We have only scratched the surface of the OpenACS templating system. ad_page_contract additionally supports verification routines that allow you to check multiple parameters or to signal errors based on information found in the database. You actually can define your own custom error messages that may appear in the case of trouble. Even outside of ad_page_contract, a .tcl page also can call the universal ad_return_complaint function, which produces error messages in nicely formatted HTML pages.
In addition, the OpenACS templating system has a complete set of form-building routines that allow a programmer to specify an HTML form using Tcl procedures. The contents of the form then can be exported to the .adp page using data sources. Not only does the form builder cut down on the amount of HTML you have to write, but it makes it easy to create a two-stage form submission process, in which users get a chance to preview their work before sending it in.
Finally, OpenACS templates include a number of additional tags, such as <if>, that allow you to include text and images conditionally, depending on the values of other data sources.
While OpenACS is often touted as a remarkable system because of its elaborate data model and advanced applications, I have found the templates to be one of the more compelling parts of OpenACS. The graphic designers with whom I work enjoy the separation between .tcl and .adp pages, and I like that I can check for errors and pass multiple values without having to remember that there is no obvious connection between them at the HTTP level.
While the learning curve for OpenACS can be quite steep, learning how the templates work is both a gratifying and interesting way to start with this system. Given the many application packages that come with OpenACS, there also are numerous examples of the templates right in the code after you download the system.
Reuven M. Lerner is a consultant specializing in web/database applications and open-source software. His book, Core Perl, was published in January 2002 by Prentice Hall. Reuven lives in Modi'in, Israel, with his wife and daughter.
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