Template::Manual::Variables - Template variables and code bindings

This section describes the different ways in which Perl data can be bound to template variables and accessed via Template Toolkit directives.

A reference to a hash array may be passed as the second argument to the process() method, containing definitions of template variables. The VARIABLES (a.k.a. PRE_DEFINE) option can also be used to pre-define variables for all templates processed by the object.

    my $tt = Template->new({
        VARIABLES => {
            version => 3.14,
            release => 'Sahara',
        },  
    });

    my $vars = {
        serial_no => 271828,
    };

    $tt->process('myfile', $vars);

'myfile':

    This is version [% version %] ([% release %]).
    Serial number: [% serial_no %]

output:

    This is version 3.14 (Sahara)
    Serial number: 271828

Variable names may contain any alphanumeric characters or underscores. They may be lower, upper or mixed case although the usual convention is to use lower case. The case is significant however, and 'foo', 'Foo' and 'FOO' are all different variables. Upper case variable names are permitted, but not recommended due to a possible conflict with an existing or future reserved word. As of version 2.00, these are:

        GET CALL SET DEFAULT INSERT INCLUDE PROCESS WRAPPER 
    IF UNLESS ELSE ELSIF FOR FOREACH WHILE SWITCH CASE
    USE PLUGIN FILTER MACRO PERL RAWPERL BLOCK META
    TRY THROW CATCH FINAL NEXT LAST BREAK RETURN STOP 
    CLEAR TO STEP AND OR NOT MOD DIV END

The variable values may be of virtually any Perl type, including simple scalars, references to lists, hash arrays, subroutines or objects. The Template Toolkit will automatically apply the correct procedure to accessing these values as they are used in the template.

Example:

    my $vars = {
        article => 'The Third Shoe',
        person  => { 
            id    => 314, 
            name  => 'Mr. Blue',
            email => 'blue@nowhere.org',
        },
        primes  => [ 2, 3, 5, 7, 11, 13 ],
        wizard  => sub { return join(' ', 'Abracadabra!', @_) },
        cgi     => CGI->new('mode=submit&debug=1'),
    };

template:

    [% article %]

    [% person.id %]: [% person.name %] <[% person.email %]>

    [% primes.first %] - [% primes.last %], including [% primes.3 %]
    [% primes.size %] prime numbers: [% primes.join(', ') %]

    [% wizard %]
    [% wizard('Hocus Pocus!') %]

    [% cgi.param('mode') %]

output:

    The Third Shoe

    314: Mr. Blue <blue@nowhere.org>

    2 - 13, including 7
    6 prime numbers: 2, 3, 5, 7, 11, 13

    Abracadabra!
    Abracadabra! Hocus Pocus!

    submit

Regular scalar variables are accessed by simply specifying their name. As these are just entries in the top-level variable hash they can be considered special cases of hash array referencing as described below, with the main namespace hash automatically implied.

    [% article %]

Members of hash arrays are accessed by specifying the hash reference and key separated by the dot '.' operator.

    my $vars = {
        'home' => 'http://www.myserver.com/homepage.html',
        'page' => {
            'this' => 'mypage.html',
            'next' => 'nextpage.html',
            'prev' => 'prevpage.html',
        },
    };

template:

    <a href="[% home %]">Home</a>
    <a href="[% page.prev %]">Previous Page</a>
    <a href="[% page.next %]">Next Page</a>

output:

    <a href="http://www.myserver.com/homepage.html">Home</a>
    <a href="prevpage.html">Previous Page</a>
    <a href="nextpage.html">Next Page</a>

Any key in a hash which starts with a '_' or '.' character will be considered private and cannot be evaluated or updated from within a template. The undefined value will be returned for any such variable accessed which the Template Toolkit will silently ignore (unless the DEBUG option is enabled).

    my $vars = {
    message => 'Hello World!',
    _secret => "On the Internet, no-one knows you're a dog",
    thing   => {
         public  => 123,
        _private => 456,
       '.hidden' => 789,
    },
    };

template:

    [% message %]       # outputs "Hello World!"
    [% _secret %]               # no output
    [% thing.public %]          # outputs "123"
    [% thing._private %]        # no output
    [% thing..hidden %]         # ERROR: unexpected token (..)

To access a hash entry using a key stored in another variable, prefix the key variable with '$' to have it interpolated before use (see Variable Interpolation).

    [% pagename = 'next' %]
    [% page.$pagename %]       # same as [% page.next %]

When you assign to a variable that contains multiple namespace elements (i.e. it has one or more '.' characters in the name), any hashes required to represent intermediate namespaces will be created automatically. In this following example, the 'product' variable automatically springs into life as a hash array unless otherwise defined.

    [% product.id    = 'XYZ-2000' 
       product.desc  = 'Bogon Generator'
       product.price = 666 
    %]

    The [% product.id %] [% product.desc %] 
    costs $[% product.price %].00

output:

    The XYZ-2000 Bogon Generator 
    costs $666.00

You can use Perl's familiar '{' ... '}' construct to explicitly create a hash and assign it to a variable. Note that commas are optional between key/value pairs and '=' can be used in place of '=>'.

    [% product = {
     id    => 'XYZ-2000',
     desc  => 'Bogon Generator',
     price => 666,
       }
    %]

Items in lists are also accessed by use of the dot operator.

    my $vars = {
    'people' => [ 'Tom', 'Dick', 'Larry' ],
    };

template:

    [% people.0 %]          # Tom
    [% people.1 %]          # Dick
    [% people.2 %]          # Larry

The FOREACH directive can be used to iterate through items in a list.

    [% FOREACH person = people %]
    Hello [% person %]
    [% END %]

output:

    Hello Tom
    Hello Dick
    Hello Larry

Lists can be constructed in-situ using the regular anonymous list '[' ... ']' construct. Commas between items are optional.

    [% cols = [ 'red', 'green', 'blue' ] %]

    [% FOREACH c = cols %]
       ...

or:

    [% FOREACH c = [ 'red', 'green', 'blue' ] %]
       ...

You can also create simple numerical sequences using the familiar '..' operator:

    [% n = [ 1 .. 4 ] %]    # n is [ 1, 2, 3, 4 ]

    [% x = 4
       y = 8
       z = [x..y]           # z is [ 4, 5, 6, 7, 8 ]
    %]

Template variables can contain references to Perl subroutines. When the variable is used, the Template Toolkit will automatically call the subroutine, passing any additional arguments specified. The return value from the subroutine is used as the variable value and inserted into the document output.

    my $vars = {
    wizard  => sub { return join(' ', 'Abracadabra!', @_) },
    };

template:

    [% wizard %]            # Abracadabra!
    [% wizard('Hocus Pocus!') %]    # Abracadabra! Hocus Pocus!

Template variables can also contain references to Perl objects. Methods are called using the dot operator to specify the method against the object variable. Additional arguments can be specified as with subroutines.

    use CGI;

    ...

    my $vars = {
    # hard coded CGI params for purpose of example
    cgi  => CGI->new('mode=submit&debug=1'),
    };

template:

    [% FOREACH p = cgi.param %]     # returns list of param keys
    [% p %] => [% cgi.param(p) %]   # fetch each param value
    [% END %]

output:

    mode => submit
    debug => 1

Object methods can also be called as lvalues. That is, they can appear on the left side of an assignment. The method will be called passing the assigning value as an argument.

    [% myobj.method = 10 %]

equivalent to:

    [% myobj.method(1) %]

Subroutines and methods will be passed any arguments specified in the template. Any template variables in the argument list will first be evaluated and their resultant values passed to the code.

    my $vars = {
    mycode => sub { return 'received ' . join(', ', @_) },
    };

template:

    [% foo = 10 %]
    [% mycode(foo, 20) %]       # received 10, 20

Named parameters may also be specified. These are automatically collected into a single hash array which is passed by reference as the last parameter to the sub-routine. Named parameters can be specified using either '=>' or '=' and can appear anywhere in the argument list.

    my $vars = {
    myjoin => \&myjoin,
    };

    sub myjoin {
    # look for hash ref as last argument
    my $params = ref $_[-1] eq 'HASH' ? pop : { };
    return join($params->{ joint } || ' + ', @_);
    }

template:

    [% myjoin(10, 20, 30) %]
    [% myjoin(10, 20, 30, joint = ' - ' %]
    [% myjoin(joint => ' * ', 10, 20, 30 %]

output:

    10 + 20 + 30
    10 - 20 - 30
    10 * 20 * 30

Parenthesised parameters may be added to any element of a variable, not just those that are bound to code or object methods. At present, parameters will be ignored if the variable isn't callable but are supported for future extensions. Think of them as hints to that variable, rather than just arguments passed to a function.

    [% r = 'Romeo' %]
    [% r(100, 99, s, t, v) %]       # outputs "Romeo"

User code should return a value for the variable it represents. This can be any of the Perl data types described above: a scalar, or reference to a list, hash, subroutine or object. Where code returns a list of multiple values the items will automatically be folded into a list reference which can be accessed as per normal.

    my $vars = {
    # either is OK, first is recommended
    items1 => sub { return [ 'foo', 'bar', 'baz' ] },
    items2 => sub { return ( 'foo', 'bar', 'baz' ) },
    };

template:

    [% FOREACH i = items1 %]
       ...
    [% END %]

    [% FOREACH i = items2 %]
       ...
    [% END %]

Errors can be reported from user code by calling die(). Errors raised in this way are caught by the Template Toolkit and converted to structured exceptions which can be handled from within the template. A reference to the exception object is then available as the 'error' variable.

    my $vars = {
    barf => sub { 
        die "a sick error has occurred\n";
    },
    };

template:

    [% TRY %]
       [% barf %]       # calls sub which throws error via die()
    [% CATCH %]
       [% error.info %]     # outputs "a sick error has occurred\n"
    [% END %]

Error messages thrown via die() are converted to exceptions of type 'undef'. Exceptions of user-defined types can be thrown by calling die() with a reference to a Template::Exception object.

    use Template::Exception;

    ...

    my $vars = {
    login => sub { 
        ...
        die Template::Exception->new('badpwd',
                     'password too silly');
    },
    };

template:

    [% TRY %]
       [% login %]
    [% CATCH badpwd %]
       Bad password: [% error.info %]
    [% CATCH %]
       Some other '[% error.type %]' error: [% error.info %]
    [% END %]

The exception types 'stop' and 'return' are used to implement the STOP and RETURN directives. Throwing an exception as:

    die (Template::Exception->new('stop'));

has the same effect as the directive:

    [% STOP %]

Subroutines and methods can also raise errors by returning a list or reference to a list containing the undefined value (undef) followed by an exception object or error message. This is supported for backwards compatibility with version 1 but may be deprecated in some future version.

    my $vars = {
    # currently equivalent
    barf => sub {
        die "I'm sorry Dave, I can't do that";
    },
    yack => sub {
        return (undef, "I'm sorry Dave, I can't do that");
    },
    };

The Template Toolkit implements a number of virtual methods which can be applied to scalars, hashes or lists. For example:

    [% mylist = [ 'foo', 'bar', 'baz' ] %]
    [% newlist = mylist.sort %]

Here 'mylist' is a regular reference to a list, and 'sort' is a virtual method that returns a new list of the items in sorted order. You can chain multiple virtual methods together. For example:

    [% mylist.sort.join(', ') %]

Here the 'join' virtual method is called to join the sorted list into a single string, generating the following output:

    bar, baz, foo

See Template::Manual::VMethods for details of all the virtual methods available.

The Template Toolkit uses '$' consistently to indicate that a variable should be interpolated in position. Most frequently, you see this in double-quoted strings:

    [% fullname = "$honorific $firstname $surname" %]

Or embedded in plain text when the INTERPOLATE option is set:

    Dear $honorific $firstname $surname,

The same rules apply within directives. If a variable is prefixed with a '$' then it is replaced with its value before being used. The most common use is to retrieve an element from a hash where the key is stored in a variable.

    [% uid = 'abw' %]
    [% userlist.$uid %]         # same as 'userlist.abw'

Curly braces can be used to delimit interpolated variable names where necessary.

    [% userlist.${me.id}.name %]

Directives such as INCLUDE, PROCESS, etc., that accept a template name as the first argument, will automatically quote it for convenience.

    [% INCLUDE foo/bar.txt %]

equivalent to:

    [% INCLUDE "foo/bar.txt" %]

To INCLUDE a template whose name is stored in a variable, simply prefix the variable name with '$' to have it interpolated.

    [% myfile = 'header' %]
    [% INCLUDE $myfile %]

equivalent to:

    [% INCLUDE header %]

Note also that a variable containing a reference to a Template::Document object can also be processed in this way.

    my $vars = {
    header => Template::Document->new({ ... }),
    };

template:

    [% INCLUDE $header %]

Any simple variables that you create, or any changes you make to existing variables, will only persist while the template is being processed. The top-level variable hash is copied before processing begins and any changes to variables are made in this copy, leaving the original intact. The same thing happens when you INCLUDE another template. The current namespace hash is cloned to prevent any variable changes made in the included template from interfering with existing variables. The PROCESS option bypasses the localisation step altogether making it slightly faster, but requiring greater attention to the possibility of side effects caused by creating or changing any variables within the processed template.

    [% BLOCK change_name %]
       [% name = 'bar' %]
    [% END %]

    [% name = 'foo' %] 
    [% INCLUDE change_name %]
    [% name %]              # foo
    [% PROCESS change_name %]
    [% name %]              # bar

Dotted compound variables behave slightly differently because the localisation process is only skin deep. The current variable namespace hash is copied, but no attempt is made to perform a deep-copy of other structures within it (hashes, arrays, objects, etc). A variable referencing a hash, for example, will be copied to create a new reference but which points to the same hash. Thus, the general rule is that simple variables (undotted variables) are localised, but existing complex structures (dotted variables) are not.

    [% BLOCK all_change %]
       [% x = 20 %]         # changes copy
       [% y.z = 'zulu' %]       # changes original
    [% END %]

    [% x = 10
       y = { z => 'zebra' }
    %]
    [% INCLUDE all_change %]
    [% x %]             # still '10'
    [% y.z %]               # now 'zulu'

If you create a complex structure such as a hash or list reference within a local template context then it will cease to exist when the template is finished processing.

    [% BLOCK new_stuff %]
       [% # define a new 'y' hash array in local context
          y = { z => 'zulu' }
       %]
    [% END %]

    [% x = 10 %]
    [% INCLUDE new_stuff %]
    [% x %]             # outputs '10'
    [% y %]             # nothing, y is undefined

Similarly, if you update an element of a compound variable which doesn't already exists then a hash will be created automatically and deleted again at the end of the block.

    [% BLOCK new_stuff %]
       [% y.z = 'zulu' %]
    [% END %]

However, if the hash does already exist then you will modify the original with permanent effect. To avoid potential confusion, it is recommended that you don't update elements of complex variables from within blocks or templates included by another.

If you want to create or update truly global variables then you can use the 'global' namespace. This is a hash array automatically created in the top-level namespace which all templates, localised or otherwise see the same reference to. Changes made to variables within this hash are visible across all templates.

    [% global.version = 123 %]

In addition to variables that get resolved each time a template is processed, you can also define variables that get resolved just once when the template is compiled. This generally results in templates processing faster because there is less work to be done.

To define compile-time constants, specify a CONSTANTS hash as a constructor item as per VARIABLES. The CONSTANTS hash can contain any kind of complex, nested, or dynamic data structures, just like regular variables.

    my $tt = Template->new({
    CONSTANTS => {
        version => 3.14,
        release => 'skyrocket',
        col     => {
        back => '#ffffff',
        fore => '#000000',
        },
        myobj => My::Object->new(),
        mysub => sub { ... },
        joint => ', ',
    },
    });

Within a template, you access these variables using the 'constants' namespace prefix.

    Version [% constants.version %] ([% constants.release %])

    Background: [% constants.col.back %]

When the template is compiled, these variable references are replaced with the corresponding value. No further variable lookup is then required when the template is processed.

You can call subroutines, object methods, and even virtual methods on constant variables.

    [% constants.mysub(10, 20) %]
    [% constants.myobj(30, 40) %]
    [% constants.col.keys.sort.join(', ') %]

One important proviso is that any arguments you pass to subroutines or methods must also be literal values or compile time constants.

For example, these are both fine:

    # literal argument
    [% constants.col.keys.sort.join(', ') %]

    # constant argument
    [% constants.col.keys.sort.join(constants.joint) %]

But this next example will raise an error at parse time because 'joint' is a runtime variable and cannot be determined at compile time.

    # ERROR: runtime variable argument!
    [% constants.col.keys.sort.join(joint) %]

The CONSTANTS_NAMESPACE option can be used to provide a different namespace prefix for constant variables. For example:

    my $tt = Template->new({
    CONSTANTS => {
        version => 3.14,
        # ...etc...
    },
    CONSTANTS_NAMESPACE => 'const',
    });

Constants would then be referenced in templates as:

    [% const.version %]

A number of special variables are automatically defined by the Template Toolkit.

template

The 'template' variable contains a reference to the main template being processed, in the form of a Template::Document object. This variable is correctly defined within PRE_PROCESS, PROCESS and POST_PROCESS templates, allowing standard headers, footers, etc., to access metadata items from the main template. The 'name' and 'modtime' metadata items are automatically provided, giving the template name and modification time in seconds since the epoch. Note that the 'template' variable always references the top-level template, even when processing other template components via INCLUDE, PROCESS, etc.

component

The 'component' variable is like 'template' but always contains a reference to the current, innermost template component being processed. In the main template, the 'template' and 'component' variable will reference the same Template::Document object. In any other template component called from the main template, the 'template' variable will remain unchanged, but 'component' will contain a new reference to the current component. This example should demonstrate the difference:

    $template->process('foo')
    || die $template->error(), "\n";

'foo':

    [% template.name %]         # foo
    [% component.name %]        # foo
    [% PROCESS footer %]

'footer':

    [% template.name %]         # foo
    [% component.name %]        # footer

Additionally, the 'component' variable has two special fields: 'caller' and 'callers'. 'caller' contains the name of the template that called the current template (or undef if the values of 'template' and 'component' are the same). 'callers' contains a reference to a list of all the templates that have been called on the road to calling the current component template (like a call stack), with the outer-most template first. Here's an example: 'outer.tt2':

    [% component.name %]        # 'outer.tt2'
    [% component.caller %]      # undef
    [% component.callers %]     # undef
    [% PROCESS 'middle.tt2' %]

'middle.tt2':

    [% component.name %]        # 'middle.tt2'
    [% component.caller %]      # 'outer.tt2'
    [% component.callers %]     # [ 'outer.tt2' ]
    [% PROCESS 'inner.tt2' %]

'inner.tt2':

    [% component.name %]        # 'inner.tt2'
    [% component.caller %]      # 'middle.tt2'
    [% component.callers %]     # [ 'outer.tt2', 'middle.tt2' ]

loop

Within a FOREACH loop, the 'loop' variable references the Template::Iterator object responsible for controlling the loop.

    [% FOREACH item = [ 'foo', 'bar', 'baz' ] -%]
       [% "Items:\n" IF loop.first -%]
       [% loop.count %]/[% loop.size %]: [% item %]
    [% END %]

error

Within a CATCH block, the 'error' variable contains a reference to the Template::Exception object thrown from within the TRY block. The 'type' and 'info' methods can be called or the variable itself can be printed for automatic stringification into a message of the form $type error - CW$info. See Template::Exception for further details.

    [% TRY %]
       ...
    [% CATCH %]
       [% error %]
    [% END %]

content

The WRAPPER method captures the output from a template block and then includes a named template, passing the captured output as the 'content' variable.

    [% WRAPPER box %]
    Be not afeard; the isle is full of noises,
    Sounds and sweet airs, that give delight and hurt not.
    [% END %]

    [% BLOCK box %]
    <table border=1>
    <tr>
      <td>
      [% content %]
      </td>
    </tr>
    </table>
    [% END %]

Compound 'dotted' variables may contain any number of separate elements. Each element may evaluate to any of the permitted variable types and the processor will then correctly use this value to evaluate the rest of the variable. Arguments may be passed to any of the intermediate elements.

    [% myorg.people.sort('surname').first.fullname %]

Intermediate variables may be used and will behave entirely as expected.

    [% sorted = myorg.people.sort('surname') %]
    [% sorted.first.fullname %]

This simplified dotted notation has the benefit of hiding the implementation details of your data. For example, you could implement a data structure as a hash array one day and then change it to an object the next without requiring any change to the templates.

Andy Wardley <abw@andywardley.com>

<http://www.andywardley.com/|http://www.andywardley.com/>

Template Toolkit version 2.14, released on 04 October 2004.

  Copyright (C) 1996-2004 Andy Wardley.  All Rights Reserved.
  Copyright (C) 1998-2002 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.