OVERVIEW

The Template Toolkit is a collection of Perl modules which collectively implement a fast, powerful and generic template processing system. In this context, a template is a text document which contains embedded processing "directives". These instruct the template processor to perform certain actions such as; inserting the value of a variable; processing and including another template file or user-defined block; testing some condition and generating output of one kind or another accordingly; iterating through a set of values; and so on. Anything not marked as a template directive is treated as plain text and gets passed through unaltered. By default, directives look something like this:

   [% INCLUDE header %]

The "mini-language" that the toolkit uses is designed to be clear, concise, regular in structure and simple in syntax. It is a specialised language which boasts many powerful features for constructing dynamic content but it is not a general purpose programming language. Instead, it supports a plugin interface which allows separate modules of application specific code to be written in the language of choice (i.e. Perl, C, C++, etc) and then loaded, used and re-used as required.

This gives you the facility to add any programmable functionality you require but without having to write the code directly into the document. In other words, it promotes the separation of application code (the implementation) from the user interface (the presentation). It tries to keep templates clutter-free, concentrating on what the document looks like, and hides away the implementation specific details of how the various items of content are stored, retrieved or calculated.

Development and subsequent maintenance become easier and less error prone when the "back-end" is separated from the "front-end". You (or your web designer) can first decide how the page(s) should look by creating the output templates, perhaps by re-using other common elements that you've already defined for your web site such as headers, footers, menus, etc. Then, you (or your progammer) can create any Perl code required to required to generate the dynamic parts of the content, perhaps by using CPAN modules, one of the existing Template Toolkit "plugins" or by re-using some code you've previously written for another web application running on your site.

The important thing is that the code is developed separately from the template documents. This is a key feature of the Template Toolkit with the emphasis being on moving code out of documents, rather than moving it in. It doesn't matter if you put every function in a separate file or every bit of application code into one big module - just tell the Template Toolkit where to find the code and leave it to handle the rest. The new functionality is then available without distraction or discourse in any and all of your template documents.

This isn't the only way to do it. There are times when the "best solution" in terms of getting the job done quickly and efficiently is to simply embed Perl code directly into the document. The Template Toolkit has an option (EVAL_PERL - disabled by default) to allow Perl code to be embedded into special 'PERL' sections which are then evaluated when the template is processed. These Perl code fragments have full access to the Template Toolkit interface and may read or update variables, process other template files, load plugins, and so on.

If you prefer the embedded Perl approach and have a need to get some functionality into a document with the minimum fuss, setup time and effort, then you might want to consider using a dedicated embedded Perl processor. In those cases, Mark-Jason Dominus' Text::Template module, available from CPAN, comes highly recommended.

In many ways, this is like CGI scripting in reverse. Instead of embedding HTML within the Perl script, you are embeddding Perl within the HTML document. The Template Toolkit tries to better separate the two in attempt to create a more structured approach to constructing dynamic content. There's a slight startup cost to using this approach because, like life in general, it often takes a little longer to get yourself properly organised. The payoff comes in terms of scalability and subsequent ease of re-use - the more you add, the more you benefit from having structure. That's not to say that you can't use it in many interesting ways with just a single document, but if you only have a single document to worry about then the chances are that you haven't got a lot to worry about.

So rather than emphasing the raw programming power embeddable within any single document, it focuses on tools and techniques to help better partition and subsequently re-integrate the different components that constitute the many documents in a system. It offers speed and simplicity in constructing document systems and delegates "real programming" to a more powerful, general purpose language - Perl.

There are many benefits to be gained from keeping Perl code separate from template documents. You could, for example, write a single web application with a dozen different sets of templates to represent alternate user interfaces. These would represent multiple "views" on the same underlying "model" and might differ in layout or design style, contain text only or go hard on the graphics, contain HTML frames or not, show "novice" or "expert" functionality, allow per-user customisation, present an internationalised or localised interface, and so on. On the other side of the same coin, you could change your underlying code to implement a faster algorithm or more efficient storage method, but you wouldn't have to change the template files. They still get their "content" from the same "place" and don't need to concern themselves with what happens inside each black box.

The end result is that complex, dynamic content systems can be built easily and quickly from a number of small, reusable components. Some of these components are template files representing user interface "chunks" and others may be data structures, library code, user-defined sub-routines or objects that implement various functionalities of the system. The Template Toolkit's role is to help pull all the different pieces together as quickly and simply as possible, hiding as many of the unnecessary details as it can.

The Template Toolkit is ideally suited for generating web content, but it is by no means limited or specific to this or any other application area. The plugin interface means that it doesn't have to be - it can just concentrate on the task of constructing documents and doesn't care if you subsequently use it to generate HTML, XML, LaTeX, RTF or plain text documents from a command-line, CGI script or an in-server web process such as Apache/mod_perl using data extracted from a CGI form, defined in a file, retrieved from a database or based on what Jim Morrison told you in a dream. You choose what do to with it and how to do it. Simply load additional functionality as you need it from CPAN modules, Template Toolkit plugins, generic web applications such as chat rooms, FAQ lists, bulletin boards, etc., or any other code you can beg, borrow or write yourself.

It is worth noting that there is nothing to stop you from adopting a well structured approach to content construction using Text::Template or one of the other fine template processing modules available. Many people do indeed develop content using these tools in such a way as to promote reusability and maintainability every bit as much as the Template Toolkit does. The benefit of using the toolkit is that much of this is done for you. You get to reap the benefits of template caching (resulting in fast runtime processing), automatic error detection and handling, variable localisation and namespacing, dynamic loading of external modules, and so on, without having to code it yourself or even worry about the specifics of implementation. The details are hidden away, the directive syntax is simplified and you can get a lot done very quickly with the minimum of fuss.

The Template Toolkit is a direct descendant of, and replacement for the Text::MetaText module. It has been designed and rebuilt from scratch based on several years of experience gained from developing, supporting and using Text::MetaText and other template processing applications and tools. It is an Open Source project in which contribution and collaboration are encouraged.