Embedding Perl in HTML with Mason Chapter 1: Introduction At its heart, Mason is simply a mechanism for embedding Perl code into plain text It is only one of many such mechanisms that all more or less the same thing However, Mason represents a particular set of choices about how this embedding should be done, and many people have found that the way Mason does things is very straightforward and extremely conducive to getting jobs done In this chapter we'll introduce you to some of Mason's key features and strengths, show you a couple of examples of how to use Mason, and talk about some alternatives to Mason After reading this chapter, you should have a fairly good idea of how Mason relates to its peers and what kinds of tasks you can accomplish with Mason The most common application of Mason is in building large dynamic web sites, and this book focuses mostly on web site building Mason is broadly applicable to any situation in which fine control over document content is required, however, such as generating mail-merged form letters, creating custom configuration file sets, and even building dynamic GIF images based on varying input parameters We intend to give you enough facility with Mason that after reading this book, you can imagine Mason-based solutions to problems we haven't ever thought of Before we get into the details of Mason and comparisons with its alternatives, we'll just briefly mention some of its guiding design principles Mason was designed to help you build, organize, and maintain large web sites or other groups of dynamically generated documents It cooperates fully with Perl, leveraging all the solutions and techniques that Perl developers have come to depend on and that have made Perl such a powerful and widespread tool It encourages thinking about your site in structural terms rather than as a collection of procedural scripts or modules All of these things are conducive to getting your job done effectively, letting you concentrate on your goals while Mason takes care of the details A First Example To help make this discussion a little more concrete (this thing is called Mason, after all), let's look at an example We'll give more in-depth treatment to the details of Mason's syntax later in the book; these examples are just to put some Mason code in front of your eyes and show you what it looks like The following code is a complete chunk of Mason code, called a component: % my $planet = "World"; Hello, ! When Mason runs this code, the output is: Hello, World! We'll talk more about the details of component syntax in Chapter 2, but two basic elements in the preceding example deserve mention here The first is that any line that begins with a % character tells Mason that the line contains Perl code The Perl code can be any syntactically correct Perl Mason doesn't care what it is or what it does In this case, it simply sets the value of a variable that will be used later in the component The other element in the previous Mason component is the substitution tag, denoted by the sequence Mason will evaluate the contents of any such tag and insert the result into the surrounding text In this case, the variable $planet evaluates to World, and the output of the entire component is Hello, World! Note that any text that isn't a special Mason construct simply becomes part of the output of the component These two lines are relatively simple and not particularly exciting, but they should give you a taste for how Mason code looks in its simplest form The Main Features of Mason There are more templating systems written in Perl than you could possibly keep in your head all at once To help you make sense of Mason's place in the world, this section presents Mason's most important and distinctive features By the end of this section, you should see that Mason pushes the boundaries of the term " templating system," with lots of features aimed at helping you manage the larger tasks of site design and maintenance Components: Modular Design Elements As we mentioned before, the basic unit of Mason code is called a component It is a chunk of Mason code that can accept input parameters and generate output text An important feature of Mason is that any component may call any other component at any point during its execution, much like a Perl subroutine calling another Perl subroutine Because of this feature, a component may represent a single web page, a part of a web page (like a side navigation bar), or even a shared utility function that generates no output of its own This separation of design elements allows you to use Mason as a sort of glorified server-side include (SSI) mechanism, as in Example 1-1, Example 1-2, and Example 1-3 Executing mainpage.mas will produce a full page of HTML with the header and footer inserted in place Example 1-1 header.mas Welcome to Wally World! Example 1-2 footer.mas Home Example 1-3 introduces the component call tag syntax, , which is used to call another component and insert its output into the surrounding text The component tag can also accept arguments, which in this case can help unify site design by moving the page header text into the header.mas component Example 1-3 mainpage.mas Wally World Home Here at Wally World you'll find all the finest accoutrements The header.mas component in Example 1-4 now accepts an argument called $head that contains the text that should get inserted into the tags A component's arguments are declared by using an block, which you'll see in more detail later in the book The $head argument becomes an honest-to-goodness Perl variable that can be used throughout the rest of the component It's lexically scoped in the header.mas component using Perl's my() function Example 1-4 header.mas $head Welcome to Wally World! The footer.mas component in Example 1-5 is fairly straightforward It just provides a link to the document root Example 1-5 footer.mas Home In the mainpage.mas component in Example 1-6, the arguments are passed to the header.mas component by using standard Perl syntax (i.e., commas, quotes, and the => operator) In fact, any Perl syntax for passing a list can be used, because the argument list is specified in real Perl syntax Example 1-6 mainpage.mas "Wally World Home" &> Here at Wally World you'll find all the latest accoutrements Mason will take the list of arguments passed to the header.mas component and assign the proper values to the variables specified in the block Object-Style Component Inheritance Aside from the fact that there's a little bit of Perl thrown into the mix for passing parameters, the examples we've seen don't really show anything that you couldn't using standard server-side include (SSI) techniques In fact, the usage demonstrated in these examples is relatively uncommon in building Mason sites, because there are better ways to get the job done One of the greatest features of Mason is that components can inherit behavior from other components, much like classes and objects in an object-oriented hierarchy.1 Typically, each component will inherit from a single component called the autohandler The autohandler implements general behavior for all components, such as the content of headers and footers Individual components implement specific behavior, such as the body text of the individual pages Using component inheritance, we can rewrite Example 1-4 through Example 1-6 in a more common Mason idiom, as shown in Example 1-7 and Example 1-8 Example 1-7 autohandler Welcome to Wally World! base_comp->attr('head') %> % $m->call_next; Home Example 1-8 mainpage.mas head => "Wally World Home" Here at Wally World you'll find all the finest accoutrements Notice that the header and footer are now both all in one file, the autohandler Visually, this helps unify the page content, because tags like and that are opened in the header are closed in the same file The other important difference here is that mainpage.mas no longer has to call the header and footer components explicitly, but rather Mason calls the parent component automatically and it wraps its header and footer around the main content The page header is now specified by an attributes block, one of Mason's object-oriented mechanisms An attribute is a component property that inherits via Mason's component inheritance chain There are zillions of other uses for Mason's inheritance mechanism, which will be further explored in Chapter Intelligent Caching Mechanisms Anyone who has built any dynamically generated web sites knows that sometimes certain portions of a site can take longer to generate and serve than you want to make your users wait Furthermore, portions of a site might be only "semidynamic," meaning that their content changes periodically but stays static for a long time between changes Alternatively, as might happen on a news site or for an online poll, content may change continually, but a lag time of a few minutes in updating the content would be acceptable if it improves site performance For cases like these, Mason provides a very sophisticated caching mechanism that you can use to control how often the output of a component is rebuilt You can base the expiration decision on time, on certain key parameters like username or content ID, or on an explicit agent that decides when specific data has expired The caching mechanism can be used for the output of a component, for an arbitrary block of text, or for any Perl data structure you might want to cache The first-class support for caching is one of Mason's most endearing qualities, and you'll learn to appreciate it the first time it saves you from spending hours optimizing sluggish code To aid overall performance, Mason also has an intelligent internal caching mechanism During execution, Mason turns each component into Perl source code on disk, then compiles the Perl code into bytecode, then executes the bytecode to produce the component's output It would be a waste of computing resources to repeat this cycle every time a component needs to be executed, so Mason caches at each stage As an aid to rapid development, Mason will check your components' modification times and invalidate its cache when you make changes to your components, ensuring that any changes you make to your site take effect immediately When your site moves from development to production, you probably won't be making frequent changes to your site, so you can disable the freshness checks in order to improve your site's responsiveness Integration with Apache and mod_perl As mentioned before, the most common use of Mason is in building large, dynamic, data-driven web sites The most popular web server around is Apache, and one of Apache's most powerful features is mod_perl, which lets you use the full power of Perl within the Apache server process Therefore, it should come as no surprise that Mason is designed to cooperate fully with mod_perl Mason comes with drop-in mod_perl handlers that let Apache serve your Mason components directly It lets you take advantage of the sophisticated decision-making mechanisms that Apache has evolved to support, such as custom authentication methods, content negotiation, and dynamic URL rewriting Mason's caching mechanism and other performance considerations are designed specifically for the task of serving dynamic content efficiently and with enough flexibility to let you design creative solutions to your specific problems Although Mason lets you build a site without relying very much on assumptions about the server environment, learning about mod_perl and Apache's request cycle can help you use Mason to create slick and powerful features Alternatives to Mason Much of this chapter so far may have sounded like a sales pitch, because we wanted you to know about Mason's biggest strengths so you could have some solid reasons for reading the rest of this book However, you should also be aware that there are many alternatives to using Mason, and an awareness of these alternatives will help you form an accurate picture of the context for which each system was created It will also help you decide which system to use for each individual project, because no single system was designed to be a solution to all the problems you might encounter Just as importantly, different people find that different systems suit the way they think better than others There are generally two kinds of systems that people consider to be alternatives to Mason: lightweight solutions and heavyweight solutions Lightweight solutions generally have the goal of being small and fast and leave much of the major work up to you They are often simple templating modules like Text::Template or HTML::Template or even homegrown templating schemes Using templates is certainly a good idea, and it is one of the core ideas in Mason itself However, when designing an entire site, you're usually going to need some more sophisticated system that helps you manage your site-building resources; if you choose a templatingonly solution, you'll probably end up writing this management code yourself You may have a good idea of what such a system would entail only after writing and maintaining dozens of complicated web sites, so you'd likely spend more time working on your management code than on building your sites This is the main trade-off with lightweight solutions: you gain flexibility because you can manage your site however you want, but since the burden rests entirely on you, you might end up preferring to use a tool that handles many of these management issues for you % } The output of both examples is: Font sizes: Size = 1 Size = 2 Size = 3 Size = 4 Size = 5 Because it is built on the ASP model, Apache::ASP is a natural choice when porting an ASP/IIS application to the Apache platform You will need to translate the scripting language from VBScript or JScript into Perl, but the overall structure of the site should remain unchanged Besides the standard ASP feature set, Apache::ASP supports several additional features, including extra event handlers, XML/XSLT processing, and component output caching It also supports a cookieless mode for maintaining session data, which can be very handy for end users unwilling or unable to store session cookies The XSLT support is particularly interesting, since combining it with the output caching features means that you can use dynamic XSLT transformations in web publishing, a technique that might be computationally prohibitive without caching For more information on Apache::ASP, please visit http://www.apacheasp.org/ HTML::Template Sam Tregar's HTML::Template module falls into the lightweight category Its chief goal is to allow site builders to separate a site's HTML from its Perl controlling code, and it enforces this division quite strictly No Perl code is ever embedded within templates, and control structures like if, include, and various loops are all implemented by custom HTML::Template tags Any variables to be interpolated into the HTML template are explicitly fed to the template by the controlling Perl code Philosophically, the reason for this strict division of HTML and Perl is so that a division of labor can be enforced in an organization, with HTML designers and Perl coders working independently, not worrying about whether they're editing the same files It is also possible to apply the same controlling code to several different templates, so that designers can create different look-and-feel templates without involving programmers HTML::Template is generally very fast at filling in HTML templates, especially if you use its just-in-time compiler HTML::Template::JIT It also provides a caching mechanism, somewhat similar to Mason's, that allows for caching templates in private memory, on disk, in memory shared between processes, or in combinations thereof Unlike Mason, HTML::Template's caching handles the caching only of templates themselves, not of their output or arbitrary data Example syntax: Employee listing: Name: Job: To make this actually something, you need to write Perl code to call the template For this template, we might write something like this: my $template = HTML::Template->new( filename => 'emp_list.tmpl' ); $template->param( EMPLOYEE_INFO => [ { NAME => 'Dave', JOB => 'Grouper of Bumpers' }, { NAME => 'Ken', JOB => 'Bumper of Groupers'} ] ); print "Content-Type: text/html\n\n"; print $template->output; Note that the top layer for the system is a Perl script Some people love this, some people hate it You may know by now which category you fall into Text::Template Text::Template , written by Mark-Jason Dominus, is a lightweight solution similar to HTML::Template, but has some philosophical differences First, it does not assume that the template contains HTML but is designed to work with any kind of text in the template (in truth, HTML::Template can work with arbitrary text, too, but it was designed specifically to work with HTML) Second, it uses native Perl control structures instead of its own custom macros, so Perl programmers have less to keep in their heads This has the effect of breaking down the barrier that HTML::Template maintains between code and HTML, so in an environment in which designers and programmers need exclusive control over their own work products, HTML::Template may be the better choice Like Embperl, Text::Template also supports code sequestering via the Safe.pm module, with the same caveats as mentioned earlier Template variables can be passed explicitly to the template for substitution or drawn from Perl variables in a specified Perl package Text::Template also allows the user to customize what delimiters are used to indicate the special Perl code or variable substitution sections The default delimiters are curly braces: my $string = q[ Dear {$recipient}, Congratulations! You have won {$amount} dollar{$plural}! ]; my $template = Text::Template->new(TYPE => 'STRING', SOURCE => $string ); $T::recipient = int(rand 2) ? 'Mary' : 'John'; $T::amount = int(rand 10) - 5; $T::plural = $T::amount == ? '' : 's'; print $template->fill_in(PACKAGE => 'T'); Text::Template was first released in 1995 and has undergone many revisions in its life cycle It is considered a mature product, and its author is ... any line that begins with a % character tells Mason that the line contains Perl code The Perl code can be any syntactically correct Perl Mason doesn''t care what it is or what it does In this... property that inherits via Mason'' s component inheritance chain There are zillions of other uses for Mason'' s inheritance mechanism, which will be further explored in Chapter Intelligent Caching Mechanisms... full power of Perl within the Apache server process Therefore, it should come as no surprise that Mason is designed to cooperate fully with mod _perl Mason comes with drop -in mod _perl handlers