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          <h1 class="headline">Template::Manual::Internals</h1>
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                  <li class=""><a href="#Introduction">Introduction</a></li>
                  <li class=""><a href="#Outside_Looking_In">Outside Looking In</a></li>
                  <li class=""><a href="#Inside_Looking_Out">Inside Looking Out</a></li>
                  <li class=""><a href="#Hacking_on_the_Template_Toolkit">Hacking on the Template Toolkit</a></li>
              
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                <h1 id="Introduction" onclick="switch_section(this)" title="Click title to show/hide section content.">Introduction</h1>
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              <div class="body">
                <p>
                      This section of the documentation is aimed at developers wishing to know
                      more about how the Template Toolkit works on the inside in order to
                      extend or adapt it to their own needs.
                    </p>
                    <p>
                      If that doesn't sound like you then you probably don't need to read this.
                      There is no test afterwards.
                    </p>
              </div>
            </div>
            <div class="section">
              <div class="head">
                <h1 id="Outside_Looking_In" onclick="switch_section(this)" title="Click title to show/hide section content.">Outside Looking In</h1>
                <a href="#body" class="top" title="Back up to the top of the page" >Top</a>
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              <div class="body">
                <p>
                      The <a href="../modules/Template.html">Template</a> module is simply
                      a front end module which creates and uses a <a href="../modules/Template/Service.html">Template::Service</a> and pipes the
                      output wherever you want it to go (<code>STDOUT</code> by default, or
                      maybe a file, scalar, etc). The <code>Apache::Template</code> module
                      (available separately from CPAN) is another front end. That creates a
                      <code>Template::Service::Apache</code> object, calls on it as required
                      and sends the output back to the relevant <code>Apache::Request</code>
                      object.
                    </p>
                    <p>
                      These front-end modules are really only there to handle any specifics of
                      the environment in which they're being used. The
                      <code>Apache::Template</code> front end, for example, handles
                      <code>Apache::Request</code> specifics and configuration via the
                      <i>httpd.conf</i>. The regular <a href="../modules/Template.html">Template</a> front-end deals with <code>STDOUT</code>, variable
                      refs, etc. Otherwise it is <a href="../modules/Template/Service.html">Template::Service</a> (or subclass) which does all the work.
                    </p>
                    <p>
                      The <a href="../modules/Template/Service.html">Template::Service</a> module provides a high-quality template
                      delivery service, with bells, whistles, signed up service level agreement
                      and a 30-day no quibble money back guarantee. "Have a good time, all the
                      time", that's our motto.
                    </p>
                    <p>
                      Within the lower levels of the Template Toolkit, there are lots of messy
                      details that we generally don't want to have to worry about most of the
                      time. Things like templates not being found, or failing to parse
                      correctly, uncaught exceptions being thrown, missing plugin modules or
                      dependencies, and so on. <a href="../modules/Template/Service.html">Template::Service</a> hides that all away and makes everything look
                      simple to the outsider. It provides extra features, like
                      <code>PRE_PROCESS</code>, <code>PROCESS</code> and
                      <code>POST_PROCESS</code>, and also provides the error recovery mechanism
                      via <code>ERROR</code>. You ask it to process a template and it takes
                      care of everything for you. The <code>Template::Service::Apache</code>
                      module goes a little bit further, adding some extra headers to the <a
                      href="http://search.cpan.org/search?query=Apache::Request&mode=all">Apache::Request</a>, setting a
                      few extra template variables, and so on.
                    </p>
                    <p>
                      For the most part, the job of a service is really just one of scheduling
                      and dispatching. It receives a request in the form of a call to its <a
                      href="../modules/Template/Service.html#method_process">process()</a> method and schedules the named template
                      specified as an argument, and possibly several other templates
                      (<code>PRE_PROCESS</code>, etc) to be processed in order. It doesn't
                      actually process the templates itself, but instead makes a <a href="../modules/Template/Context.html#method_process">process()</a> call
                      against a <a href="../modules/Template/Context.html">Template::Context</a> object.
                    </p>
                    <p>
                      <a href="../modules/Template/Context.html">Template::Context</a> is
                      the runtime engine for the Template Toolkit - the module that hangs
                      everything together in the lower levels of the Template Toolkit and that
                      one that does most of the real work, albeit by crafty delegation to
                      various other friendly helper modules.
                    </p>
                    <p>
                      Given a template name (or perhaps a reference to a scalar or file handle)
                      the context process() method must load and compile, or fetch a cached
                      copy of a previously compiled template, corresponding to that name. It
                      does this by calling on a list of one or more <a href="../modules/Template/Provider.html">Template::Provider</a> objects (the
                      <code>LOAD_TEMPLATES</code> posse) who themselves might get involved with
                      a <a href="../modules/Template/Parser.html">Template::Parser</a> to
                      help turn source templates into executable Perl code (but more on that
                      later).
                    </p>
                    <p>
                      Thankfully, all of this complexity is hidden away behind a simple <a
                      href="../modules/Template/Context.html#method_template">template()</a> method. You call it passing a
                      template name as an argument, and it returns a compiled template in the
                      form of a <a href="../modules/Template/Document.html">Template::Document</a> object, or otherwise raises an exception.
                    </p>
                    <p>
                      A <a href="../modules/Template/Document.html">Template::Document</a> is a thin object wrapper around a compiled
                      template subroutine. The object implements a <a href="../modules/Template/Document.html#method_process">process()</a> method
                      which performs a little bit of housekeeping and then calls the template
                      subroutine. The object also defines template metadata (defined in
                      <code>[% META ... %]</code> directives) and has a <a href="../modules/Template/Document.html#method_block">block()</a> method
                      which returns a hash of any additional <code>[% BLOCK xxxx %]</code>
                      definitions found in the template source.
                    </p>
                    <p>
                      So the context fetches a compiled document via its own <a href="../modules/Template/Context.html#method_template">template()</a> method
                      and then gets ready to process it. It first updates the stash (the place
                      where template variables get defined - more on that shortly) to set any
                      template variable definitions specified as the second argument by
                      reference to hash array. Then, it calls the document <a href="../modules/Template/Document.html#method_process">process()</a> method,
                      passing a reference to itself, the context object, as an argument. In
                      doing this, it provides itself as an object against which template code
                      can make callbacks to access runtime resources and Template Toolkit
                      functionality.
                    </p>
                    <p>
                      What we're trying to say here is this: not only does the <a href="../modules/Template/Context.html">Template::Context</a> object receive
                      calls from the <i>outside</i>, i.e. those originating in user code
                      calling the process() method on a Template object, but it also receives
                      calls from the <i>inside</i>, i.e. those originating in template
                      directives of the form <code>[% PROCESS template %]</code>.
                    </p>
                    <p>
                      Before we move on to that, here's a simple structure diagram showing the
                      outer layers of the Template Toolkit heading inwards, with pseudo code
                      annotations showing a typical invocation sequence.
                    </p>
                    <pre> ,--------.
 | Caller |     use Template;
 `--------'     my $tt = Template-&gt;new( ... );
      |         $tt-&gt;process($template, \%vars);
      |                                                     Outside
- - - | - - - - - - - - - - - - - - - - - - - - - - - - - - - - T T 
      |         package Template;                            Inside
      V
+----------+    sub process($template, \%vars) {
| Template |        $out = $self-&gt;SERVICE-&gt;process($template, $vars);
+----------+        print $out or send it to $self-&gt;OUTPUT;
      |         }
      |
      |         package Template::Service;
      |
      |         sub process($template, \%vars) {
      |             try {
+----------+            foreach $p in @self-&gt;PRE_PROCESS
| Service  |                $self-&gt;CONTEXT-&gt;process($p, $vars);
+----------+
      |                 $self-&gt;CONTEXT-&gt;process($template, $vars);
      |
      |                 foreach $p @self-&gt;POST_PROCESS
      |                     $self-&gt;CONTEXT-&gt;process($p, $vars);
      |             }
      |             catch {
      |                 $self-&gt;CONTEXT-&gt;process($self-&gt;ERROR);
      |             }
      |         }
      |
      V         package Template::Context;
+----------+    
| Context  |    sub process($template, \%vars) {
+----------+        # fetch compiled template
      |             $template = $self-&gt;template($template)
      |             # update stash
      |             $self-&gt;STASH-&gt;update($vars);
      |             # process template
      |             $template-&gt;process($self)
      |         }
      V     
+----------+    package Template::Document;
| Document |    
+----------+    sub process($context) {
                    $output = &amp;{ $self-&gt;BLOCK }($context);
                }</pre>
              </div>
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            <div class="section">
              <div class="head">
                <h1 id="Inside_Looking_Out" onclick="switch_section(this)" title="Click title to show/hide section content.">Inside Looking Out</h1>
                <a href="#body" class="top" title="Back up to the top of the page" >Top</a>
              </div>
              <div class="body">
                <p>
                      To understand more about what's going on in these lower levels, we need
                      to look at what a compiled template looks like. In fact, a compiled
                      template is just a regular Perl sub-routine. Here's a very simple one.
                    </p>
                    <pre>sub my_compiled_template {
    return "This is a compiled template.\n";
}</pre>
                    <p>
                      You're unlikely to see a compiled template this simple unless you wrote
                      it yourself but it is entirely valid. All a template subroutine is
                      obliged to do is return some output (which may be an empty of course). If
                      it can't for some reason, then it should raise an error via
                      <code>die()</code>.
                    </p>
                    <pre>sub my_todo_template {
    die "This template not yet implemented\n";
}</pre>
                    <p>
                      If it wants to get fancy, it can raise an error as a <a href="../modules/Template/Exception.html">Template::Exception</a> object. An
                      exception object is really just a convenient wrapper for the
                      '<code>type</code>' and '<code>info</code>' fields.
                    </p>
                    <pre>sub my_solilique_template {
    die (Template::Exception-&gt;new('yorrick', 'Fellow of infinite jest'));
}</pre>
                    <p>
                      Templates generally need to do a lot more than just generate static
                      output or raise errors. They may want to inspect variable values, process
                      another template, load a plugin, run a filter, and so on. Whenever a
                      template subroutine is called, it gets passed a reference to a <a
                      href="../modules/Template/Context.html">Template::Context</a>
                      object. It is through this context object that template code can access
                      the features of the Template Toolkit.
                    </p>
                    <p>
                      We described earlier how the <a href="../modules/Template/Service.html">Template::Service</a> object calls on <a href="../modules/Template/Context.html">Template::Context</a> to handle a <a
                      href="../modules/Template/Context.html#method_process">process()</a> request from the <i>outside</i>. We can
                      make a similar request on a context to process a template, but from
                      within the code of another template. This is a call from the
                      <i>inside</i>.
                    </p>
                    <pre>sub my_process_template {
    my $context = shift;
    my $output = $context-&gt;process('header', { title =&gt; 'Hello World' })
               . "\nsome content\n"
               . $context-&gt;process('footer');
}</pre>
                    <p>
                      This is then roughly equivalent to a source template something like this:
                    </p>
                    <pre><span class="tt">[% PROCESS header
    title = 'Hello World'
%]</span>
some content
<span class="tt">[% PROCESS footer %]</span></pre>
                    <p>
                      Template variables are stored in, and managed by a <a href="../modules/Template/Stash.html">Template::Stash</a> object. This is a
                      blessed hash array in which template variables are defined. The object
                      wrapper provides <a href="../modules/Template/Stash.html#method_get">get()</a> and <a href="../modules/Template/Stash.html#method_set">set()</a> method which implement all the
                      <i>magical.variable.features</i> of the Template Toolkit.
                    </p>
                    <p>
                      Each context object has its own stash, a reference to which can be
                      returned by the appropriately named <a href="../modules/Template/Context.html#method_stash">stash()</a> method. So
                      to print the value of some template variable, or for example, to
                      represent the following source template:
                    </p>
                    <pre>&lt;title&gt;<span class="tt">[% title %]</span>&lt;/title&gt;</pre>
                    <p>
                      we might have a subroutine definition something like this:
                    </p>
                    <pre>sub {
    my $context = shift;
    my $stash = $context-&gt;stash();
    return '&lt;title&gt;' . $stash-&gt;get('title') . '&lt;/title&gt;';
}</pre>
                    <p>
                      The stash <a href="../modules/Template/Stash.html#method_get">get()</a> method hides the details of the underlying
                      variable types, automatically calling code references, checking return
                      values, and performing other such tricks. If '<code>title</code>' happens
                      to be bound to a subroutine then we can specify additional parameters as
                      a list reference passed as the second argument to get().
                    </p>
                    <pre><span class="tt">[% title('The Cat Sat on the Mat') %]</span></pre>
                    <p>
                      This translates to the stash call:
                    </p>
                    <pre>$stash-&gt;get([ 'title', ['The Cat Sat on the Mat'] ]);</pre>
                    <p>
                      Dotted compound variables can be requested by passing a single list
                      reference to the <code>get()</code> method in place of the variable name.
                      Each pair of elements in the list should correspond to the variable name
                      and reference to a list of arguments for each dot-delimited element of
                      the variable.
                    </p>
                    <pre><span class="tt">[% foo(1, 2).bar(3, 4).baz(5) %]</span></pre>
                    <p>
                      is thus equivalent to
                    </p>
                    <pre>$stash-&gt;get([ foo =&gt; [1,2], bar =&gt; [3,4], baz =&gt; [5] ]);</pre>
                    <p>
                      If there aren't any arguments for an element, you can specify an empty,
                      zero or null argument list.
                    </p>
                    <pre><span class="tt">[% foo.bar %]</span>
$stash-&gt;get([ 'foo', 0, 'bar', 0 ]);</pre>
                    <p>
                      The <a href="../modules/Template/Stash.html#method_set">set()</a>
                      method works in a similar way. It takes a variable name and a variable
                      value which should be assigned to it.
                    </p>
                    <pre><span class="tt">[% x = 10 %]</span>         
$stash-&gt;set('x', 10);

<span class="tt">[% x.y = 10 %]</span>
$stash-&gt;set([ 'x', 0, 'y', 0 ], 10);</pre>
                    <p>
                      So the stash gives us access to template variables and the context
                      provides the higher level functionality.
                    </p>
                    <p>
                      Alongside the <a href="../modules/Template/Context.html#method_process">process()</a> method lies the <a href="../modules/Template/Context.html#method_include">include()</a> method.
                      Just as with the <code>PROCESS</code> / <code>INCLUDE</code> directives,
                      the key difference is in variable localisation. Before processing a
                      template, the <code>process()</code> method simply updates the stash to
                      set any new variable definitions, overwriting any existing values. In
                      contrast, the <code>include()</code> method creates a copy of the
                      existing stash, in a process known as <i>cloning</i> the stash, and then
                      uses that as a temporary variable store. Any previously existing
                      variables are still defined, but any changes made to variables, including
                      setting the new variable values passed aas arguments will affect only the
                      local copy of the stash (although note that it's only a shallow copy, so
                      it's not foolproof). When the template has been processed, the
                      <code>include()</code> method restores the previous variable state by
                      <i>decloning</i> the stash.
                    </p>
                    <p>
                      The context also provides an <a href="../modules/Template/Context.html#method_insert">insert()</a> method to implement the
                      <code>INSERT</code> directive, but no <code>wrapper()</code> method. This
                      functionality can be implemented by rewriting the Perl code and calling
                      <code>include()</code>.
                    </p>
                    <pre><span class="tt">[% WRAPPER foo -%]</span>
   blah blah <span class="tt">[% x %]</span>
<span class="tt">[%- END %]</span>

$context-&gt;include('foo', {
    content =&gt; 'blah blah ' . $stash-&gt;get('x'),
});</pre>
                    <p>
                      Other than the template processing methods <code>process()</code>,
                      <code>include()</code> and <code>insert()</code>, the context defines
                      methods for fetching plugin objects, <a href="../modules/Template/Context.html#method_plugin">plugin()</a>, and
                      filters, <a href="../modules/Template/Context.html#method_filter">filter()</a>.
                    </p>
                    <pre># TT USE directive
<span class="tt">[% USE foo = Bar(10) %]</span>

# equivalent Perl
$stash-&gt;set('foo', $context-&gt;plugin('Bar', [10]));

# TT FILTER block
<span class="tt">[% FILTER bar(20) %]</span>
   blah blah blah
<span class="tt">[% END %]</span>

# equivalent Perl
my $filter = $context-&gt;filter('bar', [20]);
&amp;$filter('blah blah blah');</pre>
                    <p>
                      Pretty much everything else you might want to do in a template can be
                      done in Perl code. Things like <code>IF</code>, <code>UNLESS</code>,
                      <code>FOREACH</code> and so on all have direct counterparts in Perl.
                    </p>
                    <pre># TT IF directive
<span class="tt">[% IF msg %]</span>
   Message: <span class="tt">[% msg %]</span>
<span class="tt">[% END %]</span>;

# equivalent Perl
if ($stash-&gt;get('msg')) {
    $output .=  'Message: ';
    $output .= $stash-&gt;get('msg');
}</pre>
                    <p>
                      The best way to get a better understanding of what's going on underneath
                      the hood is to set the <code>$Template::Parser::DEBUG</code> flag to a
                      true value and start processing templates. This will cause the parser to
                      print the generated Perl code for each template it compiles to
                      <code>STDERR</code>. You'll probably also want to set the
                      <code>$Template::Directive::PRETTY</code> option to have the Perl
                      pretty-printed for human consumption.
                    </p>
                    <pre>use Template;
use Template::Parser;
use Template::Directive;

$Template::Parser::DEBUG = 1;
$Template::Directive::PRETTY = 1;

my $template = Template-&gt;new();
$template-&gt;process(\*DATA, { cat =&gt; 'dog', mat =&gt; 'log' });

__DATA__
The <span class="tt">[% cat %]</span> sat on the <span class="tt">[% mat %]</span></pre>
                    <p>
                      The output sent to <code>STDOUT</code> remains as you would expect:
                    </p>
                    <pre>The dog sat on the log</pre>
                    <p>
                      The output sent to <code>STDERR</code> would look something like this:
                    </p>
                    <pre>compiled main template document block:
sub {
    my $context = shift || die "template sub called without context\n";
    my $stash   = $context-&gt;stash;
    my $output  = '';
    my $error;

    eval { BLOCK: {
        $output .=  "The ";
        $output .=  $stash-&gt;get('cat');
        $output .=  " sat on the ";
        $output .=  $stash-&gt;get('mat');
        $output .=  "\n";
    } };
    if ($@) {
        $error = $context-&gt;catch($@, \$output);
        die $error unless $error-&gt;type eq 'return';
    }

    return $output;
}</pre>
              </div>
            </div>
            <div class="section">
              <div class="head">
                <h1 id="Hacking_on_the_Template_Toolkit" onclick="switch_section(this)" title="Click title to show/hide section content.">Hacking on the Template Toolkit</h1>
                <a href="#body" class="top" title="Back up to the top of the page" >Top</a>
              </div>
              <div class="body">
                <p>
                      Please feel free to hack on the Template Toolkit. If you find a bug that
                      needs fixing, if you have an idea for something that's missing, or you
                      feel inclined to tackle something on the TODO list, then by all means go
                      ahead and do it!
                    </p>
                    <p>
                      If you're contemplating something non-trivial then you'll probably want
                      to bring it up on the mailing list first to get an idea about the current
                      state of play, find out if anyone's already working on it, and so on.
                    </p>
                    <p>
                      When you start to hack on the Template Toolkit, please make sure you
                      start from the latest developer release. Stable releases are uploaded to
                      CPAN and have all-numerical version numbers, e.g. 2.04, 2.05. Developer
                      releases are available from the Template Toolkit web site and have a
                      character suffix on the version, e.g. 2.04a, 2.04b, etc.
                    </p>
                    <p>
                      Once you've made your changes, please remember to update the test suite
                      by adding extra tests to one of the existing test scripts in the
                      <code>t</code> sub-directory, or by adding a new test script of your own.
                      And of course, run <code>make test</code> to ensure that all the tests
                      pass with your new code.
                    </p>
                    <p>
                      Don't forget that any files you do add will need to be added to the
                      MANIFEST. Running <code>make manifest</code> will do this for you, but
                      you need to make sure you haven't got any other temporary files lying
                      around that might also get added to it.
                    </p>
                    <p>
                      Documentation is often something that gets overlooked but it's just as
                      important as the code. If you're adding a new module, a plugin module,
                      for example, then it's OK to include the POD documentation in with the
                      module, but <i>please</i> write it all in one piece at the end of the
                      file, <i>after</i> the code (just look at any other
                      <code>Template::*</code> module for an example). It's a religious issue,
                      I know, but I have a strong distaste for POD documentation interspersed
                      throughout the code. In my not-so-humble opinion, it makes both the code
                      and the documentation harder to read (same kinda problem as embedding
                      Perl in HTML).
                    </p>
                    <p>
                      To share your changes with the rest of the world, you'll need to prepare
                      a patch file. To do this you should have 2 directories side-by-side, one
                      which is the original, unmodified distribution directory for the latest
                      developer release, and the other is a copy of that same directory which
                      includes your changes.
                    </p>
                    <p>
                      The following example shows a typical hacking session. First we unpack
                      the latest developer release.
                    </p>
                    <pre>$ tar zxf Template-Toolkit-2.05c.tar.gz</pre>
                    <p>
                      At this point, it's a good idea to rename the directory to give some
                      indicate of what it contains.
                    </p>
                    <pre>$ mv Template-Toolkit-2.05c Template-Toolkit-2.05c-abw-xyz-hack</pre>
                    <p>
                      Then go hack!
                    </p>
                    <pre>$ cd Template-Toolkit-2.05c-abw-xyz-hack

  [ hacking ]

$ cd ..</pre>
                    <p>
                      When you're all done and ready to prepare a patch, unpack the
                      distribution archive again so that you've got the original to
                      <code>diff</code> against your new code.
                    </p>
                    <pre>$ tar zxf Template-Toolkit-2.05c.tar.gz</pre>
                    <p>
                      You should now have an original distribution directory and a modified
                      version of that same directory, side-by-side.
                    </p>
                    <pre>$ ls
Template-Toolkit-2.05c  Template-Toolkit-2.05c-abw-xyz-hack</pre>
                    <p>
                      Now run <code>diff</code> and save the output into an appropriately named
                      patch file.
                    </p>
                    <pre>$ diff -Naur Template-Toolkit-2.05c Template-Toolkit-2.05c-abw-xyz-hack &gt; patch-TT205c-abw-xyz-hack</pre>
                    <p>
                      You can then post the generated patch file to the mailing list,
                      describing what it does, why it does it, how it does it and any other
                      relevant information.
                    </p>
                    <p>
                      If you want to apply someone else's patch then you should start with the
                      same original distribution source on which the patch is based. From
                      within the root of the distribution, run <code>patch</code> feeding in
                      the patch file as standard input. The '<code>p1</code>' option is
                      required to strip the first element of the path name (e.g.
                      <code>Template-Toolkit-2.05c/README</code> becomes <code>README</code>
                      which is then the correct path).
                    </p>
                    <pre>$ tar zxf Template-Toolkit-2.05c.tar.gz
$ cd Template-Toolkit-2.05c
$ patch -p1 &lt; ../patch-TT205c-abw-xyz-hack</pre>
                    <p>
                      The output generated by <code>patch</code> should be something like the
                      following:
                    </p>
                    <pre>patching file README
patching file lib/Template.pm
patching file lib/Template/Provider.pm
patching file t/provider.t</pre>
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