=head1 NAME

dTemplate - A powerful template handling logic with advanced features.

=head1 SYNOPSIS

  use dTemplate;

  $mail_template = define dTemplate "mail_tmpl.txt";# definition

  $mail = $mail_template->parse(                    # parsing
    TO      => "foo@bar.com",
    SUBJECT => $subject,
    BODY    => 
      sub { $email_type==3 ? $body_for_type_3 : $body_for_others },
    SIGNATURE=> $signature_template->parse( KEY => "value" )
  );

  print "Please send this mail: $mail";

  where mail_tmpl.txt is:

    From    : Me
    To      : $TO$
    Subject : $SUBJECT$

    Message body:
    $BODY$

    $SIGNATURE$

  # Changing placeholder special characters:

  $dTemplate::START_DELIMITER     = '<%\s*'; # default: \$
  $dTemplate::END_DELIMITER       = '\s*%>'; # default: \$
  $dTemplate::VAR_PATH_SEP        = '\/';    # default: \.
  $dTemplate::PRINTF_SEP          = '\$';    # default: %+
  $dTemplate::ENCODER_SEP         = '\@';    # default: \*
  $dTemplate::ENCODER_PARAM_START = '\(';    # default: \/
  $dTemplate::ENCODER_PARAM_END   = '\)';    # default:
  
  # Advanced feature: Styling

  $style={lang =>'hungarian',color=>'white'};     # Style definition

  $html_template = choose dTemplate( $style,      # Selector definition
    'hungarian+white' => 
            define dTemplate("hun_white_template.html"),
    'spanish'         => 
            define dTemplate("spanish.html"),
    'black+hungarian' => 
            define dTemplate("hun_black_template.html"),
    'english'         => 
            define dTemplate("english_template.html"),
    'empty'           => 
      "<html>This is a text, $BODY$ is NOT substituted!!!!"</html>",
    ''                => 
            text dTemplate "<html>$BODY$</html>",  # default
  );

  $body_template= choose dTemplate( $style,       # Selector definition
    'hungarian'       => 
            define dTemplate("sziasztok_emberek.html"),
    'spanish'         => 
            define dTemplate("adios_amigos.html"),
    ''                => 
            define dTemplate("bye_bye.html"),
  );

  print $html_template->parse(BODY => $body_template->parse());
    # will print "sziasztok_emberek.html" in the
    #"hun_white_template.html"

  %$style=();
  print $html_template->parse(BODY => $body_template->parse());
    # will print "bye_bye.html" surrounded by "<html>" and "</html>" tags.

  %$style=( lang => 'english' );
  print $html_template->parse(BODY => $body_template->parse());
    # will print the "bye_bye.html" in of the "english_template.html"

=head1 DESCRIPTION

This module is aimed to be a simple, general-purpose, lightweight, 
but very powerful templating system.

You can write template-parsing routines in the way the templates are
structured logically. Starting from the biggest to the smallest.
Your program code will be very clear, structured and easy to understand.
This logic can be attained by using inline subroutines as values of template
variables. (Look at the example at the end of the document)

=head1 USAGE

First, you need to know how a template looks like, then you need to know how
to define a template in a perl program, then you can parse it.

After that you can see how to make new encoders.

=head2 How a template looks like

A template is a simple text file, which contains template variable placeholders.

The full format of a placeholder is:

  $Template_Variable%printf_style_format_string*encoder1/par1*encoder2/par2$

Note: The special characters ($, % and *) can be changed, look below.

Where:

=over 4 

=item Template_Variable

It is a mandatory part of the placeholder. Can contain any (locale-aware) 
alphanumeric characters and '.' .

=item %printf_style_format_string

This is an optional part. Used when you want to format the output. You can
use as many '%' as you want, it can be good to pad the variable, for example
when you parse a table. E.g:  $MONEY%%%%%%011d$ is a valid placeholder.

=item *encoder/par

There are predefined encoders in the module, which can be used to format 
the input data. 
These are:

  - u       : url-encoder
  - h       : HTML-encoder (converts > to &gt;, etc)
  - ha      : Advanced HTML encoder (\n => <BR>, tabs => spaces)
  - uc      : convert the string to uppercase
  - lc      : convert the string to lowercase
  - eq/par  : returns true if the encoded string is "par"
  - if/par  : returns "par" if the encoded string is true

"/par" is an optional scalar parameter.

You can use zero or more of these:

  $TITLE*uc*h$

or

  $weight*eq/7*if/CHECKED$

(the last example returns CHECKED if the weight is equal to "7", useful for
radio-buttons or drop-down menus in HTML)

Read more on encoders (and how to make new encoders) in the Encoders part.

=back

=head2 Definition of a template

There are 3 ways to define a template.

=over 4

=item $template = define dTemplate $filename

This reads the template from a file.

=item $template = text dTemplate $scalar

This creates a template from a scalar

=item $template = choose dTemplate $hash, "style1" => $template1, "style2" => ...

It is the definition of the template chooser. It is the way how you can
create styled templates.

=back

=head2 Parsing

Parsing means substituting the variables which are defined in the template.
It can be done by simply calling the "parse" method of a dTemplate object.

The parameters of the "parse" method are the substitution definitions.

You can provide substitution parameters in two form: 

  - list of name => value pairs
  - with a hash reference

You can mix them if you want:

  $template->parse(
    name => $value,
    { name2 => $value2, name3 => $value3 },
    name4 => $value4,
    { name5 => $value5 },
    ...
  )

The "value" can be:

=over 4

=item scalar or scalar ref.

If a value is scalar, it will be substituted. Scalar refs can be used to save
some memory if you use them more than one time.

=item code reference ( sub { }, or \&subroutine )

The sub will be evaluated at each occurence of the template variable.

=item hash

You can assign a hash to a template variable if you want.
In this way, you can use structured data in the
templates, e.g you assign a { name => "Greg", "zip" => 111 } to the template
variable "person", and if you use "person.name" in the template, you will
get "Greg" back. Nesting (more "."-s) will also work.

=back

You can use %dTemplate::parse hash to assign global parse parameters.

The return value of the parse method is a dTemplate::Scalar object, which can
be used as a simple scalar, but really it is a scalar reference to save some
memory. It is useful if you use large templates.

=head2 Encoders

The global hash, %dTemplate::ENCODERS contains the defined encoders.

The hash keys are the names, the values are subroutine references. These subs
get the encodable data as the first parameter and returns the encoded value.
The second parameter is the optional parameter, which can provided by the "/"
separator after the encoder name.

=head2 $dTemplate::parse{''}

$dTemplate::parse{''} is a special hash key in the %dTemplate::parse hash. 
This value is parsed into the place of the unassigned variable.

By default, it is an empty scalar, so you won't even notice if you forget to
assign a variable.

If you want to be warned if a variable is not assigned, you can use the
following code reference:

  use Carp qw(cluck);
  $dTemplate::parse{''} = sub {
      cluck "$_[0] is not assigned";
      return "";
  }

By this, the output of the further "parse" calls will call this sub if a
variable is not assigned.

=head2 Magical (tied) hashes

You can use magical hashes everywhere in the "parse" method parameter list, 
where you can to use normal hashes, but because I redesign the "parse" method 
with the speed with the first priority, it works quite a bit different than
the older ( <= 2.0 ) versions. 

At template-compile time, the "compile" method collects the variable names, 
which are found in that template, and the "parse" method knows which 
variables are required for processing this template.

When the "parse" method realizes that the given parameter is a hash reference, 
then it always tries all the remaining template variables (which are not 
assigned in the preceding part of the parameter list) in that hash reference.

Imagine the following situation:

  $template->parse( name => "blow", \%hash);

... where %hash is a magical hash, and the $template contains the "name",
"address" and "method.type" variables. When the parse method meets with the
\%hash, the "name" is already assigned, so it olny tries to read the
$hash{address} and $hash{method} variables.

This is not a problem with normal hashes, but if you use magical hashes, you
may have a very expensive FETCH function, and this effect can cause problems.

There are two way to work around it:

=over 4

=item *

Use qualified variable names. If you use the following form of parse:

  $template->parse( name => "blow", data => \%hash);

... then the %hash is called only when the template parser finds the
"data.address" and "data.method.type" variable references in the template. Of
course, you have to change the template variable names also.

=item *

Use the hash instead of a hash reference:

  $template->parse( name => "blow", %hash);

In this case, the magical hash is iterated through when the parameter list
is assembled. This is better only if you are afraid of random
key retrieval, but it can be also slow, if the FIRSTKEY, NEXTKEY and FETCH
operations are slow.

But if the random-key retrieval is not a problem for your magical hash, 
then use the default form instead of this, because that requires less
operation (only one FETCH).

=back

=head2 Changing placeholder special characters

You can change the placeholder parameters by changing (or localizing) the
following variables.

  $dTemplate::START_DELIMITER     = '\$';
  $dTemplate::END_DELIMITER       = '\$';
  $dTemplate::VAR_PATH_SEP        = '\.';
  $dTemplate::PRINTF_SEP          = '%+';
  $dTemplate::ENCODER_SEP         = '\*';
  $dTemplate::ENCODER_PARAM_START = '\/';
  $dTemplate::ENCODER_PARAM_END   = '';

If you change these variables, then these are used in the compilation of the
following templates. Currently there is no supported way to recompile an
already compiled template, so you need to create a new instance.

Although if you want to compile a variable, which is not compiled, then you
can call the $template->compile method on them by hand.

These variables can be set to any regular expressions, not just one single 
character.

If you don't want to use any of the features, then set the corresponding
variable to any regexp that is too complex to be true.
  
=head1 HINTS

=over 4

=item *

In the first parse of every template, the templates will be compiled. 
It is used to speed up parsing.

=item *

Don't forget that %dTemplate::parse can be localized. This means you can 
define local-only variable assignments in a subroutine:

  local %dTemplate::parse=( 
    %dTemplate::parse, 
    local_name => $value 
  );

=item *

You don't need to use text as the input value of an encoder, you can
use any scalar, even referenes! If you want (for example) print a date by a 
date encoder, which
expects the date to be an array ref of [ year, month, day ], then you can do
this, e.g:

   $dTemplate::ENCODERS{date}=sub {
     return "" if ref($_[0]) ne 'ARRAY';
     return $_[0]->[0]."-".$_[0]->[1]."-".$_[0]->[2];
   }

Then, when you put $START_DATE*date$ to a template, you can parse this template:

  $template->parse(
    START_DATE => [2000,11,13],
    ...
  );

=back

=head1 EXAMPLE

It is an example, which contains most of the features this module has. It is not
intended to be a real-world example, but it can show the usage of this module.

This example consists of one program, and some template modules.

The executable version of this program can be found in the example
directory of the module distribution.

  use dTemplate;

  ### definition of the standard templates

  my @TEMPLATE_LIST=qw(page table_row table_cell);
  my $templates={};
  foreach my $template (@TEMPLATE_LIST) {
    $templates->{$template} = 
      define dTemplate("templates/$template.htm");
  }

  ### definition of the styled templates (styles = languages)

  my @STYLES=qw(eng hun);
  my @STYLED_TEMPLATE_LIST=qw(table_title);

  my $style_select={ lang => 'hun' }; 

  foreach my $template (@STYLED_TEMPLATE_LIST) {
    my @array=();
    foreach my $style (@STYLES) {
      push @array, $style => 
        define dTemplate("text/$style/$template.txt");
    }
    $templates->{$template} = 
      choose dTemplate $style_select, @array;
  }

  ### setting up input data

  my $table_to_print=[
    [ "Buwam",   3, 6, 9 ],
    [ "Greg",    8, 4, 2 ],
    [ "You're",  8, 3, 4 ],
    [ "HTML chars: <>", 3],
  ];

  ### setting up the global parse hash with parse parameters;

  $dTemplate::parse{PAGENO}=7;

  ### settings up a hash with personal data.

  my $person_hash={
    name => { first_name => "Greg" },
    zip  => "9971",
  };

  ### this hash is simply added to other parse parameters

  my $parse_hash={
    "unknown" => { data => 157 },
  };

  ### the main page parse routine

  print $templates->{page}->parse(
    TABLE_TITLE =>             # name => value pair
      $templates->{table_title}->parse(),
    TABLE => sub {             # name => value pair. value is a sub
      my $ret="";
      foreach my $row (@$table_to_print) {
        $ret .= $templates->{table_row}->parse(
          BODY => sub {
            my $ret="";
            foreach my $cell (@$row) {
              $ret .= $templates->{table_cell}->parse(
                TEXT => $cell,
              )
            }
            return $ret;
          }
        )
      }
      return $ret;
    },
    "person" => $person_hash,  # name => value pair. value is a href
    $parse_hash,               # only a hash with parse parameters
  );

And the templates:

=over 4

=item templates/page.htm:

  <html>
  <body>

  <h1>$TABLE_TITLE*h$</h1>

  <table>
  $TABLE$
  </table>

  <br>
  Person name: $person.name*h$, zip code: $person.zip*h$
  <br>

  Unknown data: $unknown.data*h$
  <br>

  Page: $PAGENO%02d*h$

  </body>
  </html>

=item templates/table_row.htm:

  <tr>$BODY$</tr>

=item templates/table_cell.htm:

  <td>$TEXT*h$</td>

=item text/eng/table_title.txt:

  Table 1

=item text/hun/table_title.txt:

  1. táblázat

=back

=head1 COPYRIGHT

Copyrigh (c) 2000-2001 Szabó, Balázs (dLux)

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

=head1 AUTHOR

dLux (Szabó, Balázs) <dlux@kapu.hu>

=head1 SEE ALSO

perl(1), HTML::Template, Text::Template, CGI::FastTemplate, Template.

=cut

