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Variables.pod
#============================================================= -*-perl-*- # # Template::Manual::Variables # # 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::Variables - Template variables and code bindings =head1 Template Variables A reference to a hash array may be passed as the second argument to the L<process()|Template#process()> method, containing definitions of template variables. The C<VARIABLES> (a.k.a. C<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); F<myfile> template: This is version [% version %] ([% release %]). Serial number: [% serial_no %] Generated 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 I<is> significant however, and 'C<foo>', 'C<Foo>' and 'C<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 data: 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'), }; Example 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') %] Generated 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 =head2 Scalar Values 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 %] =head2 Hash Array References Members of hash arrays are accessed by specifying the hash reference and key separated by the dot 'C<.>' operator. Example data: my $vars = { 'home' => 'http://www.myserver.com/homepage.html', 'page' => { 'this' => 'mypage.html', 'next' => 'nextpage.html', 'prev' => 'prevpage.html', }, }; Example template: <a href="[% home %]">Home</a> <a href="[% page.prev %]">Previous Page</a> <a href="[% page.next %]">Next Page</a> Generated 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 'C<_>' or 'C<.>' 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 C<DEBUG> option is enabled). Example data: my $vars = { message => 'Hello World!', _secret => "On the Internet, no-one knows you're a dog", thing => { public => 123, _private => 456, '.hidden' => 789, }, }; Example template: [% message %] # outputs "Hello World!" [% _secret %] # no output [% thing.public %] # outputs "123" [% thing._private %] # no output [% thing..hidden %] # ERROR: unexpected token (..) You can disable this feature by setting the C<$Template::Stash::PRIVATE> package variable to a false value. $Template::Stash::PRIVATE = undef; # now you can thing._private To access a hash entry using a key stored in another variable, prefix the key variable with 'C<$>' to have it interpolated before use (see L<Variable Interpolation>). [% pagename = 'next' %] [% page.$pagename %] # same as [% page.next %] You can also access hash entry using C<item()> method. This might be helpful if you have complex key name. <pre> [% files.item('example.txt').content %] </pre> See L<Template::Manual::VMethods|Template::Manual::VMethods/"item"> for more info. When you assign to a variable that contains multiple namespace elements (i.e. it has one or more 'C<.>' characters in the name), any hashes required to represent intermediate namespaces will be created automatically. In this following example, the C<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 Generated output: The XYZ-2000 Bogon Generator costs $666.00 You can use Perl's familiar C<{> ... C<}> construct to explicitly create a hash and assign it to a variable. Note that commas are optional between key/value pairs and C<=> can be used in place of C<=E<gt>>. # minimal TT style [% product = { id = 'XYZ-2000' desc = 'Bogon Generator' price = 666 } %] # perl style [% product = { id => 'XYZ-2000', desc => 'Bogon Generator', price => 666, } %] =head2 List References Items in lists are also accessed by use of the dot operator. Example data: my $vars = { people => [ 'Tom', 'Dick', 'Larry' ], }; Example template: [% people.0 %] # Tom [% people.1 %] # Dick [% people.2 %] # Larry The C<FOREACH> directive can be used to iterate through items in a list. [% FOREACH person IN people %] Hello [% person %] [% END %] Generated output: Hello Tom Hello Dick Hello Larry Lists can be constructed in-situ using the regular anonymous list C<[> ... C<]> construct. Commas between items are optional. [% cols = [ 'red', 'green', 'blue' ] %] [% FOREACH c IN cols %] [% c %] [% END %] or: [% FOREACH c IN [ 'red', 'green', 'blue' ] %] [% c %] [% END %] You can also create simple numerical sequences using the C<..> range operator: [% n = [ 1 .. 4 ] %] # n is [ 1, 2, 3, 4 ] [% x = 4 y = 8 z = [x..y] # z is [ 4, 5, 6, 7, 8 ] %] =head2 Subroutines 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!', @_) }, }; Example template: [% wizard %] # Abracadabra! [% wizard('Hocus Pocus!') %] # Abracadabra! Hocus Pocus! =head2 Objects 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'), }; Example template: [% FOREACH p IN cgi.param %] # returns list of param keys [% p %] => [% cgi.param(p) %] # fetch each param value [% END %] Generated 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(10) %] =head2 Passing Parameters and Returning Values 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 B<last> parameter to the sub-routine. Named parameters can be specified using either C<=E<gt>> or C<=> 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 } || ' + ', @_); } Example template: [% myjoin(10, 20, 30) %] [% myjoin(10, 20, 30, joint = ' - ' %] [% myjoin(joint => ' * ', 10, 20, 30 %] Generated 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' ) }, }; Example template: [% FOREACH i IN items1 %] ... [% END %] [% FOREACH i IN items2 %] ... [% END %] =head2 Error Handling Errors can be reported from user code by calling C<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 C<error> variable. my $vars = { barf => sub { die "a sick error has occurred\n"; }, }; Example 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 C<die()> are converted to exceptions of type C<undef> (the literal string "undef" rather than the undefined value). Exceptions of user-defined types can be thrown by calling C<die()> with a reference to a L<Template::Exception> object. use Template::Exception; my $vars = { login => sub { ...do something... die Template::Exception->new( badpwd => 'password too silly' ); }, }; Example template: [% TRY %] [% login %] [% CATCH badpwd %] Bad password: [% error.info %] [% CATCH %] Some other '[% error.type %]' error: [% error.info %] [% END %] The exception types C<stop> and C<return> are used to implement the C<STOP> and C<RETURN> directives. Throwing an exception as: die (Template::Exception->new('stop')); has the same effect as the directive: [% STOP %] =head1 Virtual Methods The Template Toolkit implements a number of "virtual methods" which can be applied to scalars, hashes or lists. For example: [% mylist = [ 'foo', 'bar', 'baz' ] %] [% newlist = mylist.sort %] Here C<mylist> is a regular reference to a list, and 'sort' is a virtual method that returns a new list of the items in sorted order. You can chain multiple virtual methods together. For example: [% mylist.sort.join(', ') %] Here the C<join> virtual method is called to join the sorted list into a single string, generating the following output: bar, baz, foo See L<Template::Manual::VMethods> for details of all the virtual methods available. =head1 Variable Interpolation The Template Toolkit uses C<$> 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 C<INTERPOLATE> option is set: Dear $honorific $firstname $surname, The same rules apply within directives. If a variable is prefixed with a C<$> 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' %] [% users.$uid %] # same as 'users.abw' Curly braces can be used to delimit interpolated variable names where necessary. [% users.${me.id}.name %] Directives such as C<INCLUDE>, C<PROCESS>, etc., that accept a template name as the first argument, will automatically quote it for convenience. [% INCLUDE foo/bar.txt %] The above example is equivalent to: [% INCLUDE "foo/bar.txt" %] To C<INCLUDE> a template whose name is stored in a variable, simply prefix the variable name with C<$> to have it interpolated. [% myfile = 'header' %] [% INCLUDE $myfile %] This is equivalent to: [% INCLUDE header %] Note also that a variable containing a reference to a L<Template::Document> object can also be processed in this way. my $vars = { header => Template::Document->new({ ... }), }; Example template: [% INCLUDE $header %] =head1 Local and Global Variables 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 C<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 C<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 I<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 I<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 %] =head1 Compile Time Constant Folding 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 C<CONSTANTS> hash as a constructor item as per C<VARIABLES>. The C<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 C<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 C<joint> is a runtime variable and cannot be determined at compile time. # ERROR: runtime variable argument! [% constants.col.keys.sort.join(joint) %] The C<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 %] =head1 Special Variables A number of special variables are automatically defined by the Template Toolkit. =head2 template The C<template> variable contains a reference to the main template being processed, in the form of a L<Template::Document> object. This variable is correctly defined within C<PRE_PROCESS>, C<PROCESS> and C<POST_PROCESS> templates, allowing standard headers, footers, etc., to access metadata items from the main template. The C<name> and C<modtime> metadata items are automatically provided, giving the template name and modification time in seconds since the epoch. Note that the C<template> variable always references the top-level template, even when processing other template components via C<INCLUDE>, C<PROCESS>, etc. =head2 component The C<component> variable is like C<template> but always contains a reference to the current, innermost template component being processed. In the main template, the C<template> and C<component> variable will reference the same L<Template::Document> object. In any other template component called from the main template, the C<template> variable will remain unchanged, but C<component> will contain a new reference to the current component. This example should demonstrate the difference: $template->process('foo') || die $template->error(), "\n"; F<foo> template: [% template.name %] # foo [% component.name %] # foo [% PROCESS footer %] F<footer> template: [% template.name %] # foo [% component.name %] # footer Additionally, the C<component> variable has two special fields: C<caller> and C<callers>. C<caller> contains the name of the template that called the current template (or undef if the values of C<template> and C<component> are the same). C<callers> contains a reference to a list of all the templates that have been called on the road to calling the current component template (like a call stack), with the outer-most template first. Here's an example: F<outer.tt2> template: [% component.name %] # 'outer.tt2' [% component.caller %] # undef [% component.callers %] # undef [% PROCESS 'middle.tt2' %] F<middle.tt2> template: [% component.name %] # 'middle.tt2' [% component.caller %] # 'outer.tt2' [% component.callers %] # [ 'outer.tt2' ] [% PROCESS 'inner.tt2' %] F<inner.tt2> template: [% component.name %] # 'inner.tt2' [% component.caller %] # 'middle.tt2' [% component.callers %] # [ 'outer.tt2', 'middle.tt2' ] =head2 loop Within a C<FOREACH> loop, the C<loop> variable references the L<Template::Iterator> object responsible for controlling the loop. [% FOREACH item = [ 'foo', 'bar', 'baz' ] -%] [% "Items:\n" IF loop.first -%] [% loop.count %]/[% loop.size %]: [% item %] [% END %] =head2 error Within a C<CATCH> block, the C<error> variable contains a reference to the L<Template::Exception> object thrown from within the C<TRY> block. The C<type> and C<info> methods can be called or the variable itself can be printed for automatic stringification into a message of the form "C<$type error - $info>". See L<Template::Exception> for further details. [% TRY %] ... [% CATCH %] [% error %] [% END %] =head2 content The C<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 %] <blockquote class="prose"> [% content %] </blockquote> [% END %] =head1 Compound Variables 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. =cut # Local Variables: # mode: perl # perl-indent-level: 4 # indent-tabs-mode: nil # End: # # vim: expandtab shiftwidth=4:
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