Template::Manual::IntrUser Contributed Perl DocumentTemplate::Manual::Intro(3)NAMETemplate::Manual::Intro - Introduction to the Template Toolkit
Introduction
The Template Toolkit is a collection of Perl modules which implement a
fast, flexible, powerful and extensible template processing system. It
is most often used for generating dynamic web content, although it can
be used equally well for processing any kind of text documents.
At the simplest level it provides an easy way to process template
files, filling in embedded variable references with their equivalent
values. Here's an example of a template.
Dear [% name %],
It has come to our attention that your account is in
arrears to the sum of [% debt %].
Please settle your account before [% deadline %] or we
will be forced to revoke your Licence to Thrill.
The Management.
By default, template directives are embedded within the character
sequences "[%" ... "%]" but you can change these and various other
options to configure how the Template Toolkit looks, feels and works.
You can set the "INTERPOLATE" option, for example, if you prefer to
embed your variables in Perl style:
Dear $name,
It has come to our attention that your account is in
arrears to the sum of $debt.
...etc...
The Template Perl Module
The Template Perl module is the front end to the Template Toolkit for
Perl programmers, providing access to the full range of functionality
through a single module with a simple interface. It loads the other
modules as required and instantiates a default set of objects to handle
subsequent template processing requests. Configuration parameters may
be passed to the Template constructor method, new(), which are then
used to configure the generate object.
use Template;
my $tt = Template->new({
INCLUDE_PATH => '/usr/local/templates',
INTERPOLATE => 1,
}) || die "$Template::ERROR\n";
The Template object implements a process() method for processing
template files or text. The name of the input template (or various
other sources) is passed as the first argument, followed by a reference
to a hash array of variable definitions for substitution in the
template.
my $vars = {
name => 'Count Edward van Halen',
debt => '3 riffs and a solo',
deadline => 'the next chorus',
};
$tt->process('letters/overdrawn', $vars)
|| die $tt->error(), "\n";
The process() method returns a true value (1) on success and prints the
template output to "STDOUT", by default. On error, the process() method
returns a false value ("undef"). The error() method can then be called
to retrieve details of the error.
Component Based Content Construction
A number of special directives are provided, such as "INSERT",
"INCLUDE" and "PROCESS", which allow content to be built up from
smaller template components. This permits a modular approach to
building a web site or other content repository, promoting reusability,
cross-site consistency, ease of construction and subsequent
maintenance. Common elements such as headers, footers, menu bars,
tables, and so on, can be created as separate template files which can
then be processed into other documents as required. All defined
variables are inherited by these templates along with any additional
"local" values specified.
[% PROCESS header
title = "The Cat Sat on the Mat"
%]
[% PROCESS menu %]
The location of the missing feline has now been established.
Thank you for your assistance.
[% INSERT legal/disclaimer %]
[% PROCESS footer %]
You can also define a template as a BLOCK within the same file and
PROCESS it just like any other template file. This can be invaluable
for building up repetitive elements such as tables, menus, etc.
[% BLOCK tabrow %]
<tr><td>[% name %]</td><td>[% email %]</td></tr>
[% END %]
<table>
[% PROCESS tabrow name="tom" email="tom@here.org" %]
[% PROCESS tabrow name="dick" email="disk@there.org" %]
[% PROCESS tabrow name="larry" email="larry@where.org" %]
</table>
Data and Code Binding
One of the key features that sets the Template Toolkit apart from other
template processors is the ability to bind template variables to any
kind of Perl data: scalars, lists, hash arrays, sub-routines and
objects.
my $vars = {
root => 'http://here.com/there',
menu => [ 'modules', 'authors', 'scripts' ],
client => {
name => 'Doctor Joseph von Satriani',
id => 'JVSAT',
},
checkout => sub { my $total = shift; ...; return $something },
shopcart => My::Cool::Shopping::Cart->new(),
};
The Template Toolkit will automatically Do The Right Thing to access
the data in an appropriate manner to return some value which can then
be output. The dot operator '"."' is used to access into lists and
hashes or to call object methods. The "FOREACH" directive is provided
for iterating through lists, and various logical tests are available
using directives such as "IF", "UNLESS", "ELSIF", "ELSE", "SWITCH",
"CASE", etc.
[% FOREACH section = menu %]
<a href="[% root %]/[% section %]/index.html">[% section %]</a>
[% END %]
<b>Client</a>: [% client.name %] (id: [% client.id %])
[% IF shopcart.nitems %]
Your shopping cart contains the following items:
<ul>
[% FOREACH item = shopcart.contents %]
<li>[% item.name %] : [% item.qty %] @ [% item.price %]
[% END %]
</ul>
[% checkout(shopcart.total) %]
[% ELSE %]
No items currently in shopping cart.
[% END %]
Advanced Features: Filters, Macros, Exceptions, Plugins
The Template Toolkit also provides a number of additional directives
for advanced processing and programmatical functionality. It supports
output filters (FILTER), allows custom macros to be defined (MACRO),
has a fully-featured exception handling system (TRY, THROW, CATCH,
FINAL) and supports a plugin architecture (USE) which allows special
plugin modules and even regular Perl modules to be loaded and used with
the minimum of fuss. The Template Toolkit is "just" a template
processor but you can trivially extend it to incorporate the
functionality of any Perl module you can get your hands on. Thus, it
is also a scalable and extensible template framework, ideally suited
for managing the presentation layer for application servers, content
management systems and other web applications.
Separating Presentation and Application Logic
Rather than embedding Perl code or some other scripting language
directly into template documents, it encourages you to keep functional
components (i.e. Perl code) separate from presentation components (e.g.
HTML templates). The template variables provide the interface between
the two layers, allowing data to be generated in code and then passed
to a template component for displaying (pipeline model) or for sub-
routine or object references to be bound to variables which can then be
called from the template as and when required (callback model).
The directives that the Template Toolkit provide implement their own
mini programming language, but they're not really designed for serious,
general purpose programming. Perl is a far more appropriate language
for that. If you embed application logic (e.g. Perl or other scripting
language fragments) in HTML templates then you risk losing the clear
separation of concerns between functionality and presentation. It
becomes harder to maintain the two elements in isolation and more
difficult, if not impossible, to reuse code or presentation elements by
themselves. It is far better to write your application code in
separate Perl modules, libraries or scripts and then use templates to
control how the resulting data is presented as output. Thus you should
think of the Template Toolkit language as a set of layout directives
for displaying data, not calculating it.
Having said that, the Template Toolkit doesn't force you into one
approach or the other. It attempts to be pragmatic rather than
dogmatic in allowing you to do whatever best gets the job done. Thus,
if you enable the EVAL_PERL option then you can happily embed real Perl
code in your templates within PERL ... END directives.
Performance
The Template Toolkit uses a fast YACC-like parser which compiles
templates into Perl code for maximum runtime efficiency. It also has
an advanced caching mechanism which manages in-memory and on-disk (i.e.
persistent) versions of compiled templates. The modules that comprise
the toolkit are highly configurable and the architecture around which
they're built is designed to be extensible. The Template Toolkit
provides a powerful framework around which content creation and
delivery systems can be built while also providing a simple interface
through the Template front-end module for general use.
perl v5.14.3 2011-12-20 Template::Manual::Intro(3)
