NAME
Template::Plex - (P)erl (L)exical and (EX)tendable Templating
SYNOPSIS
Write a template:
__DATA__
@{[
init {
use Time::HiRes qw<time>;
$title="Mr.";
}
]}
Dear $title Connery,
Ordered a $size pizza with $slices slices to share between @$people and
myself. That averages @{[$slices/(@$people+1)]} slices each.
Use a template:
use Template::Plex;
my $vars={
size=>"large",
slices=>8,
people=>[qw<Kim Sam Harry Sally>]
};
my $template= Template::Plex->load(\*DATA, $vars);
print $template->render;
#OUTPUT
Dear Mr. Connery,
Ordered a large pizza with 8 slices to share between Kim, Sam, Harry,
Sally and myself. That averages 1.6 slices each.
Change values and render it again:
$vars->{size}="extra large";
$vars->{slices}=12;
print $template->render;
#OUTPUT
Dear Mr. Connery,
Ordered a extra large pizza with 12 slices to share between Kim, Sam,
Harry, Sally and myself. That averages 2.4 slices each.
DESCRIPTION
"Template::Plex" facilitates the use of perl (not embedded perl) as a
template language. It implements bootstrapping and a system to load,
cache, inherit and render templates with minimal code.
The 'lexical' part of this module refers to the lexical aliasing of
input variables into the template. This reduces the amount of markup
required to reference variables and thus improves the style and
readability of a template.
Templates can be extended and reused by way of inclusion, sub templates
and inheritance. The template system itself can be extended by sub
classing "Template::Plex" and implementing customised load routines and
other helper methods.
The short tutorial in this document plus the examples included in the
distribution cover the basics to get you started. Reading through the
"load" API options is also recommended to get a better understanding on
how templates are processed.
MOTIATION
Many templating systems are available, yet none use perl as the template
language? Perl already has a great text interpolation, so why not use
it?
Lexical aliasing allows the input variables to be accessed directly by
name (i.e. $name) instead of as a member of a hash ref
(i.e."$fields->{name}") or by delimiting with custom syntax (i.e.
"<%=name%>")
I like the idea of Jekyll's 'Front Matter', but think its potential is
limited as it can only support variables and not code. With perl's
flexible syntax introducing code is doable.
TUTORIAL
Syntax Genesis
We all know how to interpolate variables into a string in perl:
"This string $uses a $some{variables}"
But how can we easily interpolate a statement, function or method call?
We can use the "@{[]}" construct.
"This is a perl string interpolating @{[ map uc, qw<a b c d>]}"
If we need multiple statements, we can combine with a "do" statement.
Like always the last statement executed in a "do" block is returned (
and interpolated into the string):
"This is a perl string interpolating
@{[ do {
my $result="STATEMENTS";
...
lc $result;
}
]}
"
Combining the above examples, we make a "Template::Plex" template simply
by removing the outer quoting operators:
This string $uses a $some{variables}
This is a perl string interpolating @{[ map uc, qw<a b c d>]}
This is a perl string interpolating
@{[ do {
my $result="STATEMENTS";
lc $result;
}
]}
A "Template::Plex" template is just perl! The above is the literal text
you can save to a file and load as a template.
Specifically, a "Template::Plex" template it is the subset of perl
that's valid between double quotation operators.
Smart Meta Data and Code
Templates can include an "init{}" block at the beginning which is
executed (only once) during the setup stage of the template. In some
ways this is similar to Jekyll 'Front Matter', but more powerful. You
can manipulate input variables, define helper subroutines, or import
modules:
@{[ init {
use Time::HiRes qw<time>;
sub my_func{ 1+2 };
}
]}
Calculated @{[my_func]} at @{[time]}
The "init" block does not inject any content into a template, but
manipulates the state of a template.
Each template has access to it's self, using the $self variable. This
comes in very handy when loading sub templates and doing more advanced
task or even extending the template system.
So far we have seen the "do" and "init" directives. General code can
also be executed with a "pl" block. This does the same as "do" but does
not inject the result into the template.
Loading and Rendering
There are a few ways of executing a template from your application. Each
of them are accessible via class methods:
#Load a template and render later
my $template=Template::Plex->load($path, $vars, %options);
my $result=$template->render;
#Load a template from cache and render later
my $template=Template::Plex->cache(undef, $path, $vars, %options);
my $result=$template->render;
#Load from cache and render now
my $result= Template::Plex->immediate(undef, $path,$vars,%options);
A "load" call returns a new template object each time, where a "cache"
call returns a template already loaded (or loads it once), for a user
defined key. The returned template is then rendered by calling the
"render" method.
The "immediate" call loads and caches a template with a user defined key
and then immediately renders it, returning the results.
An important option when loading templates is the root option. This is
the directory (relative to the working directory) which is prepended to
all paths. This makes it easy for templates to refer to other templates
with relative paths, regardless of the working directory.
Template Reuse
Reusing templates can reduce the repetitive nature of content.
"Template::Plex" provides multiple facilities for template reuse.
Sub Templates
A sub template is just another template. While you can load a sub
template with the class methods shown previously, it's not recommended.
This is because you would need to specify all the variables and options
again manually.
You normally would like to pass on the same variables and options to sub
templates, so a better way is to call the same method on the $self
object:
@{[$self->load(...)]}
@{[$self->cache(...)]}
@{[$self->immediate(...)]}
This will automatically link the variables and relevant option to be the
same as the current template.
Better still, these methods are made available within a template simply
as a subroutine call:
@{[load ... ]}
@{[cache ... ]}
@{[immediate ...]}
Slots and Inheritance
A sub template can be used at any location within a template. However
there are special locations called slots. These are defined with the
"slot" directive:
@{[slot slot1=>"some text"]}
@{[slot slot_abc=> cache "sub template"]}
@{[slot]}
The slot name can be any string and the value can either be text or a
template object. This value is the default value, which is used when no
child template wants to fill the slot.
A slot named 'default' (or no name) is special and is the location at
which a child template body will be rendered.
A child template can also fill other slots in the parent by explicitly
using the "fill_slot" directive. The value can be text or a loaded
template
@{[fill_slot name=>"override content"]}
@{[fill_slot another=>load "path to template"]}
Child can setup inheritance by using the "inherit" directive within a
"init" block, specifying the template to use as the parent:
@{[ init {
inherit "my_parent.plex";
}
]}
The following is an example showing a child template inheriting from a
parent. The child will provide content to the default slot in the parent
and also override the 'header' slot with another template which it
loads:
Parent Template:
@{[slot header=>"==HEADER=="]}
More parent content...
@{[slot]}
@{[slot footer=>"==FOOTER=="]}
Sub template (header):
-=-=-=Fancy header=-=-=-
Child template:
@{[ init {
inherit "parent.plex";
}
]}
@{[slot header=> load "header.plex";
This content will render into default slot
Inclusion
Much like the C language preprocessor, including an other template or
other file will do a literal copy of its contents into the calling
template. The resulting text is processed again and again as long as
more include statements are present:
@{[include("...")]}
This basically makes a single large template. As such the included
templates will use the same aliased variables.
In simple use cases, it is similar to loading a sub template. However it
lacks the flexibility of sub templates.
Logging and Error Handling
As templates are executed, they may throw an exception. If a syntax
error or file can not be read, an exception is also thrown during load.
In the case of a syntax error, "die" is called with a summary of
template, prefixed with line numbers which caused the error. Currently 5
line before and after the error are included for context generated by
Error::Show. Deliberately breaking the synopsis example (see examples
dir) gives the following error output:
perl -I lib examples/synopsis_syntax_error.pl
GLOB(0x7f8510025928)
1 {@{[
2 init {
3 use Time::HiRes qw<time>;
4 a+1
5=> $title="Mr.";
6 }
7 ]}
8 Dear $title Connery,
9 Ordered a $size pizza with $slices slices to share between @$people and
10 myself. That averages @{[$slices/(@$people+1)]} slices each.}
syntax error at GLOB(0x7f8510025928) 5 near "1
It is recommended to use a try/catch block to process the errors.
Currently Log::ger combined with "Log::OK" is utilised for logging and
debugging purposes. This comes in very handy when developing sub
classes.
Also note that line numbers reported in errors can be inaccurate when
the block_fix and include features are in use, as the content of the
source file is altered
Filters
Unlike other template system, there are no built in filters. However as
"Template::Plex" templates are just perl you are free to use builtin
string routines or import other modules in to your template.
API
"load"
#Class method. Used by top level applciation
Template::Plex->load($path, $vars, %opts);
#Object method. Used within a template
@{[$self->load($path, $vars, %opts);
#Subroutine. Prefered within a template.
@{[load $path, $vars, %opts]}
#Reuse existing $vars and %opts from withing a template
@{[load $path]}
A factory method, returning a new instance of a template, loaded from a
scalar, file path or an existing file handle.
From a top level user application, the class method must be used. From
within a template, either the object method form or subroutine form can
be used.
If now variables or options are specified when loading a sub templates,
the variables and options from the calling templates are reused.
Arguments to this function:
$path
This is a required argument.
If $path is a string, it is treated as a file path to a template
file. The file is opened and slurped with the content being used as
the template.
If $path is a filehandle, or GLOB ref, it is slurped with the
content being used as the template. Can be used to read template
stored in "__DATA__" for example
If $path is an array ref, the items of the array are joined into a
string, which is used directly as the template.
$vars
This is an optional argument but if present must be an empty hash
ref "{}" or "undef".
The top level items of the $vars hash are aliased into the template
using the key name (key names must be valid for a variable name for
this to operate). This allows an element such as "$fields{name"}> to
be directly accessible as $name in the template and sub templates.
External modification of the items in $vars will be visible in the
template. This is thee primary mechanism change inputs for
subsequent renders of the template.
In addition, the $vars itself is aliased to %fields variable (note
the %) and directly usable in the template like a normal hash e.g.
$fields{name}
If the $vars is an empty hash ref "{}" or "undef" then no variables
will be lexically aliased. The only variables accessible to the
template will be via the "render" method call.
%options
These are non required arguments, but must be key value pairs when
used.
Options are stored lexically for access in the template in the
variable %options. This variable is automatically used as the
options argument in recursive calls to "load" or "plx", if no
options are provided.
Currently supported options are:
root
"root" is a directory path, which if present, is prepended to to
the $path parameter if $path is a string (file path).
no_include
Disables the uses of the preprocessor include feature. The
template text will not be scanned and will prevent the "include"
feature from operating. See "include" for more details
This doesn't impact recursive calls to "load" when
dynamically/conditionally loading templates.
"no_init_fix"
Disables correcting missing init blocks.
If not specified or false, a template file is scanned for a
"@{[init{..}]}" directive. If one is found, the template is not
modified. Otherwise, a 'null' block is added at the beginning of
the template.
The added block is not effected by the enabling/disabling of
block fix mechanism.
no_block_fix
Disables removing of EOL after a "@{[]}"
eg
Line 1
@{[
""
]} <-- this NL removed by default
Line 3
In the above example, the default behaviour is to remove the
newline after the closing "]}". The rendered output would be:
Line1
Line3
If block fix was disabled (i.e. "no_block_fix" was true) the
output would be:
Line1
Line3
NOTE: in the case of a syntax error present in the template, the
line numbers maybe incorrect when block_fix is active, as it
effectivly removes a line from the source
no_eof_chomp
When this key is present and value is true, the last newline in
the template file is left in place.
Most text editors insert a extra newline as the last character
in a file. By default a chomp is performed before the template
is prepared to avoid extra newlines in the output when using sub
templates.
If you really need that newline you can specify the
"no_eof_chomp => 1" key or place an extra empty line at the end
of your template.
use_comments
From v0.5.0 Enables stripping of lines that start with perl
style comments from the template before preparation. This is
disabled by default
package
Specifies a package to run the template in. Any "our" variables
defined in the template will be in this package. If a package is
not specified, a unique package name is created to prevent name
collisions
base
Specifies the base class type of the template. If not specified,
templates are of type "Template::Plex". Sub classes must inherit
from this class.
Sub classes should force always specify this field.
no_alias
Top level elements in a $vars hash are aliased into the template
by default.
If this key is present and true, aliasing is disabled and all
variables need to be accessed via the %fields.
use An array ref of packages names (as strings) to use within a
template's package. Intended to be utilised by subclasses to add
features to a template.
inject
An array ref of strings, representing perl code, to be injected
into the template package. Intended to be utilised for
subclasses to inject small pieces of code which cannot be
otherwise required/used.
Return value
The return value is "Template::Plex" (or subclass) object which can
be rendered using the "render" method.
Example Usage my $hash={ name=>"bob", age=>98 };
my $template_dir="/path/to/dir";
my $obj=Template::Plex->load("template.plex", $hash, root=>$template_dir);
$obj->render;
=back
"cache"
#Class method
Template::Plex->cache($key, $path, $vars, %options);
#Object method
$self->cache($key, $path, $vars, %options);
#Subroutine
cache $key, $path, $vars, %options;
#Use the current line/package/template as a key
cache undef, $path, $vars, $%opts;
This is a wrapper around the "load" API, to improve performance of sub
templates used in loops. The first argument is a key to identity the
template when loaded. Subsequent calls with the same key will return the
already loaded template from active cache.
If called from the top level user application, the cache is shared.
Templates have their own cache storage to prevent cross collisions.
If no key is provided, then information about the caller (including the
line number, package and target template) is used generate one. This
approach allows for a template which maybe rendered multiple times in a
loop, to only be loaded once for example.
"immediate"
#Class method
Template::Plex->immediate($key, $path, $vars, %options);
#Object method
$self->immediate($key, $path, $vars, %options);
#Subrutine
immediate $key, $path, $vars, %options;
#Use current line/package/template as key
immediate undef, $path, $vars, %options;
Loads and renders a template immediately. Uses the same arguments as
"cache". Calls the "cache" API but also calls "render" on the returned
template.
Returns the result or the rendered template.
"include"
@{[include("path")}]
This is a special directive that replaces the directive with the literal
contents of the file pointed to by path in a similar style to #include
in the C preprocessor. This is a preprocessing step which happens before
the template is prepared for execution.
If "root" was included in the options to "load", then it is prepended to
"path" if defined.
When a template is loaded by "load" the processing of this is subject to
the "no_include" option. If "no_include" is specified, any template text
that contains the "@{[include("path")}]" text will result in a syntax
error
NOTE: in the case of a syntax error present in the template, the line
numbers maybe incorrect if "include" is used, as it effectivly adds
lines to the template source.
pl
block
@{[ block { ... } ]}
# or
@{[ pl { ... } ]}
A subroutine which executes a block just like the built in "do". However
it always returns an empty string.
Only usable in a template "@{[]}" construct, to execute arbitrary
statements. However, as an empty string is returned, perl's
interpolation won't inject anything at that point in the template.
If you DO want the last statement returned into the template, use the
built in "do".
eg
@{[
# This will assign a variable for use later in the template
# but WILL NOT inject the value 1 into template when rendered
pl {
$i=1;
}
]}
@{[
# This will assign a variable for use later in the tamplate
# AND immediately inject '1' into the template when rendered
do {
$i=1
}
]}
init
@{[ init {...} ]}
It is used to configure or setup meta data for a template and return
immediately. It takes a single argument which is a perl block.
Only the first "init {...}" block in a template will be executed.
A "init {...}" block is executed once, even when the template is
rendered multiple times
Before the block is run, the "pre_init" method is called. After the
block is run, the "post_init" method is called.
After the initialisation stages have run, a initialisation flag is set
and the remainder on the template is skipped with the "skip" method.
This means only the first "init" block in a template will be executed
pre_init
Do not call this directly. It is called internally by an init block.
Implemented as an empty method designed to be overridden in a subclass.
post_init
Do not call this directly. It is called internally by an init block.
Implemented as an empty method designed to be overridden in a subclass.
inherit
@{[ init {
inherit "Path to template";
}
]}
Specifies the template which will is the current template's parent. The
current template will be rendered into the default slot of the parent.
slot
@{[slot name=>$value]}
Declares a slot in a template which can be filled by a child template
calling "fill_slot" directives.
"name" is the name of the slot to render into the template. If not
specified, the slot is the default slot which will be rendered by the
content of a child template.
$value is optional and is the default content to render in the case a
child does not provide data for the slot. It can be a scalar value or a
template loaded by "load" or "cache"
fill_slot
@{[fill_slot name=>$value]}
Fills an inherited slot of name "name" with $value, replacing the
current contents.
The default slot cannot be specified. It is filled with the rendered
result of the child template.
append_slot
@{[append_slot name=>$value]}
Appends to an inherited slot of name "name" with $value.
prepend_slot
@{[prepend_slot name=>$value]}
Prepends to an inherited slot of name "name" with $value.
clear
clear;
Subject to change. Clears the cached templates
jmap
jmap {block} $delimiter, @array
Performs a join using $delimiter between each item in the @array after
they are processed through "block"
Very handy for rendering lists:
eg
<ul>
@{[jmap {"<li>$_</li>"} "\n", @items]}
</ul>
Note the lack of comma after the block.
"skip"
Causes the template to immediately finish, with an empty string as
result. From within a template, either the class method or template
directive can be used:
@{[$self->skip]}
@{[skip]}
"meta"
Returns the options hash used to load the template. From within a
template, it is recommended to use the %options hash instead:
@{[$self->meta->{file}]}
or
@{[$options{file}]}
This can also be used outside template text to inspect a templates meta
information
$template->meta;
"args"
Returns the argument hash used to load the template. From within a
template, it is recommended to use the aliased variables or the %fields
hash instead:
@{[$self->args->{my_arg}]}
or
@{[$fields{my_arg}]}
or
$my_arg
This can also be used outside template text to inspect a templates input
variables
$template->args;
parent
$self->parent;
Returns the parent template.
render
$template->render($fields);
This object method renders a template object created by "load" into a
string. It takes an optional argument $fields which is a reference to a
hash containing field variables. "fields" is aliased into the template
as %fields which is directly accessible in the template
eg
my $more_data={
name=>"John",
};
my $string=$template->render($more_data);
#Template:
My name is $fields{John}
Note that the lexically aliased variables setup in "load" are
independent to the %fields variable and can both be used simultaneously
in a template
SUB CLASSING
Sub classing is as per the standard perl "use parent". The object
storage is actually an array.
Package constants are defined for the indexes of the fields along with
"KEY_OFFSET" and "KEY_COUNT" to aid in adding extra fields in sub
classes.
If you intend on adding additional fields in your class you will need to
do the following as the object
use parent "Template::Plex";
use constant KEY_OFFSET=>Template::Plex::KEY_OFFSET+ Template::Plex::KEY_COUNT;
use enum ("first_field_=".KEYOFFSET, ..., last_field_);
use constant KEY_COUNT=>last_field_ - first_field_ +1;
Any further sub classing will need to repeat this using using your
package name.
FEATURE CHEAT SHEET
* Templates can contain a initialisation state
@{[
init {
# Nomral perl code here will only execute once
# when templates is loaded
}
]}
* Templates can cache at caller location
Sub/template is loaded only the first time in this map/loop
@{[map {immediate undef, "path_to_template",{}} qw< a b c d e >]}
And rendereds serveral times
* Lexical and package variables accessed/created within templates
@{[
init {
$input_var//=1; #set default
}
}]
Value is $input_var;
* Call and create subroutines within templates:
@{[
init {
sub my_great_calc {
my $input=shift;
$input*2/5;
}
}
}]
Result of calculation: @{[my_great_calc(12)]}
* 'Include' Templates within templates easily:
@{[include("path_to_file")]}
* Recursive sub template loading
@{[load "path_to_sub_template"]}
* Conditional rendering
@{[ $flag and $var]}
@{[ $flag?$var:""]}
@{[
pl {
if($flag){
#do stuff
}
}
]}
* Lists/Loops/maps
template interpolates @$lists directly
Items that are ok:
@{[
do {
#Standard for loop
my $output;
for(@$items){
$output.=$_."\n" if /ok/;
}
$output;
}
}]
More ok items:
@{[map {/ok/?"$_\n":()} @$items]}
* "use" other modules directly in templates:
@{[
init {
use Time::HiRes qw<time>
}
]}
Time of day right now: @{[time]}
TIPS ON USAGE
Potential Pitfalls
* Remeber to set $" locally to your requied seperator
The default is a space, however when generating HTML lists for
example, a would make it easier to read:
#Before executing template
local $"="\n";
load ...
Or alternatively use "jmap" to explicitly set the interpolation
separator each time
* Aliasing is a two way steet
Changes made to aliased variables external to the template are
available inside the template (one of the main tenets of this
module)
Changes make to aliased variables internal to the template are
available outside the template.
* Unbalanced Delimiter Pairs
Perl double quote operators are smart and work on balanced pairs of
delimiters. This allows for the delimiters to appear in the text
body without error.
However if your template doesn't have balanced pairs (i.e. a missing
"}" in javascript/c/perl/etc), the template will fail to compile and
give a strange error.
If you know you don't have balanced delimiters, then you can escape
them with a backslash
Currently Template::Plex delimiter pair used is { }. It isn't
changeable in this version.
* Are you sure it's one statement?
If you are having trouble with "@{[...]}", remember the result of
the last statement is returned into the template.
Example of single statements
@{[time]} #Calling a sub and injecting result
@{[$a,$b,$c,time,my_sub]} #injecting list
@{[our $temp=1]} #create a variable and inject
@{[our ($a,$b,$c)=(7,8,9)]} #declaring a
If you are declaring a package variable, you might not want its
value injected into the template at that point. So instead you could
use "block{..}" or "pl{..}" to execute multiple statements and not
inject the last statement:
@{[ pl {our $temp=1;} }];
More on Input Variables
If the variables to apply to the template completely change (note:
variables not values), then the aliasing setup during a "load" call will
not reflect what you want.
However the "render" method call allows a hash ref containing values to
be used. The hash is aliased to the %fields variable in the template.
my $new_variables={name=>data};
$template->render($new_variables);
However to use this data the template must be constructed to access the
fields directly:
my $template='my name is $fields{name} and I am $fields{age}';
Note that the %field is aliased so any changes to it is reflected
outside the template
Interestingly the template can refer to the lexical aliases and the
direct fields at the same time. The lexical aliases only refer to the
data provided at preparation time, while the %fields refer to the latest
data provided during a "render" call:
my $template='my name is $fields{name} and I am $age
my $base_data={name=>"jimbo", age=>10};
my $override_data={name=>"Eva"};
my $template=load $template, $base_data;
my $string=$template->render($override_data);
#string will be "my name is Eva and I am 10
As an example, this could be used to 'template a template' with global,
slow changing variables stored as the aliased variables, and the fast
changing, per render data being supplied as needed.
ISSUES
Enabling lexically scoped features (ie "use feature "say"") is only in
the block it used in. Unfortunately that means that features enabled in
an init block will not be active in subsequent blocks. The inject or use
option would need to be utilised to achieve this currently.
Templates are completely processed in memory. A template can execute sub
templates and run general IO code, so in theory it would be possible to
break up very large data templates and stream them to disk...
This module uses "eval" to generate the code for rendering. This means
that your template, being perl code, is being executed. If you do not
know what is in your templates, then maybe this module isn't for you.
Aliasing means that the template has write access to variables outside
of it. So again if you don't know what your templates are doing, then
maybe this module isn't for you
TODO
Extending the template system has been mentioned but not elaborated on.
Probably need to make an other tutorial document.
SEE ALSO
Yet another template module right?
Do a search on CPAN for 'template' and make a cup of coffee.
REPOSITORY and BUG REPORTING
Please report any bugs and feature requests on the repo page: GitHub
<http://github.com/drclaw1394/perl-template-plex>
AUTHOR
Ruben Westerberg, <drclaw@mac.com>
COPYRIGHT AND LICENSE
Copyright (C) 2023 by Ruben Westerberg
This library is free software; you can redistribute it and/or modify it
under the same terms as Perl itself, or under the MIT license