There are a number of Perl modules that are universally recognised as The Right Thing To Use for certain tasks. If you accessed a database without using DBI, pulled data from the WWW without using one of the LWP modules or parsed XML without using XML::Parser or one of its subclasses then you’d run the risk of being shunned by polite Perl society.

I believe that the year 2000 saw the emergence of another ‘must have’ Perl module – the Template Toolkit. I don’t think I’m alone in this belief as the Template Toolkit won the ‘Best New Module’ award at the Perl Conference last summer. Version 2.0 of the Template Toolkit (known as TT2 to its friends) was recently released to the CPAN.

TT2 was designed and written by Andy Wardley. It was born out of Andy’s previous templating module, Text::Metatext, in best Fred Brooks ‘plan to throw one away’ manner; and aims to be the most useful (or, at least, the most used) Perl templating system.

TT2 provides a way to take a file of fixed boilerplate text (the template) and embed variable data within it. One obvious use of this is in the creation of dynamic web pages and this is where a lot of the attention that TT2 has received has been focussed. In this article, I hope to demonstrate that TT2 is just as useful in non-web applications.

Using the Template Toolkit

Let’s look at how we’d use TT2 to process a simple data file. TT2 is an object oriented Perl module. Having downloaded it from CPAN and installed it in the usual manner, using it in your program is as easy as putting the lines

in your code. The constructor function, new, takes a number of optional parameters which are documented in the copious manual pages that come with the module, but for the purposes of this article we’ll keep things as simple as possible.

To process the template, you would call the process method like this

We pass two parameters to process, the first is the name of the file containing the template to process (in this case, my_template) and the second is a reference to a hash which contains the data items that you want to use in the template. If processing the template gives any kind of error, the program will die with a (hopefully) useful error message.

So what kinds of things can go in %data? The answer is just about anything. Here’s an example showing data about English Premier League football teams.

This creates three data items which can be accessed within the template, called name, season and teams. Notice that teams is a complex data structure.

Here is a template that we might use to process this data.

[text]
League Standings

League Name: [% name %]
Season : [% season %]
Teams:
[% FOREACH team = teams -%]
[% team.name %] [% team.played -%]
[% team.won %] [% team.drawn %] [% team.lost %]
[% END %]
[/text]

Running this template with this data gives us the following output

[text]
League Standings

League Name: English Premier League
Season : 2000/01

Teams:
Man Utd 16 12 3 1
Bradford 16 2 5 9
[/text]

Hopefully the syntax of the template is simple enough to follow. There are a few points to note.

• Template processing directives are written using a simple language which is not Perl.
• The keys of the %data have become the names of the data variables within the template.
• Template processing directives are surrounded by [% and %] sequences.
• If these tags are replaced with [%- -%] then the preceding or following linefeed is suppressed.
• In the FOREACH loop, each element of the teams list was assigned, in turn, to the temporary variable team.
• Each item assigned to the team variable is a Perl hash. Individual values within the hash are accessed using a dot notation.

It’s probably the first and last of these points which are the most important. The first point emphasises the separation of the data acquisition logic from the presentation logic. The person creating the presentation template doesn’t need to know Perl, they only need to know the data items which will be passed into the template.

The last point demonstrates the way that TT2 protects the template designer from the implementation of the data structures. The data objects passed to the template processor can be scalars, arrays, hashes, objects or even subroutines. The template processor will just interpret your data correctly and Do The Right Thing to return the correct value to you. In this example each team was a hash, but in a larger system each team might be an object, in which case name, played, etc. would be accessor methods to the underlying object attributes. No changes would be required to the template as the template processor would realise that it needed to call methods rather than access hash values.

A more complex example

Stats about the English Football League are usually presented in a slightly more complex format than the one we used above. A full set of stats will show the number of games that a team has won, lost or drawn, the number of goals scored for and against the team and the number of points that the team therefore has. Teams gain three points for a win and one point for a draw. When teams have the same number of points they are separated by the goal difference, that is the number of goals the team has scored minus the number of team scored against them. To complicate things even further, the games won, drawn and lost and the goals for and against are often split between home and away games.

Therefore if you have a data source which lists the team name together with the games won, drawn and lost and the goals for and against split into home and away (a total of eleven data items) you can calculate all of the other items (goal difference, points awarded and even position in the league). Let’s take such a file, but we’ll only look at the top three teams. It will look something like this:

[text]
Man Utd,7,1,0,26,4,5,2,1,15,6
Arsenal,7,1,0,17,4,2,3,3,7,9
Leicester,4,3,1,10,8,4,2,2,7,4
[/text]

A simple script to read this data into an array of hashes will look something like this (I’ve simplified the names of the data columns – w, d, and l are games won, drawn and lost and f and a are goals scored for and against; h and a at the front of a data item name indicates whether it’s a home or away statistic):

We can then go thru the teams again and calculate all of the derived data items:

And then produce a list sorted in descending order:

And finally add the league position data item:

Having pulled all of our data into an internal data structure we can start to produce output using out templates. A template to create a CSV file containing the data split between home and away stats would look like this:

[text]
[% FOREACH team = teams -%]
[% team.pos %],[% team.name %],[% team.pl %],[% team.hw %],
[%- team.hd %],[% team.hl %],[% team.hf %],[% team.ha %],
[%- team.aw %],[% team.ad %],[% team.al %],[% team.af %],
[%- team.aa %],[% team.gd %],[% team.pt %]
[%- END %]
[/text]

And processing it like this:

produces the following output:

[text]
1,Man Utd,16,7,1,0,26,4,5,2,1,15,6,31,39
2,Arsenal,16,7,1,0,17,4,2,3,3,7,9,11,31
3,Leicester,16,4,3,1,10,8,4,2,2,7,4,5,29
[/text]

Notice that we’ve introduced the third parameter to process. If this parameter is missing then the TT2 sends its output to STDOUT. If this parameter TT2 sends its output to STDOUT. If this parameter is a scalar then it is taken as the name of a file to write the output to. This parameter can also be (amongst other things) a filehandle or a reference to an object which is assumed to implement a print method.

If we weren’t interested in the split between home and away games, then we could use a simpler template like this:

[text]
[% FOREACH team = teams -%]
[% team.pos %],[% team.name %],[% team.pl %],[% team.w %],
[%- team.d %],[% team.l %],[% team.f %],[% team.a %],
[%- team.aa %],[% team.gd %],[% team.pt %]
[% END -%]
[/text]

Which would produce output like this:

[text]
1,Man Utd,16,12,3,1,41,10,6,31,39
2,Arsenal,16,9,4,3,24,13,9,11,31
3,Leicester,16,8,5,3,17,12,4,5,29
[/text]

Producing XML

This is starting to show some of the power and flexibility of TT2, but you may be thinking that you could just as easily produce this output with a foreach loop and a couple of print statements in your code. This is, of course, true; but that’s because I’ve chosen a deliberately simple example to explain the concepts. What if we wanted to produce an XML file containing the data? And what if (as I mentioned earlier) the league data was held in an object? The code would then look even easier as most of the code we’ve written earlier would be hidden away in FootballLeague.pm.

And the template in league_xml.tt would look something like this:

Notice that as we’ve passed the whole object into process then we need to put an extra level of indirection on our template variables – everything is now a component of the league variable. Other than that, everything in the template is very similar to what we’ve used before. Presumably now team.name calls an accessor function rather than carrying out a hash lookup, but all of this is transparent to our template designer.

Multiple Formats

As a final example, let’s suppose that we need to create output football league tables in a number of formats. Perhaps we are passing this data on to other people and they can’t all use the same format. Some of our users need CSV files and others need XML. Some require data split between home and away matches and other just want the totals. In total, then, we’ll need four different templates, but the good news is that they can use the same data object. All the script needs to do is to establish which template is required and process it.

For example, you can call this script as

[text]
$ league.pl ‘English Premier’ xml split
[/text]

This will process a template called league_xml_split.tt and put the results in a file called league_split.xml.

This starts to show the true strength of the Template Toolkit. If we later wanted to add another file format – perhaps we wanted to create a league table HTML page or even a LaTeX document – then we would just need to create the appropriate template and name it according to our existing naming convention. We would need to make no changes to the code.

I hope you can now see why the Template Toolkit is fast becoming an essential part of many people’s Perl installation.