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✏️ Editing: Datafile.pod

#============================================================= -*-perl-*-
#
# Template::Tutorial::Datafile
#
# DESCRIPTION

#
# AUTHOR
#   Dave Cross  <dave@dave.org.uk>
#
# COPYRIGHT
#   Copyright (C) 1996-2008 Andy Wardley.  All Rights Reserved.
#
#   This module is free software; you can redistribute it and/or
#   modify it under the same terms as Perl itself.
#
#========================================================================

=head1 NAME

Template::Tutorial::Datafile - Creating Data Output Files Using the Template Toolkit

=head1 DESCRIPTION

=head1 Introducing the Template Toolkit

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 E<lt>abw@wardley.orgE<gt>. 
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 
I<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.

=head1 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

use Template;
    my $tt = Template->new;

in your code. The constructor function, C<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 C<process> method 
like this

$tt->process('my_template', \%data)
        || die $tt->error;

We pass two parameters to C<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 C<%data>? The answer is just 
about anything. Here's an example showing data about English 
Premier League football teams.

my @teams = ({ name   => 'Man Utd',
                   played => 16,
                   won    => 12,
                   drawn  => 3,
                   lost   => 1 },
                 { name   => 'Bradford',
                   played => 16,
                   won    => 2,
                   drawn  => 5,
                   lost   => 9 });

my %data = ( name   => 'English Premier League',
                 season => '2000/01',
                 teams  => \@teams );

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

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

League Standings

League Name: [% name %]
    Season     : [% season %]

Teams:
    [% FOREACH team = teams -%]
    [% team.name %] [% team.played -%] 
     [% team.won %] [% team.drawn %] [% team.lost %]
    [% END %]

Running this template with this data gives us the following 
output

League Standings

League Name: English Premier League
    Season     : 2000/01

Teams:
    Man Utd 16 12 3 1
    Bradford 16 2 5 9

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

=over 4

=item *

Template processing directives are written using a simple 
language which is not Perl.

=item *

The keys of the C<%data> have become the names of the data 
variables within the template.

=item *

Template processing directives are surrounded by C<[%> and 
C<%]> sequences.

=item *

If these tags are replaced with C<[%-> C<-%]> then the preceding 
or following linefeed is suppressed.

=item *

In the C<FOREACH> loop, each element of the C<teams> list was 
assigned, in turn, to the temporary variable C<team>.

=item *

Each item assigned to the C<team> variable is a Perl hash. 
Individual values within the hash are accessed using a dot notation.

=back

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 C<name>, C<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.

=head2 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:

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

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):

my @cols = qw(name hw hd hl hf ha aw ad al af aa);

my @teams;
    while (<>) {
        chomp;

my %team;

@team{@cols} = split /,/;

push @teams, \%team;
    }

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

foreach (@teams) {
        $_->{w} = $_->{hw} + $_->{aw};
        $_->{d} = $_->{hd} + $_->{ad};
        $_->{l} = $_->{hl} + $_->{al};

$_->{pl} = $_->{w} + $_->{d} + $_->{l};

$_->{f} = $_->{hf} + $_->{af};
        $_->{a} = $_->{ha} + $_->{aa};

$_->{gd} = $_->{f} - $_->{a};
        $_->{pt} = (3 * $_->{w}) + $_->{d};
    }

And then produce a list sorted in descending order:

@teams = sort { 
	$b->{pt} <=> $b->{pt} || $b->{gd} <=> $a->{gd} 
    } @teams;

And finally add the league position data item:

$teams[$_]->{pos} = $_ + 1 
        foreach 0 .. $#teams;

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:

[% 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 %]

And processing it like this:

$tt->process('split.tt', { teams => \@teams }, 'split.csv')
      || die $tt->error;

produces the following output:

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

Notice that we've introduced the third parameter to C<process>. 
If this parameter is missing then the TT2 sends its output to 
C<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 C<print> method.

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

[% 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 -%]

Which would produce output like this:

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

=head1 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 C<foreach> loop and a couple of C<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 C<FootballLeague.pm>.

use FootballLeague;
    use Template;

my $league = FootballLeague->new(name => 'English Premier');

my $tt = Template->new;

$tt->process('league_xml.tt', { league => $league })
        || die $tt->error;

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

<?xml version="1.0"?>
    <!DOCTYPE LEAGUE SYSTEM "league.dtd">

<league name="[% league.name %]" season="[% league.season %]">
    [% FOREACH team = league.teams -%]
      <team name="[% team.name %]"
            pos="[% team.pos %]"
            played="[% team.pl %]"
            goal_diff="[% team.gd %]"
            points="[% team.pt %]">
         <stats type="home">
        	win="[% team.hw %]"
        	draw="[%- team.hd %]"
        	lose="[% team.hl %]"
        	for="[% team.hf %]"
        	against="[% team.ha %]" />
         <stats type="away">
        	win="[% team.aw %]"
        	draw="[%- team.ad %]"
        	lose="[% team.al %]"
        	for="[% team.af %]"
        	against="[% team.aa %]" />
      </team>
    [% END -%]
    &/league>

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

=head1 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.

use FootballLeague;
    use Template;

my ($name, $type, $stats) = @_;

my $league = FootballLeague->new(name => $name);

my $tt = Template->new;

$tt->process("league_${type}_$stats.tt", 
                 { league => $league }
                 "league_$stats.$type")
        || die $tt->error;

For example, you can call this script as

league.pl 'English Premier' xml split

This will process a template called C<league_xml_split.tt> 
and put the results in a file called C<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.

=head1 AUTHOR

Dave Cross E<lt>dave@dave.org.ukE<gt>

=head1 VERSION

Template Toolkit version 2.19, released on 27 April 2007.

=head1 COPYRIGHT

This module is free software; you can redistribute it and/or
modify it under the same terms as Perl itself.

=cut

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