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Go on, admit it: You''ve written a templating system. It''s OK,
nearly everyone has at some point. You start with something
beautifully simple like $HTML =~ s/\$(\w+)/${$1}/g and end up
adding conditionals, loops and includes until you''ve created your
own unmaintainable monster.

Luckily for you, you are not the first to think it might be nice to get
the HTML out of your code. Many have come before, and more than a few
have put their contributions up on CPAN. At this time, there are so
many templating modules on CPAN that it''s almost certain you can find
one that meets your needs. This document aims to be your guide to
those modules, leading you down the path to the templating system of
your dreams.

And, if you just went straight to CPAN in the first place and never
bothered to write your own, congratulations: You''re one step ahead of
the rest of us.

On a Personal Note

Nothing can start an argument faster on the mod_perl mailing list than
a claim that one approach to templating is better than another.
People get attached to the tools they''ve chosen. Therefore, let
me say up front that I am biased. I''ve been at this for a while and I
have opinions about what works best. I''ve tried to present a balanced
appraisal of the features of various systems in this document, but it
probably won''t take you long to figure out what I like. Besides,
attempts to be completely unbiased lead to useless documents that
don''t contain any real information. So take it all with a pound of
salt and if you think I''ve been unfair to a particular tool through a
factual error or omission, let me know.

Why Use Templates?

Why bother using templates at all? Print statements and CGI.pm were
good enough for grandpa, so why should you bother learning a new way
to do things?

Consistency of Appearance

It doesn''t take a genius to see that making one navigation bar
template and using it in all of your pages is easier to manage than
hard-coding it everywhere. If you build your whole site like this,
it''s much easier to make site-wide changes in the look and feel.

Reusability

Along the same lines, building a set of commonly used components makes
it easier to create new pages.

Better Isolation From Changes

Which one changes more often: the logic of your application or the
HTML used to display it? Actually the answer doesn''t matter, as long as it''s one of them. Templates can be a great
abstraction layer between the application logic and the display logic,
allowing one to be updated without touching the other.

Division of Labor

Separating your Perl code from your HTML means that when your
marketing department decides everything should be green instead of
blue, you don''t have to lift a finger. Just send them to the HTML
coder down the hall. It''s a beautiful thing, getting out of the HTML
business.

Even if the same people in your organization write the Perl code and
the HTML, you at last have the opportunity for more people to be
working on the project in parallel.

What Are the Differences?

Before we look at the available options, let''s go through an
explanation of some of the things that make them different.

Execution Models

Although some try to be flexible about it, most templating systems
expect you to use some variation of the two basic execution models,
which I will refer to as ``pipeline'' and ``callback.'' In the callback
style, you let the template take over and it has the application''s
control flow coded into it. It uses callbacks to modules or snippets
of in-line Perl code to retrieve data for display or perform actions
such as user authentication. Some popular examples of systems using this
model include Mason, Embperl and Apache::ASP.

The pipeline style does all the work up front in a standard CGI or
mod_perl handler, then decides which template to run and passes some
data to it. The template has no control flow logic in it, just
presentation logic, e.g. show this graphic if this item is on sale.
Popular systems supporting this approach include HTML::Template and
Template Toolkit.

The callback model works well for publishing-oriented sites where
the pages are essentially mix and match sets of articles and lists.
Ideally, a site can be broken down into visual ``components'' or pieces
of pages that are general enough for an HTML coder to recombine them
into entirely new kinds of pages without any help from a programmer.

The callback model can get a bit hairy when you have to code logic
that can result in totally different content being returned. For
example, if you have a system that processes some form input and takes
the user to different pages depending on the data submitted. In these
situations, it''s easy to end up coding a spaghetti of includes and
redirects, or putting what are really multiple pages in the same file.

On the other hand, a callback approach can result in fewer files (if
the Perl code is in the HTML file), and feels easier and more
intuitive to many developers. It''s a simple step from static files to
static files with a few in-line snippets of code in them. This is part
of why PHP is so popular with new developers.

The pipeline model is more like a traditional model-view-controller
design. Working this way can provide additional performance tuning
opportunities over an approach where you don''t know what data will be
needed at the beginning of the request. You can aggregate database
queries, make smarter choices about caching, etc. It can also promote
a cleaner separation of application logic and presentation. However,
this approach takes longer to get started with since it''s a bigger
conceptual hurdle and always involves at least two files: one for the
Perl code and one for the template.

Keep in mind, many systems offer significant flexibility for
customizing their execution models. For example, Mason users could
write separate components for application logic and display, letting
the logic components choose which display component to run after
fetching their data. This allows it to be used in a pipeline style.
A Template Toolkit application could be written to use a simple
generic handler (like the Apache::Template module included in the
distribution) with all the application logic placed in the template
using object calls or in-line Perl. This would be using it in a
callback style.

HTML::Template and some of the AxKit XML processors are fairly rigid
about insisting on a pipeline approach. Neither provide methods for
calling back into Perl code during the HTML formatting stage; you have
to do the work before running the template. The authors of these tools
consider this a feature since it prevents developers from cheating on
the separation of application code and presentation.

Languages

Here''s the big issue with templating systems. This is the one that
always cranks up the flame on Web development mailing lists.

Some systems use in-line Perl statements. They may provide some extra
semantics, like Embperl''s operators for specifying whether the code''s
output should be displayed or Mason''s <%init> sections for specifying
when the code gets run, but at the end of the day your templates are
written in Perl.

Other systems provide a specialized mini-language instead of (or in
addition to) in-line Perl. These will typically have just enough
syntax to handle variable substitution, conditionals and looping.
HTML::Template and Template Toolkit are popular systems using this
approach. AxKit straddles the fence, providing both a (not-so-)
mini-language - XSLT - and an in-line Perl approach - XPathScript.

Here''s how a typical discussion of the merits of these approaches might go:

IN-LINE: Mini-languages are stupid. I already know Perl and it''s easy
enough. Why would you want to use something different?

MINI-LANG: Because my HTML coder doesn''t know Perl, and this is easier
for him.

IN-LINE: Maybe he should learn some Perl. He''d get paid more.

MINI-LANG: Whatever. You just want to use in-line Perl so you can
handle change requests by putting little hacks in the template instead
of changing your modules. That''s sloppy coding.

IN-LINE: That''s efficient coding. I can knock out data editing
screens in half the time it takes you, and then I can go back through,
putting all the in-line code into modules and just have the templates
call them.

MINI-LANG: You could, but you won''t.

IN-LINE: Is it chilly up there in that ivory tower?

MINI-LANG: Go write some VBScript, weenie.

etc.

Most people pick a side in this war and stay there. If you are one of
the few who hasn''t decided yet, you should take a moment to
think about who will be building and maintaining your templates, what
skills those people have and what will allow them to work most
efficiently.

Here''s an example of a simple chunk of template using first an in-line
style (Apache::ASP in this case) and then a mini-language style
(Template Toolkit). This code fetches an object and displays some
properties of it. The data structures used are identical in both
examples. First Apache::ASP:

<% my $product = Product->load(''sku'' => ''bar1234''); %>
<% if ($product->isbn) { %>
It''s a book!
<% } else { %>
It''s NOT a book!
<% } %>
<% foreach my $item (@{$product->related}) { %>
You might also enjoy <% $item->name %>.
<% } %>

And now Template Toolkit:

[% USE product(sku=bar1234) %]
[% IF product.isbn %]
It''s a book!
[% ELSE %]
It''s NOT a book!
[% END %]
[% FOREACH item = product.related %]
You might also enjoy [% item.name %].
[% END %]

There is a third approach, based on parsing an HTML document into a
DOM tree and then manipulating the contents of the nodes. The only
module using this approach is HTML_Tree. The idea is similar to using
a mini-language, but it doesn''t require any non-standard HTML tags and
it doesn''t embed any logic about loops or conditionals in the template
itself. This is nice because it means your templates are valid HTML
documents that can be viewed in a browser and worked with in most
standard HTML tools. It also means people working with the templates
can put placeholder data in them for testing and it will simply be
replaced when the template is used. This preview ability only breaks
down when you need an if/else type construct in the template. In that
situation, both the ``if'' and ``else'' chunks of HTML would show up when
previewing.

Parsers and Caching

The parsers for these templating systems are implemented in one of
three ways: They parse the template every time (``repeated parse''),
they parse it and cache the resulting parse tree (``cached parse
tree''), or they parse it, convert it to Perl code and compile it
(``compiled'').

Systems that compile templates to Perl take advantage of Perl''s
powerful run-time code evaluation capabilities. They examine the
template, generate a chunk of Perl code from it and eval the
generated code. After that, subsequent requests for the template can
be handled by running the compiled bytecode in memory. The complexity
of the parsing and code generation steps varies based on the number of
bells and whistles the system provides beyond straight in-line Perl
statements.

Compiling to Perl and then to Perl bytecode is slow on the first hit
but provides excellent performance once the template has been
compiled, since the template becomes a Perl subroutine call. This is
the same approach used by systems like JSP (Java ServerPages). It is
most effective in environments with a long-running Perl interpreter,
like mod_perl.

HTML::Template, HTML_Tree, and the 2.0 beta release of Embperl all use
a cached parse tree approach. They parse templates into their
respective internal data structures and then keep the parsed structure
for each processed template in memory. This is similar to the
compiled Perl approach in terms of performance and memory
requirements, but does not actually involve Perl code generation and
thus doesn''t require an eval step. Which way is faster: caching
the parse tree or compiling? It''s hard to objectively measure, but
anecdotal evidence seems to support compilation. Template Toolkit
used a cached parse tree approach for version 1, but switched to a
compilation approach for version 2 after tests showed it to offer a
significant speed increase. However, as will be discussed later,
either approach is more than fast enough.

In contrast to this, a repeated parse approach may sound slow.
However, it can be pretty fast if the tokens being parsed for are
simple enough. Systems using this approach generally use simple
tokens, which allows them to use fast and simple parsers.

Why would you ever use a system with this approach if compilation has
better performance? Well, in an environment without a persistent Perl
interpreter like vanilla CGI this can actually be faster than a
compiled approach since the startup cost is lower. The caching of
Perl bytecode done by compilation systems is useless when the Perl
interpreter doesn''t stick around for more than one request.

There are other reasons, too. Compiled Perl code takes up a lot of
memory. If you have many unique templates, they can add up fast.
Imagine how much RAM it would take up if each page that used
server-side includes (SSI) had to stay in memory after they had been
accessed. (Don''t worry, the Apache::SSI module doesn''t use
compilation so it doesn''t have this problem.)

Application Frameworks vs. Just Templates

Some of the templating tools try to offer a comprehensive solution to
the problems of Web development. Others offer just a templating
solution and assume you will fit this together with other modules to
build a complete system.

Some common features offered in the frameworks include:

URL Mapping

All of the frameworks offer a way to map a URL to a template file. In
addition to simple mappings similar to the handling of static
documents, some offer ways to intercept all requests within a certain
directory for pre-processing, or to create an object inheritance scheme
out of the directory structure of a site.

Session Tracking

Most interactive sites need to use some kind of session tracking to
associate application state data with a user. Some tools make this
simple by handling all the cookies or URL-munging for you and
allowing you simply to read and write from an object or hash that contains
the current user''s session data. A common approach is to use the
Apache::Session module for storage.

Output Caching

Caching is the key to good performance in many Web systems, and some
of these tools provide user-controlled caching of output. This is one
of the major features of both Mason and AxKit. AxKit can cache at the
page level, while Mason also offers fine-grained caching of components
within the page.

Form Handling

How will you live without CGI.pm to parse incoming form data? Many of
these tools will do it for you, making it available in a convenient
data structure. Some also validate form input, and even provide
``sticky'' form widgets that keep their selected values when
re-displayed or set up default values based on data you provide.

Debugging

Everyone knows how painful it can be to debug a CGI script.
Templating systems can make it worse, by screwing up Perl''s line
numbers with generated code. To help fix the problem they''ve created,
some offer built-in debugging support, including extra logging, or
integration with the Perl debugger.

If you want to use a system that just does templates but you need some
of these other features and don''t feel like implementing them yourself,
there are some tools on CPAN that provide a framework you can build
upon. The libservlet distribution, which provides an interface similar
to the Java servlet API, is independent of any particular templating
system. Apache::PageKit and CGI::Application are other options in this
vein, but both of these are currently tied to HTML::Template. OpenInteract
is another framework, this time tied to Template Toolkit. All of these
could be adapted for the ``just templates'' module of your choice with
fairly minimal effort.

The Contenders

OK, now that you know something about what separates these tools
from one another, let''s take a look at the top choices for Perl
templating systems. This is not an exhaustive list: I''ve only
included systems that are currently maintained, well-documented and
have managed to build up a significant user community. In short, I''ve
left out about dozen less-popular systems. At the end of this
section, I''ll mention a few systems that aren''t as commonly used but
may be worth a look.

SSI

SSI is the granddaddy of templating systems, and the first one that
many people used since it comes as a standard part of most Web
servers. With mod_perl installed, mod_include gains some additional
power. Specifically, it is able to take a new #perl directive that
allows for in-line subroutine calls. It can also efficiently include
the output of Apache::Registry scripts by using the Apache::Include
module.

The Apache::SSI module implements the functionality of mod_include
entirely in Perl, including the additional #perl directive. The main
reasons to use it are to post-process the output of another handler
(with Apache::Filter) or to add your own directives. Adding
directives is easy through subclassing. You might be tempted to
implement a complete template processor in this way, by adding loops
and other constructs, but it''s probably not worth the trouble with so
many other tools out there.

SSI follows the callback model and is mostly a mini-language, although
you can sneak in bits of Perl code as anonymous subs in #perl
directives. Because SSI uses a repeated parse implementation, it is
safe to use it on large numbers of files without worrying about memory
bloat.

SSI is a great choice for sites with fairly simple templating needs,
especially ones that just want to share some standard headers and
footers between pages. However, you should consider whether your site will eventually need to grow into something with more
flexibility and power before settling on this simple approach.

HTML::Mason

http://www.masonhq.com/

Mason has been around for a few years, and has built a loyal
following. It was originally created as a Perl clone of some of the
most interesting features from Vignette StoryServer, but has since
become its own unique animal. It comes from a publishing background,
and includes features oriented toward splitting pages into
re-useable chunks, or ``components.''

Mason uses in-line Perl with a compilation approach, but has a feature
to help keep the Perl code out of the HTML coder''s way. Components
(templates) can include a section of Perl at the end of the file that
is wrapped inside a special tag indicating that it should be run
first, before the rest of the template. This allows programmers to
put all the logic for a component down at the bottom away from the
HTML, and then use short in-line Perl snippets in the HTML to insert
values, loop through lists, etc.

Mason is a site development framework, not just a templating tool. It
includes a handy caching feature that can be used for capturing
the output of components or simply storing data that is expensive to
compute. It is currently the only tool that offers this sort of
caching as a built-in. It also implements an argument parsing scheme
that allows a component to specify the names, types and default
values that it expects to be passed, either from another component or
from the values passed in the URI query string.

While the documentation mostly demonstrates a callback execution
model, it is possible to use Mason in a pipeline style. This can be
accomplished in various ways, including building special components
called ``autohandlers,'' which run before anything else for requests
within a certain directory tree. An autohandler could do some
processing and set up data for a display template that includes only minimal in-line Perl. There is also support for an object-oriented
site approach, applying concepts such as inheritance to the site
directory structure. For example, the autohandler component at
/store/book/ might inherit a standard layout from the autohandler at
/store/, but override the background color and navigation bar. Then
/store/music/ can do the same, with a different color. This can be a
powerful paradigm for developing large sites. Note that this
inheritance is only supported at the level of methods defined in
autohandler components. You can''t override the component
/store/fool with another one at /store/book/fool.

Mason''s approach to debugging is to create ``debug files'' that run
Mason outside of a Web server environment, providing a fake Web
request and activating the debugger. This can be helpful if you''re
having trouble getting Apache::DB to behave under mod_perl, or using
an execution environment that doesn''t provide built-in debugger
support.

Another unique feature is the ability to leave the static text parts
of a large template on disk, and pull them in with a file seek when
needed rather than keeping them in RAM. This exchanges some speed for
a significant savings in memory when dealing with templates that are
mostly static text.

There are many other features in this package, including filtering of
HTML output and a page previewing utility. Session support is not
built-in, but a simple example showing how to integrate with
Apache::Session is included. Mason''s feature set can be a bit
overwhelming for newbies, but the high-quality documentation and
helpful user community go a long way.

HTML::Embperl

http://perl.apache.org/embperl/

Embperl makes its language choice known up front: embedded perl. It
is one of the most popular in-line Perl templating tools and has been
around longer than most of the others. It has a solid reputation for
speed and ease of use.

It is commonly used in a callback style, with Embperl intercepting
URIs and processing the requested file. However, it can optionally be
invoked through a subroutine call from another program, allowing it to
be used in a pipeline style. Templates are compiled to Perl bytecode
and cached.

Embperl has been around long enough to build an impressive list of
features. It has the ability to run code inside a Safe compartment,
support for automatically cleaning globals to make mod_perl coding
easier, and extensive debugging tools including the ability to e-mail
errors to an administrator.

The main thing that sets Embperl apart from other in-line Perl systems
is its tight HTML integration. It can recognize TABLE tags and
automatically iterate over them for the length of an array. It
automatically provides sticky form widgets. An array or hash
reference placed at the end of a query string in an HREF or SRC
attribute will be automatically expanded into query string
``name=value'' format. META HTTP-EQUIV tags are turned into true HTTP
headers.

Another reason people like Embperl is that it makes some of the common
tasks of Web application coding so simple. For example, all form data
is always available just by reading the magic variable %fdat.
Sessions are supported just as easily, by reading and writing to the
magic %udat hash. There is also a hash for storing persistent
application state. HTML-escaping is automatic (though it can be
toggled on and off).

Embperl includes something called EmbperlObject, which allows you to
apply OO concepts to your site hierarchy in a similar way to the
autohandler and inheritance features of Mason, mentioned above. This
is a convenient way to code sites with styles that vary by area,
and is worth checking out. EmbperlObject includes the ability to do
overrides on a file level. This means that you can have a directory
like /store/music that overrides specific templates and inherits the
rest from a parent directory.

One drawback of older versions of Embperl was the necessity to use
built-in replacements for most of Perl''s control structures like ``if''
and ``foreach'' when they are being wrapped around non-Perl sections.
For example:

[$ if ($foo) $]
Looks like a foo!
[$ else $]
Nope, it''s a bar.
[$ endif $]

These may seem out of place in a system based around in-line Perl. As
of version 1.2b2, it is possible to use Perl''s standard syntax instead:

[$ if ($foo) { $]
Looks like a foo!
[$ } else { $]
Nope, it''s a bar.
[$ } $]

At the time of this writing, a new 2.x branch of Embperl is in beta
testing. This includes some interesting features such as a more flexible
parsing scheme that can be modified to users'' tastes. It also
supports direct use of the Perl debugger on Embperl templates and
provides performance improvements.

Apache::AxKit

http://axkit.org/

AxKit is the first mod_perl page generation system to be built from the
ground up around XML. Technically, AxKit is not a templating
tool but rather a framework for stringing together different modules
that generate and transform XML data. In fact, it can optionally use
Template Toolkit as an XML transformation language. However, it
deserves coverage here since it is also the home of some templating
tools that are not represented elsewhere.

In its simplest form, AxKit maps XML files to XSL stylesheets that it
can process using commonly available XSLT modules like XML::XSLT or
XML::Sablotron. The rules for mapping a stylesheet to a request are
flexible, and they can incorporate query strings, cookies and
other attributes of the request. The idea is that you can use this
feature to handle a wide variety of clients with differing display
capabilities by choosing the right stylesheet.

Recognizing that not everyone is a fan of XSL''s somewhat obtuse
syntax, Matt Sergeant has provided an alternate stylesheet language
called XPathScript. XPathScript allows you to write a stylesheet
using text with embedded Perl code. This is similar to the other
embedded Perl templating tools, but the focus is on using the built-in
XPath functions for querying an XML document and manipulating the
retrieved data. XPathScript can also be used in a declarative
fashion, specifying the formatting of particular elements in the XML
input. For example, this snippet will change all <foo> tags
in an XML document to BAR in the output::

<%
$t->{''foo''}{pre}   = ''BAR'';
$t->{''foo''}{post}    = '';
$t->{''foo''}{showtag} = 0;
%>
<%= apply_templates() %>

By using XPathScript''s include function (which looks just like SSI),
you can build libraries of useful transformations that use this
technique.

This is all well and good if you have a bunch of XML files sitting on
a disk somewhere, but what about dynamic content? AxKit handles this
by allowing you to substitute a different data source for the default
file-based one. This can include running some dynamic code on each
request to generate the XML data that will be transformed. The
distribution includes a module for doing this called XSP. XSP is a
language for building an XML DOM using in-line Perl and tag libraries.
The tag libraries are specified as stylesheets that can turn XML tags
into Perl code. This is demonstrated through the included SQL tag
library, which allows you to write an XSP page using XML tags that
will connect to a database, execute queries and generate an XML
document with the results.

AxKit has some nice performance boosts built into it. It can cache
the full output of a page and serve it as a static file on future
requests. It can also compress output to speed up downloads for
browsers that understand gzip encoding. These can be done with other
systems, but they require you to setup additional software. With
AxKit, you just enable them in the configuration file.

If all of these languages, tag libraries and stylesheets sound
intimidating to you, AxKit may be overkill for your project. However,
AxKit has the advantage of being built on approved W3C standards, and
many of the skills used in developing for it carry over to other
languages and tools.

Apache::ASP

http://www.apache-asp.org/

Apache::ASP started as a port of Microsoft''s Active Server Pages
technology, and its basic design still follows that model. It uses
in-line Perl with a compilation approach and provides a set of simple
objects for accessing the request information and formulating a
response. Scripts written for Microsoft''s ASP using Perl (via
ActiveState''s PerlScript) can usually be run on this system without
changes. (Pages written in VBScript are not supported.)

Like the original ASP, it has hooks for calling specified code when
certain events are triggered, such as the start of a new user session.
It also provides the same easy-to-use state and session management.
Storing and retrieving state data for a whole application or a
specific user is as simple as a single method call. It can even
support user sessions without cookies -- a unique
feature among these systems.

A significant addition that did not come from Microsoft ASP is the XML
and XSLT support. There are two options provided: XMLSubs and XSLT
transforms. XMLSubs is a way of adding custom tags to your pages. It
maps XML tags to your subroutines, so that you can add something like
<site:header page="Page Title" /> to your pages and have it
translate into a subroutine call like &site::header({title =>
"Page Title"}). It can handle processing XML tags with body text as
well.

The XSLT support allows the output of ASP scripts to be filtered
through XSLT for presentation. This allows your ASP scripts to
generate XML data and then format that data with a separate XSL
stylesheet. This support is provided through integration with the
XML::XSLT module.

Apache::ASP provides sticky widgets for forms through the use of the
HTML::FillInForm module. It also has built-in support for removing
extra whitespace from generated output, gzip compressing output (for
browsers that support it), tracking performance using Time::HiRes,
automatically mailing error messages to an administrator and many
other conveniences and tuning options. This is a mature package that
has evolved to handle real-world problems.

One thing to note about the session and state management in this
system is that it currently only supports clusters through the use of
network file systems such as NFS or SMB. (Joshua Chamas, the module''s
author, has reported much better results from Samba file sharing than
from NFS.) This may be an issue for large-scale server clusters,
which usually rely on a relational database for network storage of
sessions. Support for database storage of sessions is planned in a
future release. In the meantime, instructions are provided for
hooking up to Apache::Session.

Text::Template

http://search.cpan.org/search?dist=Text-Template

This module has become the de facto standard general purpose
templating module on CPAN. It has an easy interface and thorough
documentation. The examples in the docs show a pipeline execution
style, but it''s easy to write a mod_perl handler that directly invokes
templates, allowing a callback style. The module uses in-line Perl.
It has the ability to run the in-line code in a Safe compartment, in
case you are concerned about mistakes in the code crashing your
server.

The module relies on creative uses of in-line code to provide things
that people usually expect from templating tools, like includes. This
can be good or bad. For example, to include a file you could just
call Text::Template::fill_in_file(filename). However, you''ll have to
specify the complete file path and nothing will stop you from using
/etc/passwd as the file to be included. Most of the fancier
templating tools have concepts like include paths, which allow you to
specify a list of directories to search for included files. You could
write a subroutine that works this way, and make it available in your
template''s namespace, but it''s not built in.

Each template is loaded as a separate object. Templates are compiled
to Perl and only parsed the first time they are used. However, to
take full advantage of this caching in a persistent environment like
mod_perl, your program will have to keep track of which templates have
been used, since Text::Template does not have a way of globally
tracking this and returning cached templates when possible.

Text::Template is not tied to HTML, and is just a templating module,
not a Web application framework. It is perfectly at home generating
e-mails, PDFs, etc.

Template Toolkit

http://template-toolkit.org/

One of the more recent additions to the templating scene, Template
Toolkit, is a flexible mini-language system. It has a complete
set of directives for working with data, including loops and
conditionals, and it can be extended in a number of ways. In-line
Perl code can be enabled with a configuration option, but is generally
discouraged. It uses compilation, caching the compiled bytecode in
memory and optionally caching the generated Perl code for templates on
disk. Although it is commonly used in a pipeline style, the included
Apache::Template module allows templates to be invoked directly from
URLs.

Template Toolkit has a large feature set, so we''ll only be able cover
some of the highlights here. The TT distribution sets a gold standard
for documentation thoroughness and quality, so it''s easy to learn more
if you choose to.

One major difference between TT and other systems is that it provides
simple access to complex data structures through the concept of a dot
operator. This allows people who don''t know Perl to access nested
lists and hashes or call object methods. For example, we could pass
in this Perl data structure:

$vars = {
customer => {
name    => ''Bubbles'',
address => {
city => ''Townsville'',
}
}
};

Then we can refer to the nested data in the template:

Hi there, [% customer.name %]!
How are things in [% customer.address.city %]?

This is simpler and more uniform than the equivalent syntax in Perl.
If we pass in an object as part of the data structure, we can use the
same notation to call methods within that object. If you''ve modeled
your system''s data as a set of objects, this can be convenient.

Templates can define macros and include other templates, and
parameters can be passed to either. Included templates can optionally
localize their variables so that changes made while the included
template is executing do not affect the values of variables in the
larger scope.

There is a filter directive, which can be used for post-processing
output. Uses for this range from simple HTML entity conversion to
automatic truncation (useful for pulldown menus when you want to limit
the size of entries) and printing to STDERR.

TT supports a plugin API, which can be used to add extra capabilities
to your templates. The provided plug-ins can be broadly organized into
data access and formatting. Standard data access plugins include
modules for accessing XML data or a DBI data source and using that
data within your template. There''s a plugin for access to CGI.pm as
well.

Formatting plug-ins allow you to display things like dates and prices
in a localized style. There''s also a table plugin for use in
displaying lists in a multi-column format. These formatting plug-ins
do a good job of covering the final 5 percent of data display problems that
often cause people who are using an in-house system to embed a little
bit of HTML in their Perl modules.

In a similar vein, TT includes some nice convenience features for
template writers, including eliminating white space around tags and the
ability to change the tag delimiters -- things that may sound a little
esoteric, but can sometimes make templates significantly easier to
use.

The TT distribution also includes a script called ttree, which allows
for processing an entire directory tree of templates. This is useful
for sites that pre-publish their templated pages and serve them
statically. The script checks modification times and only updates
pages that require it, providing a make-like functionality. The
distribution also includes a sample set of template-driven HTML
widgets that can be used to give a consistent look and feel to a
collection of documents.

HTML::Template

http://search.cpan.org/search?dist=HTML-Template

HTML::Template is a popular module among those looking to use a
mini-language rather than in-line Perl. It uses a simple set of tags
that allow looping (even on nested data structures) and conditionals
in addition to basic value insertion. The tags are intentionally
styled to look like HTML tags, which may be useful for some
situations.

As the documentation says, it ``does just one thing and it does quickly
and carefully'' -- there is no attempt to add application features like
form-handling or session tracking. The module follows a pipeline
execution style. Parsed templates are stored in a Perl data structure
that can be cached in any combination of memory, shared memory (using
IPC::SharedCache) and disk. The documentation is complete and
well-written, with plenty of examples.

You may be wondering how this module is different from Template
Toolkit, the other popular mini-language system. Beyond the obvious
differences in syntax, HTML::Template is faster and simpler, while
Template Toolkit has more advanced features, like plug-ins and dot
notation. Here''s a simple example comparing the syntax:

HTML::Template:

<TMPL_LOOP list>
<a href= url>"><b><TMPL_VAR name></b></A>
</TMPL_LOOP>

Template Toolkit:

[% FOREACH list %]
<a href= url %]"><b>[% name %]</a></a>
[% END %]

And now, a few honorable mentions:

HTML_Tree

http://homepage.mac.com/pauljlucas/software/html_tree/

As mentioned earlier, HTML Tree uses a fairly unique method of
templating: it loads in an HTML page, parses it to a DOM and then
programmatically modifies the contents of nodes. This allows it to
use genuine valid HTML documents as templates, something that none of
these other modules can do. The learning curve is a little steeper
than average, but this may be just the thing if you are concerned
about keeping things simple for your HTML coders. Note that the name
is ``HTML_Tree,'' not ``HTML::Tree.''

Apache::XPP

http://search.cpan.org/~cnation/Apache-XPP-2.02

XPP is an in-line Perl system that compiles to bytecode. Although it
is a perfectly good implementation, it has little to differentiate it
except for an easy mechanism to define new HTML-like tags that can be
used to replace in-line code in templates.

ePerl

http://search.cpan.org/search?dist=Apache-ePerl

Possibly the first module to embed Perl code in a text or HTML file,
ePerl is still a viable option in the form of Apache::ePerl. It
caches compiled bytecode in memory to achieve solid performance, and
some people find it refreshingly simple to use.

CGI::FastTemplate

http://search.cpan.org/search?dist=CGI-FastTemplate

This module takes a minimalistic approach to templating, which makes
it unusually well-suited to use in CGI programs. It parses templates
with a single regular expression and does not support anything in templates
beyond simple variable interpolation. Loops are handled by including
the output of other templates. Unfortunately, this leads to a Perl coding
style that is more confusing than most, and a proliferation of template
files. However, some people swear by this dirt-simple approach.

Performance

People always seem to worry about the performance of templating
systems. If you''ve ever built a large-scale application, you should
have enough perspective on the relative costs of different actions to
know that your templating system is not the first place to look for
performance gains. All of the systems mentioned here have excellent
performance characteristics in persistent execution environments like
mod_perl. Compared to such glacially slow operations as fetching data
from a database or file, the time added by the templating system is
almost negligible.

If you think your templating system is slowing you down, get the
facts: pull out Devel::DProf and see. If one of the tools mentioned
here is at the top of the list for wall clock time used, you should
pat yourself on the back -- you''ve done a great job tuning your system
and removing bottlenecks! Personally, I have only seen this happen
when I had managed to successfully cache nearly every part of the work
to handle a request except running a template.

However, if you really are in a situation where you need to squeeze a
few extra microseconds out of your page generation time, there are
performance differences between systems. They''re pretty much what you
would expect: systems that do the least run the fastest. Using
in-line print() statements is faster than using templates. Using
simple substitution is faster than using in-line Perl code. Using
in-line Perl code is faster than using a mini-language.

The only templating benchmark available at this time is one developed
by Joshua Chamas, author of Apache::ASP. It includes a ``hello world''
test, which simply checks how fast each system can spit back those
famous words, and a ``hello 2000'' test, which exercises the basic
functions used in most dynamic pages. It is available from the
following URL:

http://www.chamas.com/bench/

Results from this benchmark currently show SSI, Apache::ASP and
HTML::Embperl having the best performance. Not all of the
systems mentioned here are currently included in the test. If your
favorite was missed, you might want to download the benchmark code and
add it. As you can imagine, benchmarking people''s pet projects
is largely a thankless task and Joshua deserves some recognition and
support for this contribution to the community.

CGI Performance Concerns

If you''re running under CGI, you have bigger fish to fry than worrying
about the performance of your templating system. Nevertheless, some
people are stuck with CGI but still want to use a templating system
with reasonable performance. CGI is a tricky situation, since you
have to worry about how much time it will take for Perl to compile the
code for a large templating system on each request. CGI also breaks
the in-memory caching of templates used by most of these systems,
although the slower disk-based caching provided by Mason,
HTML::Template and Template Toolkit will still work. (HTML::Template
does provide a shared memory cache for templates, which may improve
performance, although shared memory on my Linux system is usually
slower than using the filesystem. Benchmarks and additional
information are welcome.)

Your best performance bet with CGI is to use one of the simpler tools,
like CGI::FastTemplate or Text::Template. They are small and compile
quickly, and CGI::FastTemplate gets an extra boost since it relies on
simple regex parsing and doesn''t need to eval any in-line Perl code.
Almost everything else mentioned here will add tenths of seconds to
each page in compilation time alone.

Matrix

To help you choose a system, I''ll summarize the basic characteristics
of the major systems along the decision points I''ve explained in the
beginning of the article. Keep in mind that in many cases a system
can be used in more than one way, and I''ve simply shown the dominant
method as seen in the documentation and real world use. You should
not eliminate options based on this chart without reading the more
detailed explanations above.

Updates

These modules are moving targets, and a document like this is bound to
contain some mistakes. Send your corrections to Return to Related Articles from the O''Reilly Network .

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