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#============================================================= -*-perl-*-
#
# Template::Manual::Syntax
#
# AUTHOR
#   Andy Wardley  <abw@wardley.org>
#
# COPYRIGHT
#   Copyright (C) 1996-2022 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::Manual::Syntax - Directive syntax, structure and semantics

=head1 Tag Styles

Template directives are embedded between start and end markers tags.
By default these tag markers are C<[%> and C<%]>.

    [% PROCESS header %]

    <h1>Hello World!</h1>
    <a href="[% page.next %]"><img src="[% icon.next %].gif"></a>

    [% PROCESS footer %]

You can change the tag characters using the C<START_TAG>, C<END_TAG> and
C<TAG_STYLE> configuration options. You can also use the C<TAGS> directive to
define a new tag style for the current template file.

You can also set the C<INTERPOLATE> option to allow simple variable references
to be embedded directly in templates, prefixed by a C<$>.

    # INTERPOLATE = 0
    <td>[% name %]</td>
    <td>[% email %]</td>

    # INTERPOLATE = 1
    <td>$name</td>
    <td>$email</td>

Directives may be embedded anywhere in a line of text and can be split
across several lines.  Insignificant whitespace is generally ignored
within the directive.

    [% INCLUDE header
         title = 'Hello World'
         bgcol = '#ffffff'
    %]

    [%INCLUDE menu align='right'%]

    Name: [% name %]  ([%id%])

=head1 Outline Tags

As of version 2.26, the Template Toolkit supports "outline" tags.  These have
a designated marker at the start of a line (C<%%> by default) and continue to
the end of a line.  The newline character at the end of the line is discarded
(aka "chomped").

So rather than writing something like this:

    [% IF some.list.size -%]
      <ul>
    [%   FOREACH item IN some.list -%]
        <li>[% item.html %]</li>
    [%   END -%]
      </ul>
    [% END -%]

You can write it like this instead:

    %% IF some.list.size
      <ul>
    %%   FOREACH item IN some.list
        <li>[% item.html %]</li>
    %%   END
      </ul>
    %% END

Outline tags aren't enabled by default.  There are a numbers of ways you can
enable them.  The first is to use the C<TAGS> directive to set the tag style
to C<outline> in any templates where you want to use them.  This will enable
outline tags from that point on.

    [% TAGS outline -%]
    %% INCLUDE header

You can set the C<TAGS> back to the C<default> value at some point later in the
template if you want to disable them:

    [% TAGS default -%]

You can set the C<TAG_STYLE> configuration option if you want then enabled in
all templates by default.  You can always use the C<[% TAGS default %]>
directive to disable them in any templates or parts of templates if necessary.

    my $tt = Template->new({
        TAG_STYLE => 'outline',
    });

The C<OUTLINE_TAG> option allows you to set the outline tag marker to something
else if you're not a fan of percent signs.  Setting this option will
automatically enable outline tags.

    my $tt = Template->new({
        OUTLINE_TAG => '>>',
    });

You can also use the C<TAGS> directive to define your own custom tags (start,
end and now optionally, outline) for a template or part of a template.

    [% TAGS <* *> >> %]
    >> INCLUDE header       # outline tag
    Hello <* name *>        # inline tag

If you only specify a start and end tag then outline tags will be disabled.

    [% TAGS <* *> %]        # no outline tags

=head1 Comments

The C<#> character is used to indicate comments within a directive.
When placed immediately inside the opening directive tag, it causes
the entire directive to be ignored.

    [%# this entire directive is ignored no
        matter how many lines it wraps onto
    %]

In any other position, it causes the remainder of the current line to
be treated as a comment.

    [% # this is a comment
       theta = 20      # so is this
       rho   = 30      # <aol>me too!</aol>
    %]

=head1 Chomping Whitespace

You can add C<-> or C<+> to the immediate start or end of a directive
tag to control the whitespace chomping options.  See the C<PRE_CHOMP> and
C<POST_CHOMP> options for further details.

    [% BLOCK foo -%]    # remove trailing newline
    This is block foo
    [%- END %]          # remove leading newline

=head1 Implicit Directives: GET and SET

The simplest directives are C<GET> and C<SET> which retrieve and update
variable values respectively. The C<GET> and C<SET> keywords are actually
optional as the parser is smart enough to see them for what they really are
(but note the caveat below on using side-effect notation). Thus, you'll
generally see:

    [% SET foo = 10 %]
    [% GET foo %]

written as:

    [% foo = 10 %]
    [% foo %]

You can also express simple logical statements as implicit C<GET> directives:

    [% title or template.title or 'Default Title' %]

    [% mode == 'graphics' ? "Graphics Mode Enabled" : "Text Mode" %]

All other directives should start with a keyword specified in UPPER
CASE (but see the C<ANYCASE> option).  All directives keywords are in
UPPER CASE to make them visually distinctive and to distinguish them
from variables of the same name but different case.  It is perfectly
valid, for example, to define a variable called C<stop> which is
entirely separate from the C<STOP> directive.

    [% stop = 'Clackett Lane Bus Depot' %]

    The bus will next stop at [% stop %]    # variable

    [% STOP %]                              # directive

=head1 Block Directives

Directives such as C<FOREACH>, C<WHILE>, C<BLOCK>, C<FILTER>, etc., mark the
start of a block which may contain text or other directives up to the matching
C<END> directive. Blocks may be nested indefinitely. The C<IF>, C<UNLESS>,
C<ELSIF> and C<ELSE> directives also define blocks and may be grouped together
in the usual manner.

    [% FOREACH item = [ 'foo' 'bar' 'baz' ] %]
       * Item: [% item %]
    [% END %]

    [% BLOCK footer %]
       Copyright 2000 [% me %]
       [% INCLUDE company/logo %]
    [% END %]

    [% IF foo %]
       [% FOREACH thing = foo.things %]
          [% thing %]
       [% END %]
    [% ELSIF bar %]
       [% INCLUDE barinfo %]
    [% ELSE %]
       do nothing...
    [% END %]

Block directives can also be used in a convenient side-effect notation.

    [% INCLUDE userinfo FOREACH user = userlist %]

    [% INCLUDE debugtxt msg="file: $error.info"
         IF debugging %]

    [% "Danger Will Robinson" IF atrisk %]

versus:

    [% FOREACH user = userlist %]
       [% INCLUDE userinfo %]
    [% END %]

    [% IF debugging %]
       [% INCLUDE debugtxt msg="file: $error.info" %]
    [% END %]

    [% IF atrisk %]
    Danger Will Robinson
    [% END %]

=head1 Capturing Block Output

The output of a directive can be captured by simply assigning the directive
to a variable.

    [% headtext = PROCESS header title="Hello World" %]

    [% people = PROCESS userinfo FOREACH user = userlist %]

This can be used in conjunction with the C<BLOCK> directive for defining large
blocks of text or other content.

    [% poem = BLOCK %]
       The boy stood on the burning deck,
       His fleece was white as snow.
       A rolling stone gathers no moss,
       And Keith is sure to follow.
    [% END %]

Note one important caveat of using this syntax in conjunction with side-effect
notation.  The following directive does not behave as might be expected:

    [% var = 'value' IF some_condition %]   # does not work

In this case, the directive is interpreted as (spacing added for clarity)

    [% var = IF some_condition %]
       value
    [% END %]

rather than

    [% IF some_condition %]
       [% var = 'value' %]
    [% END %]

The variable is assigned the output of the C<IF> block which returns
C<'value'> if true, but nothing if false.  In other words, the following
directive will always cause 'var' to be cleared.

    [% var = 'value' IF 0 %]

To achieve the expected behaviour, the directive should be written as:

    [% SET var = 'value' IF some_condition %]

=head1 Chaining Filters

Multiple C<FILTER> directives can be chained together in sequence.  They
are called in the order defined, piping the output of one into the
input of the next.

    [% PROCESS somefile FILTER truncate(100) FILTER html %]

The pipe character, C<|>, can also be used as an alias for C<FILTER>.

    [% PROCESS somefile | truncate(100) | html %]

=head1 Multiple Directive Blocks

Multiple directives can be included within a single tag when delimited
by semi-colons.  Note however that the C<TAGS> directive must always
be specified in a tag by itself.

    [% IF title;
          INCLUDE header;
       ELSE;
          INCLUDE other/header  title="Some Other Title";
       END
    %]

versus

    [% IF title %]
       [% INCLUDE header %]
    [% ELSE %]
       [% INCLUDE other/header  title="Some Other Title" %]
    [% END %]

=cut

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