=head1 NAME

Template::Filters - post-processing filters for template blocks

=head1 SYNOPSIS

    use Template::Filters;

    $filters = Template::Filters->new(\%config);

    ($filter, $error) = $filters->fetch($name, \@args, $context);

=head1 DESCRIPTION

The Template::Plugins module implements a provider for creating and/or
returning subroutines that implement the standard filters.  Additional 
custom filters may be provided via the FILTERS options.

=head1 METHODS

=head2 new(\%params) 

Constructor method which instantiates and returns a reference to a
Template::Filters object.  A reference to a hash array of configuration
items may be passed as a parameter.  These are described below.  

    my $filters = Template::Filters->new({
	FILTERS => { ... },
    });

    my $template = Template->new({
	LOAD_FILTERS => [ $filters ],
    });

A default Template::Filters module is created by the Template.pm module
if the LOAD_FILTERS option isn't specified.  All configuration parameters
are forwarded to the constructor.

    $template = Template->new({
        FILTERS => { ... },
    });

=head2 fetch($name, \@args, $context)

Called to request that a filter of a given name be provided.  The name
of the filter should be specified as the first parameter.  This should
be one of the standard filters or one specified in the FILTERS
configuration hash.  The second argument should be a reference to an
array containing configuration parameters for the filter.  This may be
specified as 0, or undef where no parameters are provided.  The third
argument should be a reference to the current Template::Context
object.

The method returns a reference to a filter sub-routine on success.  It
may also return (undef, STATUS_DECLINE) to decline the request, to allow
delegation onto other filter providers in the LOAD_FILTERS chain of 
responsibility.  On error, ($error, STATUS_ERROR) is returned where $error
is an error message or Template::Exception object indicating the error
that occurred. 

When the TOLERANT option is set, errors are automatically downgraded to
a STATUS_DECLINE response.


=head1 CONFIGURATION OPTIONS

The following list details the configuration options that can be provided
to the Template::Plugins new() constructor.

=over 4


=item FILTERS

The FILTERS option can be used to specify custom filters which can
then be used with the FILTER directive like any other.  These are
added to the standard filters which are available by default.  Filters
specified via this option will mask any standard filters of the same
name.

The FILTERS option should be specified as a reference to a hash array
in which each key represents the name of a filter.  The corresponding
value should contain a reference to an array containing a subroutine
reference and a flag which indicates if the filter is static (0) or
dynamic (1).  A filter may also be specified as a solitary subroutine
reference and is assumed to be static.

    $filters = Template::Filters->new({
  	FILTERS => {
  	    'sfilt1' =>   \&static_filter,      # static
            'sfilt2' => [ \&static_filter, 0 ], # same as above
  	    'dfilt1' => [ \&dyanamic_filter_factory, 1 ],
  	},
    });

Additional filters can be specified at any time by calling the 
define_filter() method on the current Template::Context object.
The method accepts a filter name, a reference to a filter 
subroutine and an optional flag to indicate if the filter is 
dynamic.

    my $context = $template->context();
    $context->define_filter('new_html', \&new_html);
    $context->define_filter('new_repeat', \&new_repeat, 1);

Static filters are those where a single subroutine reference is used
for all invocations of a particular filter.  Filters that don't accept
any configuration parameters (e.g. 'html') can be implemented
statically.  The subroutine reference is simply returned when that
particular filter is requested.  The subroutine is called to filter
the output of a template block which is passed as the only argument.
The subroutine should return the modified text.

    sub static_filter {
  	my $text = shift;
	# do something to modify $text...
  	return $text;
    }

The following template fragment:

    [% FILTER sfilt1 %]
    Blah blah blah.
    [% END %]

is approximately equivalent to:

    &static_filter("\nBlah blah blah.\n");

Filters that can accept parameters (e.g. 'truncate') should be
implemented dynamically.  In this case, the subroutine is taken to be
a filter 'factory' that is called to create a unique filter subroutine
each time one is requested.  A reference to the current
Template::Context object is passed as the first parameter, followed by
any additional parameters specified.  The subroutine should return
another subroutine reference (usually a closure) which implements the
filter.

    sub dynamic_filter_factory {
	my ($context, @args) = @_;

  	return sub {
  	    my $text = shift;
	    # do something to modify $text...
	    return $text;	    
  	}
    }

The following template fragment:

    [% FILTER dfilt1(123, 456) %] 
    Blah blah blah
    [% END %]              

is approximately equivalent to:

    my $filter = &dynamic_filter_factory($context, 123, 456);
    &$filter("\nBlah blah blah.\n");

See the FILTER directive for further examples.

=item TOLERANT

The TOLERANT flag is used by the various Template Toolkit provider
modules (Template::Provider, Template::Plugins, Template::Filters) to
control their behaviour when errors are encountered.  By default, any
errors are reported as such, with the request for the particular
resource (template, plugin, filter) being denied and an exception
raised.  When the TOLERANT flag is set to any true values, errors will
be silently ignored and the provider will instead return
STATUS_DECLINED.  This allows a subsequent provider to take
responsibility for providing the resource, rather than failing the
request outright.  If all providers decline to service the request,
either through tolerated failure or a genuine disinclination to
comply, then a '<resource> not found' exception is raised.

=head1 TEMPLATE TOOLKIT FILTERS

The following standard filters are distributed with the Template Toolkit.

=over 4

=item html

Converts the characters '<', '>' and '&' to '&lt;', '&gt;' and '&amp',
respectively, protecting them from being interpreted as representing
HTML tags or entities. 

    [% FILTER html %]
    Binary "<=>" returns -1, 0, or 1 depending on...
    [% END %]

output:

    Binary "&lt;=&gt;" returns -1, 0, or 1 depending on...

=item html_para

This filter formats a block of text into HTML paragraphs.  A sequence of 
two or more newlines is used as the delimiter for paragraphs which are 
then wrapped in HTML E<lt>pE<gt>...E<lt>/pE<gt> tags.

    [% FILTER html_para %]
    The cat sat on the mat.

    Mary had a little lamb.
    [% END %]

output:

    <p>
    The cat sat on the mat.
    </p>

    <p>
    Mary had a little lamb.
    </p>

=item html_break

Similar to the html_para filter described above, but uses the HTML tag
sequence E<lt>brE<gt>E<lt>brE<gt> to join paragraphs.

    [% FILTER html_break %]
    The cat sat on the mat.

    Mary had a little lamb.
    [% END %]

output:

    The cat sat on the mat.
    <br>
    <br>
    Mary had a little lamb.

=item format(format)

The 'format' filter takes a format string as a parameter (as per
printf()) and formats each line of text accordingly.

    [% FILTER format('<!-- %-40s -->') %]
    This is a block of text filtered 
    through the above format.
    [% END %]

output:

    <!-- This is a block of text filtered        -->
    <!-- through the above format.               -->

=item truncate(length)

Truncates the text block to the length specified, or a default length of
32.  Truncated text will be terminated with '...' (i.e. the '...' falls
inside the required length, rather than appending to it).

    [% FILTER truncate(21) %]
    I have much to say on this matter that has previously 
    been said on more than one occassion.
    [% END %]

output:

    I have much to say...

=item repeat(iterations)

Repeats the text block for as many iterations as are specified (default: 1).

    [% FILTER repeat(3) %]
    We want more beer and we want more beer,
    [% END %]
    We are the more beer wanters!

output:

    We want more beer and we want more beer,
    We want more beer and we want more beer,
    We want more beer and we want more beer,
    We are the more beer wanters!

=item remove(string) 

Searches the input text for any occurences of the specified string and 
removes them.  A Perl regular expression may be specified as the search 
string.

    [% "The  cat  sat  on  the  mat" FILTER remove('\s+') %]

output: 

    Thecatsatonthemat

=item replace(search, replace) 

Similar to the remove filter described above, but taking a second parameter
which is used as a replacement string for instances of the search string.

    [% "The  cat  sat  on  the  mat" | replace('\s+', '_') %]

output: 

    The_cat_sat_on_the_mat

=item redirect(file)

The 'redirect' filter redirects the output of the block into a separate
file, specified relative to the OUTPUT_PATH configuration item.

    [% FOREACH user = myorg.userlist %]
       [% FILTER redirect("users/${user.id}.html") %]
          [% INCLUDE userinfo %]
       [% END %]
    [% END %]

or more succinctly, using side-effect notation:

    [% INCLUDE userinfo 
         FILTER redirect("users/${user.id}.html")
	   FOREACH user = myorg.userlist 
    %]

A 'file' exception will be thrown if the OUTPUT_PATH option is undefined.

=item eval(template_text)

The 'eval' filter evaluates the block as template text, processing
any directives embedded within it.  This allows template variables to
contain template fragments, or for some method to be provided for
returning template fragments from an external source such as a
database, which can then be processed in the template as required.

    my $vars  = {
	fragment => "The cat sat on the [% place %]",
    };
    $template->process($file, $vars);

The following example:

    [% fragment | eval %]

is therefore equivalent to 

    The cat sat on the [% place %]

The 'evaltt' filter is provided as an alias for 'eval'.

=item perl(perlcode)

The 'perl' filter evaluates the block as Perl code.  The EVAL_PERL
option must be set to a true value or a 'perl' exception will be
thrown.

    [% my_perl_code | perl %]

In most cases, the [% PERL %] ... [% END %] block should suffice for 
evaluating Perl code, given that template directives are processed 
before being evaluate as Perl.  Thus, the previous example could have
been written in the more verbose form:

    [% PERL %]
    [% my_perl_code %]
    [% END %]

as well as

    [% FILTER perl %]
    [% my_perl_code %]
    [% END %]

The 'evalperl' filter is provided as an alias for 'perl' for backwards
compatability.

=item stderr

The stderr filter prints the output generating by the enclosing block to
STDERR 

=back

=head1 AUTHOR

Andy Wardley E<lt>abw@kfs.orgE<gt>

    http://www.template-toolkit.org/
    http://www.kfs.org/~abw/

=head1 REVISION

$Revision: 1.1 $

=head1 COPYRIGHT

    Copyright (C) 1996-2000 Andy Wardley.  All Rights Reserved.
    Copyright (C) 1998-2000 Canon Research Centre Europe Ltd.

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


=head1 SEE ALSO


L<Template|Template>, 
L<Template::Context|Template::Context>