) };

  % die $@ if $@;

The blank line between the two Perl lines is new output generated after the redirect. Either remove it or call "$m->clear_buffer" immediately before calling "die()".

Internal Redirects

First, you can use a Mason subrequest (see "Subrequests"). This only works if you are redirecting to another Mason page.

Second, you can use Apache's internal_redirect method, which works whether or not the new URL will be handled by Mason. Use it this way:

    $r->internal_redirect($url);
    $m->auto_send_headers(0);
    $m->clear_buffer;
    $m->abort;

The last three lines prevent the original request from accidentally generating extra headers or content.

Determining inheritance

You can use the "inherit" flag to override a component's parent:

    <%flags>
    inherit => '/foo/bar'
    </%flags>

If you specify undef as the parent, then the component inherits from no one. This is how to suppress templates.

Currently there is no way to specify a parent dynamically at run-time, or to specify multiple parents.

Content wrapping

Accessing methods and attributes

    my $self = $m->request_comp;

$self now refers to the component corresponding to the requested page (the component at the end of the chain).

To access a method for the page, use "call_method":

    $self->call_method('header');

This looks for a method named 'header' in the page component. If no such method exists, the chain of parents is searched upwards, until ultimately a "method not found" error occurs. Use 'method_exists' to avoid this error for questionable method calls:

    if ($self->method_exists('header')) { ...

The component returned by the "$m->request_comp" method never changes during request execution. In contrast, the component returned by "$m->base_comp" may change several times during request execution.

When execution starts, the base component is the same as the requested component. Whenever a component call is executed, the base component may become the component that was called. The base component will change for all component calls except in the following cases:

• A component is called via its component object rather than its path, for example:

    <& $m->fetch_comp('/some/comp'), foo => 1 &>
      

• A subcomponent (defined with "<%def>") is called.
• A method is called via the use of "SELF:", "PARENT:", or "REQUEST:". These are covered in more detail below.

In all other cases, the base component is the called component or the called component's owner component if that called component is a method.

As hinted at above, Mason provides a shortcut syntax for method calls.

If a component call path starts with "SELF:", then Mason will start looking for the method (the portion of the call after "SELF:"), in the base component.

    <& SELF:header &>
    $m->comp('SELF:header')

If the call path starts with "PARENT:", then Mason will start looking in the current component's parent for the named method.

    <& PARENT:header &>
    $m->comp('PARENT:header')

In the context of a component path, PARENT is shorthand for "$m->current_comp->parent".

If the call path begins with "REQUEST:", then Mason looks for the method in the requested component. REQUEST is shorthand for "$m->request_comp".

The rules for attributes are similar. To access an attribute for the page, use "attr":

    my $color = $self->attr('color')

This looks for an attribute named 'color' in the $self component. If no such attribute exists, the chain of parents is searched upwards, until ultimately an "attribute not found" error occurs. Use "attr_exists" or "attr_if_exist" to avoid this error for questionable attributes:

    if ($self->attr_exists('color')) { ...

    my $color = $self->attr_if_exists('color'); # if it doesn't exist $color is undef

Sharing data

To share variables, declare them either in the "<%once>" or "<%shared>" section. Both sections have an all-inclusive scope. The "<%once>" section runs once when the component loads; its variables are persistent for the lifetime of the component. The "<%shared>" section runs once per request (when needed), just before any code in the component runs; its variables last only til the end of the request.

In the following example, various sections of code require information about the logged-in user. We use a "<%shared>" section to fetch these in a single request.

    <%attr>
    title=>sub { "Account for $full_name" }
    </%attr>

    <%method lefttoc>
    <i><% $full_name %></i>
    (<a href="logout.html">Log out</a>)<br />
    ...
    </%method>

    Welcome, <% $fname %>. Here are your options:

    <%shared>
    my $dbh = DBI::connect ...;
    my $user = $r->connection->user;
    my $sth = $dbh->prepare("select lname,fname, from users where user_id = ?");
    $sth->execute($user);
    my ($lname, $fname) = $sth->fetchrow_array;
    my $full_name = "$first $last";
    </%shared>

"<%shared>" presents a good alternative to "<%init>" when data is needed across multiple scopes. Outside these situations, "<%init>" is preferred for its slightly greater speed and predictable execution model.

Example

    /autohandler
    /products/autohandler
    /products/index.html

and that a request comes in for /products/index.html.

/autohandler contains a general template for the site, referring to a number of standard methods and attributes for each page:

    <head>
    <title><& SELF:title &></title>
    </head>
    <body style="<% $self->attr('body_style') %>">
    <& SELF:header &>

    <div id="main">
    % $m->call_next;
    </div>

    <& SELF:footer &>
    </body>

    <%init>
    my $self = $m->base_comp;
    ...
    </%init>

    <%attr>
    body_style => 'standard'
    </%attr>

    <%method title>
    McGuffey Inc.
    </%method>

    <%method header>
    <h2><& SELF:title &></h2>
    </%method>

    <%method footer>
    </%method>

Notice how we provide defaults for each method and attribute, even if blank.

/products/autohandler overrides some attributes and methods for the /products section of the site.

    <%attr>
    body_style => 'plain'
    </%attr>
    <%method title>
    McGuffey Inc.: Products
    </%method>

    % $m->call_next;

Note that this component, though it only defines attributes and methods, must call "$m->call_next" if it wants the rest of the chain to run.

/products/index.html might override a few attributes, but mainly provides a primary section for the body.