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NAME
    Template::Declare - Perlish declarative templates

SYNOPSIS
    Here's an example of basic HTML usage:

        package MyApp::Templates;
        use Template::Declare::Tags; # defaults to 'HTML'
        use base 'Template::Declare';

        template simple => sub {
            html {
                head {}
                body {
                    p { 'Hello, world wide web!' }
                }
            }
        };

        package main;
        use Template::Declare;
        Template::Declare->init( dispatch_to => ['MyApp::Templates'] );
        print Template::Declare->show( 'simple' );

    And here's the output:

     <html>
      <head></head>
      <body>
       <p>Hello, world wide web!
       </p>
      </body>
     </html>

DESCRIPTION
    "Template::Declare" is a pure-Perl declarative HTML/XUL/RDF/XML
    templating system.

    Yes. Another one. There are many others like it, but this one is ours.

    A few key features and buzzwords:

    *   All templates are 100% pure Perl code

    *   Simple declarative syntax

    *   No angle brackets

    *   "Native" XML namespace and declaration support

    *   Mixins

    *   Inheritance

    *   Delegation

    *   Public and private templates

GLOSSARY
    template class
        A subclass of Template::Declare in which one or more templates are
        defined using the "template" keyword, or that inherits templates
        from a super class.

    template
        Created with the "template" keyword, a template is a subroutine that
        uses "tags" to generate output.

    attribute
        An XML element attribute. For example, in "<img src="foo.png" />",
        "src" is an attribute of the "img" element.

    tag A subroutine that generates XML element-style output. Tag
        subroutines execute blocks that generate the output, and can call
        other tags to generate a properly hierarchical structure.

    tag set
        A collection of related tags defined in a subclass of
        Template::Declare::TagSet for a particular purpose, and which can be
        imported into a template class. For example,
        Template::Declare::TagSet::HTML defines tags for emitting HTML
        elements.

    wrapper
        A subroutine that wraps the output from a template. Useful for
        wrapping template output in common headers and footers, for example.

    dispatch class
        A template class that has been passed to "init()" via the
        "dispatch_to" parameter. When show is called, only templates defined
        in or mixed into the dispatch classes will be executed.

    path
        The name specified for a template when it is created by the
        "template" keyword, or when a template is mixed into a template
        class.

    mixin
        A template mixed into a template class via "mix". Mixed-in templates
        may be mixed in under prefix paths to distinguish them from the
        templates defined in the dispatch classes.

    alias
        A template aliased into a template class via "alias". Aliased
        templates may be added under prefix paths to distinguish them from
        the templates defined in the dispatch classes.

    package variable
        Variables defined when mixing templates into a template class. These
        variables are available only to the mixed-in templates; they are not
        even accessible from the template class in which the templates were
        defined.

    helper
        A subroutine used in templates to assist in the generation of
        output, or in template classes to assist in the mixing-in of
        templates. Output helpers include "outs()" for rending text output
        and "xml_decl()" for rendering XML declarations. Mixin helpers
        include "into" for specifying a template class to mix into, and
        "under" for specifying a path prefix under which to mix templates.

USAGE
    Like other Perl templating systems, there are two parts to
    Template::Declare: the templates and the code that loads and executes
    the templates. Unlike other template systems, the templates are written
    in Perl classes. A simple HTML example is in the "SYNOPSIS".

  A slightly more advanced example
    In this example, we'll show off how to set attributes on HTML tags, how
    to call other templates, and how to declare a *private* template that
    can't be called directly. We'll also show passing arguments to
    templates. First, the template class:

        package MyApp::Templates;
        use base 'Template::Declare';
        use Template::Declare::Tags;

        private template 'util/header' => sub {
            head {
                title { 'This is a webpage' };
                meta  {
                    attr { generator => "This is not your father's frontpage" }
                }
            }
        };

        private template 'util/footer' => sub {
            my $self = shift;
            my $time = shift || gmtime;

            div {
                attr { id => "footer"};
                "Page last generated at $time."
            }
        };

        template simple => sub {
            my $self = shift;
            my $user = shift || 'world wide web';

            html {
                show('util/header');
                body {
                    img { src is 'hello.jpg' }
                    p {
                        attr { class => 'greeting'};
                        "Hello, $user!"
                    };
                };
                show('util/footer', 'noon');
            }
        };

    A few notes on this example:

    *   Since no parameter was passed to "use Template::Declare::Tags", the
        HTML tags are imported by default.

    *   The "private" keyword indicates that a template is private. That
        means that it can only be executed by other templates within the
        template class in which it's declared. By default,
        "Template::Declare->show" will not dispatch to it.

    *   The two private templates have longer paths than we've seen before:
        "util/header" and "util/footer". They must of course be called by
        their full path names. You can put any characters you like into
        template names, but the use of Unix filesystem-style paths is the
        most common (following on the example of HTML::Mason).

    *   The first argument to a template is a class name. This can be useful
        for calling methods defined in the class.

    *   The "show" sub executes another template. In this example, the
        "simple" template calls "show('util/header')" and
        "show('util/footer')" in order to execute those private templates in
        the appropriate places.

    *   Additional arguments to "show" are passed on to the template being
        executed. here, "show('util/footer', 'noon')" is passing "noon" to
        the "util/footer" template, with the result that the "last generated
        at" string will display "noon" instead of the default "gmtime".

    *   In the same way, note that the "simple" template expects an
        additional argument, a user name.

    *   In addition to using "attr" to declare attributes for an element,
        you can use "is", as in

            img { src is 'hello.jpg' }

    Now for executing the template:

        package main;
        use Template::Declare;
        Template::Declare->init( dispatch_to => ['MyApp::Templates'] );
        print Template::Declare->show( '/simple', 'TD user');

    We've told Template::Declare to dispatch to templates defined in our
    template class. And note how an additional argument is passed to
    "show()"; that argument, "TD user", will be passed to the "simple"
    template, where it will be used in the $user variable.

    The output looks like this:

     <html>
      <head>
       <title>This is a webpage</title>
       <meta generator="This is not your father&#39;s frontpage" />
      </head>
      <body>
       <img src="hello.jpg" />
       <p class="greeting">Hello, TD user!</p>
      </body>
      <div id="footer">Page last generated at Thu Sep  3 20:56:14 2009.</div>
     </html>

    Note that the single quote in "father's" was quoted for you. We sanitize
    your output for you to help prevent cross-site scripting attacks.

  XUL
    Template::Declare isn't limited to just HTML. Let's do XUL!

        package MyApp::Templates;
        use base 'Template::Declare';
        use Template::Declare::Tags 'XUL';

        template main => sub {
            xml_decl { 'xml', version => '1.0' };
            xml_decl {
                'xml-stylesheet',
                href => "chrome://global/skin/",
                type => "text/css"
            };
            groupbox {
                caption { attr { label => 'Colors' } }
                radiogroup {
                    for my $id ( qw< orange violet yellow > ) {
                        radio {
                            attr {
                                id    => $id,
                                label => ucfirst($id),
                                $id eq 'violet' ? (selected => 'true') : ()
                            }
                        }
                    } # for
                }
            }
        };

    The first thing to do in a template class is to subclass
    Template::Declare itself. This is required so that Template::Declare
    always knows that it's dealing with templates. The second thing is to
    "use Template::Declare::Tags" to import the set of tag subroutines you
    need to generate the output you want. In this case, we've imported tags
    to support the creation of XUL. Other tag sets include HTML (the
    default), and RDF.

    Templates are created using the "template" keyword:

        template main => sub { ... };

    The first argument is the name of the template, also known as its
    *path*. In this case, the template's path is "main" (or "/main", both
    are allowed (to keep both PHP and HTML::Mason fans happy). The second
    argument is an anonymous subroutine that uses the tag subs (and any
    other necessary code) to generate the output for the template.

    The tag subs imported into your class take blocks as arguments, while a
    number of helper subs take other arguments. For example, the "xml_decl"
    helper takes as its first argument the name of the XML declaration to be
    output, and then a hash of the attributes of that declaration:

        xml_decl { 'xml', version => '1.0' };

    Tag subs are used by simply passing a block to them that generates the
    output. Said block may of course execute other tag subs in order to
    represent the hierarchy required in your output. Here, the "radiogroup"
    tag calls the "radio" tag for each of three different colors:

        radiogroup {
            for my $id ( qw< orange violet yellow > ) {
                radio {
                    attr {
                        id    => $id,
                        label => ucfirst($id),
                        $id eq 'violet' ? (selected => 'true') : ()
                    }
                }
            } # for
        }

    Note the "attr" sub. This helper function is used to add attributes to
    the element created by the tag in which they appear. In the previous
    example, the the "id", "label", and "selected" attributes are added to
    each "radio" output.

    Once you've written your templates, you'll want to execute them. You do
    so by telling Template::Declare what template classes to dispatch to and
    then asking it to show you the output from a template:

        package main;
        Template::Declare->init( dispatch_to => ['MyApp::Templates'] );
        print Template::Declare->show( 'main' );

    The path passed to "show" can be either "main" or </main>, as you
    prefer. In either event, the output would look like this:

     <?xml version="1.0"?>
     <?xml-stylesheet href="chrome://global/skin/" type="text/css"?>

     <groupbox>
      <caption label="Colors" />
      <radiogroup>
       <radio id="orange" label="Orange" />
       <radio id="violet" label="Violet" selected="true" />
       <radio id="yellow" label="Yellow" />
      </radiogroup>
     </groupbox>

  Postprocessing
    Sometimes you just want simple syntax for inline elements. The following
    shows how to use a postprocessor to emphasize text _like this_.

        package MyApp::Templates;
        use Template::Declare::Tags;
        use base 'Template::Declare';

        template before => sub {
            h1 {
                outs "Welcome to ";
                em { "my" };
                outs " site. It's ";
                em { "great" };
                outs "!";
            };
        };

        template after => sub {
            h1  { "Welcome to _my_ site. It's _great_!" };
            h2  { outs_raw "This is _not_ emphasized." };
            img { src is '/foo/_bar_baz.png' };
        };

    Here we've defined two templates in our template class, with the paths
    "before" and "after". The one new thing to note is the use of the "outs"
    and "outs_raw" subs. "outs" XML-encodes its argument and outputs it. You
    can also just specify a string to be output within a tag call, but if
    you need to mix tags and plain text within a tag call, as in the
    "before" template here, you'll need to use "outs" to get things to
    output as you would expect. "outs_raw" is the same, except that it does
    no XML encoding.

    Now let's have a look at how we use these templates with a
    post-processor:

        package main;
        use Template::Declare;
        Template::Declare->init(
            dispatch_to   => ['MyApp::Templates'],
            postprocessor => \&emphasize,
            strict        => 1,
        );

        print Template::Declare->show( 'before' );
        print Template::Declare->show( 'after'  );

        sub emphasize {
            my $text = shift;
            $text =~ s{_(.+?)_}{<em>$1</em>}g;
            return $text;
        }

    As usual, we've told Template::Declare to dispatch to our template
    class. A new parameter to "init()" is "postprocessor", which is a code
    reference that should expect the template output as an argument. It can
    then transform that text however it sees fit before returning it for
    final output. In this example, the "emphasize" subroutine looks for text
    that's emphasized using _underscores_ and turns them into
    "<em>emphasis</em>" HTML elements.

    We then execute both the "before" and the "after" templates with the
    output ending up as:

     <h1>Welcome to
      <em>my</em> site. It&#39;s
      <em>great</em>!</h1>
     <h1>Welcome to <em>my</em> site. It&#39;s <em>great</em>!</h1>
     <h2>This is _not_ emphasized.</h2>
     <img src="/foo/_bar_baz.png" />

    The thing to note here is that text passed to "outs_raw" is not passed
    through the postprocessor, and neither are attribute values (like the
    "img"'s "src").

  Inheritance
    Templates are really just methods. You can subclass your template
    packages to override some of those methods:

        package MyApp::Templates::GenericItem;
        use Template::Declare::Tags;
        use base 'Template::Declare';

        template 'list' => sub {
            my ($self, @items) = @_;
            div {
                show('item', $_) for @items;
            }
        };
        template 'item' => sub {
            my ($self, $item) = @_;
            span { $item }
        };

        package MyApp::Templates::BlogPost;
        use Template::Declare::Tags;
        use base 'MyApp::Templates::GenericItem';

        template 'item' => sub {
            my ($self, $post) = @_;
            h1  { $post->title }
            div { $post->body }
        };

    Here we have two template classes; the second,
    "MyApp::Templates::BlogPost", inherits from the first,
    "MyApp::Templates::GeniricItem". Note also that
    "MyApp::Templates::BlogPost" overrides the "item" template. So execute
    these templates:

        package main;
        use Template::Declare;

        Template::Declare->init( dispatch_to => ['MyApp::Templates::GenericItem'] );
        print Template::Declare->show( 'list', 'foo', 'bar', 'baz' );

        Template::Declare->init( dispatch_to => ['MyApp::Templates::BlogPost'] );
        my $post = My::Post->new(title => 'Hello', body => 'first post');
        print Template::Declare->show( 'item', $post );

    First we execute the "list" template in the base class, passing in some
    items, and then we re-"init()" Template::Declare and execute *its*
    "list" template with an appropriate argument. Here's the output:

     <div>
      <span>foo</span>
      <span>bar</span>
      <span>baz</span>
     </div>

     <h1>Hello</h1>
     <div>first post</div>

    So the override of the "list" template in the subclass works as
    expected. For another example, see Jifty::View::Declare::CRUD.

  Wrappers
    There are two levels of wrappers in Template::Declare: template wrappers
    and smart tag wrappers.

   Template Wrappers
    "create_wrapper" declares a wrapper subroutine that can be called like a
    tag sub, but can optionally take arguments to be passed to the wrapper
    sub. For example, if you wanted to wrap all of the output of a template
    in the usual HTML headers and footers, you can do something like this:

        package MyApp::Templates;
        use Template::Declare::Tags;
        use base 'Template::Declare';

        BEGIN {
            create_wrapper wrap => sub {
                my $code = shift;
                my %params = @_;
                html {
                    head { title { outs "Hello, $params{user}!"} };
                    body {
                        $code->();
                        div { outs 'This is the end, my friend' };
                    };
                }
            };
        }

        template inner => sub {
            wrap {
                h1 { outs "Hello, Jesse, s'up?" };
            } user => 'Jesse';
        };

    Note how the "wrap" wrapper function is available for calling after it
    has been declared in a "BEGIN" block. Also note how you can pass
    arguments to the function after the closing brace (you don't need a
    comma there!).

    The output from the "inner" template will look something like this:

     <html>
      <head>
       <title>Hello, Jesse!</title>
      </head>
      <body>
       <h1>Hello, Jesse, s&#39;up?</h1>
       <div>This is the end, my friend</div>
      </body>
     </html>

   Tag Wrappers
    Tag wrappers are similar to template wrappers, but mainly function as
    syntax sugar for creating subroutines that behave just like tags but are
    allowed to contain arbitrary Perl code and to dispatch to other tag. To
    create one, simply create a named subroutine with the prototype "(&)" so
    that its interface is the same as tags. Within it, use
    "smart_tag_wrapper" to do the actual execution, like so:

        package My::Template;
        use Template::Declare::Tags;
        use base 'Template::Declare';

        sub myform (&) {
            my $code = shift;

            smart_tag_wrapper {
                my %params = @_; # set using 'with'
                form {
                    attr { %{ $params{attr} } };
                    $code->();
                    input { attr { type => 'submit', value => $params{value} } };
                };
            };
        }

        template edit_prefs => sub {
            with(
                attr  => { id => 'edit_prefs', action => 'edit.html' },
                value => 'Save'
            ), myform {
                label { 'Time Zone' };
                input { type is 'text'; name is 'tz' };
            };
        };

    Note in the "edit_prefs" template that we've used "with" to set up
    parameters to be passed to the smart wrapper. "smart_tag_wrapper()" is
    the device that allows you to receive those parameters, and also handles
    the magic of making sure that the tags you execute within it are
    properly output. Here we've used "myform" similarly to "form", only
    "myform" does something different with the "with()" arguments and
    outputs a submit element.

    Executing this template:

        Template::Declare->init( dispatch_to => ['My::Template'] );
        print Template::Declare->show('edit_prefs');

    Yields this output:

     <form action="edit.html" id="edit_prefs">
      <label>Time Zone</label>
      <input type="text" name="tz" />
      <input type="submit" value="Save" />
     </form>

  Class Search Dispatching
    The classes passed via the "dispatch_to" parameter to "init()" specify
    all of the templates that can be executed by subsequent calls to
    "show()". Template searches through these classes in order to find those
    templates. Thus it can be useful, when you're creating your template
    classes and determining which to use for particular class to "show()",
    to have templates that override other templates. This is similar to how
    an operating system will search all the paths in the $PATH environment
    variable for a program to run, and to HTML::Mason component roots or
    Template::Toolkit's "INCLUDE_PATH" parameter.

    For example, say you have this template class that defines a template
    that you'll use for displaying images on your Web site.

        package MyApp::UI::Standard;
        use Template::Declare::Tags;
        use base 'Template::Declare';

        template image => sub {
            my ($self, $src, $title) = @_;
            img {
                src is $src;
                title is $title;
            };
        };

    As usual, you can use it like so:

        my @template_classes = 'MyApp::UI::Standard';
        Template::Declare->init( dispatch_to => \@template_classes );
        print Template::Declare->show('image', 'foo.png', 'Foo');

    We're explicitly using a reference to @template_classes so that we can
    manage this list ourselves.

    The output of this will be:

     <div class="std">
      <img src="foo.png" title="Foo" />
      <p class="caption"></p>
     </div>

    But say that in some sections of your site you need to have a more
    formal treatment of your photos. Maybe you publish photos from a wire
    service and need to provide an appropriate credit. You might write the
    template class like so:

        package MyApp::UI::Formal;
        use Template::Declare::Tags;
        use base 'Template::Declare';

        template image => sub {
            my ($self, $src, $title, $credit, $caption) = @_;
            div {
                class is 'formal';
                img {
                    src is $src;
                    title is $title;
                };
                p {
                    class is 'credit';
                    outs "Photo by $credit";
                };
                p {
                    class is 'caption';
                    outs $caption;
                };
            };
        };

    This, too, will work as expected, but the useful bit that comes in when
    you're mixing and matching template classes to pass to "dispatch_to"
    before rendering a page. Maybe you always pass have MyApp::UI::Standard
    to "dispatch_to" because it has all of your standard formatting
    templates. But when the code realizes that a particular page needs the
    more formal treatment, you can prepend the formal class to the list:

        unshift @template_classes, 'MyApp::UI::Formal';
        print Template::Declare->show(
            'image',
            'ap.png',
            'AP Photo',
            'Clark Kent',
            'Big news'
        );
        shift @template_classes;

    In this way, made the formal "image" template will be found first,
    yielding this output:

     <div class="formal">
      <img src="ap.png" title="AP Photo" />
      <p class="credit">Photo by Clark Kent</p>
      <p class="caption">Big news</p>
     </div>

    At the end, we've shifted the formal template class off the
    "dispatch_to" list in order to restore the template classes the default
    configuration, ready for the next request.

  Template Composition
    There are two methods of template composition: mixins and delegation.
    Their interfaces are very similar, the only difference being the
    template invocant.

   Mixins
    Let's start with a mixin.

        package MyApp::UtilTemplates;
        use Template::Declare::Tags;
        use base 'Template::Declare';

        template content => sub {
            my $self  = shift;
            my @paras = @_;
            h1 { $self->get_title };
            div {
                id is 'content';
                p { $_ } for @paras;
            };
        };

        package MyApp::Templates;
        use Template::Declare::Tags;
        use base 'Template::Declare';
        mix MyApp::UtilTemplates under '/util';

        sub get_title { 'Kashmir' }

        template story => sub {
            my $self = shift;
            html {
              head {
                  title { "My Site: " . $self->get_title };
              };
              body {
                  show( 'util/content' => 'first paragraph', 'second paragraph' );
              };
            };
        };

    The first template class, "MyApp::UtilTemplates", defines a utility
    template, called "content", for outputting the contents of page. Note
    its call to "$self->get_title" even though it doesn't have a "get_title"
    method. This is part of the mixin's "contract": it requires that the
    class it's mixed into have a "get_title()" method.

    The second template class, "MyApp::Templates", mixes
    "MyApp::UtilTemplates" into itself under the path "/util" and defines a
    "get_title()" method as required by the mixin. Then, its "story"
    template calls the mixed-in template as "util/content", because the
    "content" template was mixed into the current template under "/util".
    Get it?

    Now we can use the usual template invocation:

        package main;
        Template::Declare->init( dispatch_to => ['MyApp::Templates'] );
        print Template::Declare->show('story');

    To appreciate our output:

     <html>
      <head>
       <title>My Site: Kashmir</title>
      </head>
      <body>
       <h1>Kashmir</h1>
       <div id="content">
        <p>fist paragraph</p>
        <p>second paragraph</p>
       </div>
      </body>
     </html>

    Mixins are a very useful tool for template authors to add reusable
    functionality to their template classes. But it's important to pay
    attention to the mixin contracts so that you're sure to implement the
    required API in your template class (here, the "get_title()" method).

   Aliases
    Aliases are very similar to mixins, but implement delegation as a
    composition pattern, rather than mixins. The upshot is that there is no
    contract provided by an aliased class: it just works. This is because
    the invocant is the class from which the aliases are imported, and
    therefore it will dispatch to methods defined in the aliased class.

    For example, say that you wanted to output a sidebar on pages that need
    one (perhaps your CMS has sidebar things). We can define a template
    class that has a template for that:

        package MyApp::UI::Stuff;
        use Template::Declare::Tags;
        use base 'Template::Declare';

        sub img_path { '/ui/css' }

        template sidebar => sub {
            my ($self, $thing) = @_;
            div {
                class is 'sidebar';
                img { src is $self->img_path . '/sidebar.png' };
                p { $_->content } for $thing->get_things;
            };
        };

    Note the use of the "img_path()" method defined in the template class
    and used by the "sidebar" template. Now let's use it:

        package MyApp::Render;
        use Template::Declare::Tags;
        use base 'Template::Declare';
        alias MyApp::UI::Stuff under '/stuff';

        template page => sub {
            my ($self, $page) = @_;
            h1 { $page->title };
            for my $thing ($page->get_things) {
                if ($thing->is('paragraph')) {
                    p { $thing->content };
                } elsif ($thing->is('sidebar')) {
                    show( '/stuff/sidebar' => $thing );
                }
            }
        };

    Here our rendering template class has aliased "MyApp::UI::Stuff" under
    "/stuff". So the "page" template calls "show('/stuff/sidebar')" to
    invoke the sidebar template. If we run this:

        Template::Declare->init( dispatch_to => ['MyApp::Render'] );
        print Template::Declare->show( page => $page );

    We get output as you might expect:

     <h1>My page title</h1>
     <p>Page paragraph</p>
     <div class="sidebar">
      <img src="/ui/css/sidebar.png" />
      <p>Sidebar paragraph</p>
      <p>Another paragraph</p>
     </div>

    Now, let's say that you have political stuff that you want to use a
    different image for in the sidebar. If that's the only difference, we
    can subclass "MyApp::UI::Stuff" and just override the "img_path()"
    method:

        package MyApp::UI::Stuff::Politics;
        use Template::Declare::Tags;
        use base 'MyApp::UI::Stuff';

        sub img_path { '/politics/ui/css' }

    Now let's mix that into a politics template class:

        package MyApp::Render::Politics;
        use Template::Declare::Tags;
        use base 'Template::Declare';
        alias MyApp::UI::Stuff::Politics under '/politics';

        template page => sub {
            my ($self, $page) = @_;
            h1 { $page->title };
            for my $thing ($page->get_things) {
                if ($thing->is('paragraph')) {
                    p { $thing->content };
                } elsif ($thing->is('sidebar')) {
                    show( '/politics/sidebar' => $thing );
                }
            }
        };

    The only difference between this template class and "MyApp::Render" is
    that it aliases "MyApp::UI::Stuff::Politics" under "/politics", and then
    calls "show('/politics/sidebar')" in the "page" template. Running this
    template:

        Template::Declare->init( dispatch_to => ['MyApp::Render::Politics'] );
        print Template::Declare->show( page => $page );

    Yields output using the value of the subclass's "img_path()" method --
    that is, the sidebar image is now /politics/ui/css/sidebar.png instead
    of /ui/css/sidebar.png:

     <h1>My page title</h1>
     <p>Page paragraph</p>
     <div class="sidebar">
      <img src="/politics/ui/css/sidebar.png" />
      <p>Sidebar paragraph</p>
      <p>Another paragraph</p>
     </div>

   Other Tricks
    The delegation behavior of "alias" actually makes it a decent choice for
    template authors to mix and match libraries of template classes as
    appropriate, without worrying about side effects. You can even alias
    templates in one template class into another template class if you're
    not the author of that class by using the "into" keyword:

        alias My::UI::Widgets into Your::UI::View under '/widgets';

    Now the templates defined in "Your::UI::View" are available in
    "My::UI::Widgets" under "/widgets". The "mix" method supports this
    syntax as well, though it's not necessarily recommended, given that you
    would not be able to fulfill any contracts unless you re-opened the
    class into which you mixed the templates. But in any case, authors of
    framework view classes might find this functionality useful for
    automatically aliasing template classes into a single dispatch template
    class.

    Another trick is to alias or mix your templates with package variables
    specific to the composition. Do so via the "setting" keyword:

        package My::Templates;
        mix Some::Mixin under '/mymix', setting { name => 'Larry' };

    The templates mixed from "Some::Mixin" into "My::Templates" have package
    variables set for them that are accessible *only* from their mixed-in
    paths. For example, if this template was defined in "Some::Mixin":

        template howdy => sub {
            my $self = shift;
            outs "Howdy, " . $self->package_variable('name') || 'Jesse';
        };

    Then "show('mymix/howdy')" called on "My::Templates" will output "Howdy,
    Larry", while the output from "show('howdy')" will output "Howdy,
    Jesse". In other words, package variables defined for the mixed-in
    templates are available only to the mixins and not to the original. The
    same functionality exists for "alias" as well.

  Indentation configuration
    by default, Template::Declare renders a readable XML adding end of lines
    and a one column indentation. This behavior could break a webpage design
    or add a significant amount of chars to your XML output. This could be
    changed by overwriting the default values. so

        $Template::Declare::Tags::TAG_INDENTATION  = 0;
        $Template::Declare::Tags::EOL              = "";
        say Template::Declare->show('main');

    will render

        <html><body><p>hi</p></body></html>

METHODS
  init
    This *class method* initializes the "Template::Declare" system.

    dispatch_to
        An array reference of classes to search for templates.
        Template::Declare will search this list of classes in order to find
        a template path.

    roots
        Deprecated. Just like "dispatch_to", only the classes are searched
        in reverse order. Maintained for backward compatibility and for the
        pleasure of those who want to continue using Template::Declare the
        way that Jesse's "crack-addled brain" intended.

    postprocessor
        A coderef called to postprocess the HTML or XML output of your
        templates. This is to alleviate using Tags for simple text markup.

    around_template
        A coderef called instead of rendering each template. The coderef
        will receive three arguments: a coderef to invoke to render the
        template, the template's path, an arrayref of the arguments to the
        template, and the coderef of the template itself. You can use this
        for instrumentation. For example:

            Template::Declare->init(around_template => sub {
                my ($orig, $path, $args, $code) = @_;
                my $start = time;
                $orig->();
                warn "Rendering $path took " . (time - $start) . " seconds.";
            });

    strict
        Die in exceptional situations, such as when a template can't be
        found, rather than just warn. False by default for backward
        compatibility. The default may be changed in the future, so
        specifying the value explicitly is recommended.

  show TEMPLATE_NAME
        Template::Declare->show( 'howdy', name => 'Larry' );
        my $output = Template::Declare->show('index');

    Call "show" with a "template_name" and "Template::Declare" will render
    that template. Subsequent arguments will be passed to the template.
    Content generated by "show()" can be accessed via the "output()" method
    if the output method you've chosen returns content instead of outputting
    it directly.

    If called in scalar context, this method will also just return the
    content when available.

  Template Composition
    Sometimes you want to mix templates from one class into another class,
    or delegate template execution to a class of templates. "alias()" and
    "mix()" are your keys to doing so.

   mix
        mix Some::Clever::Mixin      under '/mixin';
        mix Some::Other::Mixin       under '/otmix', setting { name => 'Larry' };
        mix My::Mixin into My::View, under '/mymix';

    Mixes templates from one template class into another class. When the
    mixed-in template is called, its invocant will be the class into which
    it was mixed. This type of composition is known as a "mixin" in
    object-oriented parlance. See Template Composition for extended examples
    and a comparison to "alias".

    The first parameter is the name of the template class to be mixed in.
    The "under" keyword tells "mix" where to put the templates. For example,
    a "foo" template in "Some::Clever::Mixin" will be mixed in as
    "mymixin/foo".

    The "setting" keyword specifies package variables available only to the
    mixed-in copies of templates. These are available to the templates as
    "$self->package_variable($varname)".

    The "into" keyword tells "mix" into what class to mix the templates.
    Without this keyword, "mix" will mix them into the calling class.

    For those who prefer a direct OO syntax for mixins, just call "mix()" as
    a method on the class to be mixed in. To replicate the above three
    examples without the use of the sugar:

        Some::Clever::Mixin->mix( '/mixin' );
        Some::Other::Mixin->mix( '/otmix', { name => 'Larry' } );
        My::Mixin->mix( 'My::View', '/mymix' );

   alias
        alias Some::Clever:Templates   under '/delegate';
        alias Some::Other::Templates   under '/send_to', { name => 'Larry' };
        alias UI::Stuff into My::View, under '/mystuff';

    Aliases templates from one template class into another class. When an
    alias called, its invocant will be the class from which it was aliased.
    This type of composition is known as "delegation" in object-oriented
    parlance. See Template Composition for extended examples and a
    comparison to "mix".

    The first parameter is the name of the template class to alias. The
    "under" keyword tells "alias" where to put the templates. For example, a
    "foo" template in "Some::Clever::Templates" will be aliased as
    "delegate/foo".

    The "setting" keyword specifies package variables available only to the
    aliases. These are available to the templates as
    "$self->package_variable($varname)".

    The "into" keyword tells "alias" into what class to alias the templates.
    Without this keyword, "alias" will alias them into the calling class.

    For those who prefer a direct OO syntax for mixins, just call "alias()"
    as a method on the class to be mixed in. To replicate the above three
    examples without the use of the sugar:

        Some::Clever:Templates->alias( '/delegate' );
        Some::Other::Templates->alias( '/send_to', { name => 'Larry' } );
        UI::Stuff->alias( 'My::View', '/mystuff' );

   package_variable( VARIABLE )
      $td->package_variable( $varname => $value );
      $value = $td->package_variable( $varname );

    Returns a value set for a mixed-in template's variable, if any were
    specified when the template was mixed-in. See "mix" for details.

   package_variables( VARIABLE )
        $td->package_variables( $variables );
        $variables = $td->package_variables;

    Get or set a hash reference of variables for a mixed-in template. See
    "mix" for details.

  Templates registration and lookup
   resolve_template TEMPLATE_PATH INCLUDE_PRIVATE_TEMPLATES
        my $code = Template::Declare->resolve_template($template);
        my $code = Template::Declare->has_template($template, 1);

    Turns a template path ("TEMPLATE_PATH") into a "CODEREF". If the boolean
    "INCLUDE_PRIVATE_TEMPLATES" is true, resolves private template in
    addition to public ones. "has_template()" is an alias for this method.

    First it looks through all the valid Template::Declare classes defined
    via "dispatch_to". For each class, it looks to see if it has a template
    called $template_name directly (or via a mixin).

   has_template TEMPLATE_PATH INCLUDE_PRIVATE_TEMPLATES
    An alias for "resolve_template".

   register_template( TEMPLATE_NAME, CODEREF )
        MyApp::Templates->register_template( howdy => sub { ... } );

    This method registers a template called "TEMPLATE_NAME" in the calling
    class. As you might guess, "CODEREF" defines the template's
    implementation. This method is mainly intended to be used internally, as
    you use the "template" keyword to create templates, right?

   register_private_template( TEMPLATE_NAME, CODEREF )
        MyApp::Templates->register_private_template( howdy => sub { ... } );

    This method registers a private template called "TEMPLATE_NAME" in the
    calling class. As you might guess, "CODEREF" defines the template's
    implementation.

    Private templates can't be called directly from user code but only from
    other templates.

    This method is mainly intended to be used internally, as you use the
    "private template" expression to create templates, right?

   buffer
    Gets or sets the String::BufferStack object; this is a class method.

    You can use it to manipulate the output from tags as they are output.
    It's used internally to make the tags nest correctly, and be output to
    the right place. We're not sure if there's ever a need for you to frob
    it by hand, but it does enable things like the following:

        template simple => sub {
           html {
               head {}
               body {
                   Template::Declare->buffer->set_filter( sub {uc shift} );
                   p { 'Whee!' }
                   p { 'Hello, world wide web!' }
                   Template::Declare->buffer->clear_top if rand() < 0.5;
               }
           }
        };

    ...which outputs, with equal regularity, either:

     <html>
      <head></head>
      <body>
       <P>WHEE!</P>
       <P>HELLO, WORLD WIDE WEB!</P>
      </body>
     </html>

    ...or:

     <html>
      <head></head>
      <body></body>
     </html>

    We'll leave it to you to judge whether or not that's actually useful.

  Helpers
    You don't need to call any of this directly.

   into
        $class = into $class;

    "into" is a helper method providing semantic sugar for the "mix" method.
    All it does is return the name of the class on which it was called.

  Old, deprecated or just better to avoid
   import_templates
        import_templates MyApp::Templates under '/something';

    Like "mix()", but without support for the "into" or "setting" keywords.
    That is, it mixes templates into the calling template class and does not
    support package variables for those mixins.

    Deprecated in favor of "mix". Will be supported for a long time, but new
    code should use "mix()".

   new_buffer_frame
        $td->new_buffer_frame;
        # same as 
        $td->buffer->push( private => 1 );

    Creates a new buffer frame, using "push" in String::BufferStack with
    "private".

    Deprecated in favor of dealing with "buffer" directly.

   end_buffer_frame
        my $buf = $td->end_buffer_frame;
        # same as
        my $buf = $td->buffer->pop;

    Deletes and returns the topmost buffer, using "pop" in
    String::BufferStack.

    Deprecated in favor of dealing with "buffer" directly.

   path_for $template
        my $path = Template::Declare->path_for('index');

    Returns the path for the template name to be used for show, adjusted
    with paths used in "mix". Note that this will only work for the last
    class into which you imported the template. This method is, therefore,
    deprecated.

PITFALLS
    We're reusing the perl interpreter for our templating language, but Perl
    was not designed specifically for our purpose here. Here are some known
    pitfalls while you're scripting your templates with this module.

    *   It's quite common to see tag sub calling statements without trailing
        semi-colons right after "}". For instance,

            template foo => sub {
                p {
                    a { attr { src => '1.png' } }
                    a { attr { src => '2.png' } }
                    a { attr { src => '3.png' } }
                }
            };

        is equivalent to

            template foo => sub {
                p {
                    a { attr { src => '1.png' } };
                    a { attr { src => '2.png' } };
                    a { attr { src => '3.png' } };
                };
            };

        But "xml_decl" is a notable exception. Please always put a trailing
        semicolon after "xml_decl { ... }", or you'll mess up the order of
        output.

    *   Another place that requires trailing semicolon is the statements
        before a Perl looping statement, an if statement, or a "show" call.
        For example:

            p { "My links:" };
            for (@links) {
                with ( src => $_ ), a {}
            }

        The ";" after " p { ... } " is required here, or Perl will complain
        about syntax errors.

        Another example is

            h1 { 'heading' };  # this trailing semicolon is mandatory
            show 'tag_tag'

    *   The "is" syntax for declaring tag attributes also requires a
        trailing semicolon, unless it is the only statement in a block. For
        example,

            p { class is 'item'; id is 'item1'; outs "This is an item" }
            img { src is 'cat.gif' }

    *   Literal strings that have tag siblings won't be captured. So the
        following template

            p { 'hello'; em { 'world' } }

        produces

         <p>
          <em>world</em>
         </p>

        instead of the desired output

         <p>
          hello
          <em>world</em>
         </p>

        You can use "outs" here to solve this problem:

            p { outs 'hello'; em { 'world' } }

        Note you can always get rid of "outs" if the string literal is the
        only element of the containing block:

            p { 'hello, world!' }

    *   Look out! If the if block is the last block/statement and the
        condition part is evaluated to be 0:

            p { if ( 0 ) { } }

        produces

         <p>0</p>

        instead of the more intuitive output:

         <p></p>

        This is because "if ( 0 )" is the last expression, so 0 is returned
        as the value of the whole block, which is used as the content of <p>
        tag.

        To get rid of this, just put an empty string at the end so it
        returns empty string as the content instead of 0:

            p { if ( 0 ) { } '' }

BUGS
    Crawling all over, baby. Be very, very careful. This code is so cutting
    edge, it can only be fashioned from carbon nanotubes. But we're already
    using this thing in production :) Make sure you have read the "PITFALLS"
    section above :)

    Some specific bugs and design flaws that we'd love to see fixed.

    Output isn't streamy.

    If you run into bugs or misfeatures, please report them to
    "bug-template-declare@rt.cpan.org".

SEE ALSO
    Template::Declare::Tags
    Template::Declare::TagSet
    Template::Declare::TagSet::HTML
    Template::Declare::TagSet::XUL
    Jifty

AUTHOR
    Jesse Vincent <jesse@bestpractical.com>

LICENSE
    Template::Declare is Copyright 2006-2010 Best Practical Solutions, LLC.

    Template::Declare is distributed under the same terms as Perl itself.