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Parse::Template - Processor for templates containing Perl expressions |
Parse::Template - Processor for templates containing Perl expressions
use Parse::Template;
my %template =
(
'TOP' => q!Text before %%$self->eval('DATA')%% text after!,
'DATA' => q!Insert data: ! .
q!1. List: %%"@list$N"%%! .
q!2. Hash: %%"$hash{'key_value'}$N"%%! .
q!3. File content: %%print <FH>%%! .
q!4. Sub: %%&SUB()$N%%!
);
my $tmplt = new Parse::Template (%template); open FH, "< foo";
$tmplt->env('var' => '(value!)');
$tmplt->env('list' => [1, 2, 10],
'N' => "\n",
'FH' => \*FH,
'SUB' => sub { "->content generated by a sub<-" },
'hash' => { 'key_value' => q!It\'s an hash value! });
print $tmplt->eval('TOP'), "\n";
The Parse::Template class evaluates Perl expressions
placed within a text. This class can be used as a code generator,
or a generator of documents in various document formats (HTML, XML,
RTF, etc.).
The principle of template-based text generation is simple. A template
consists of a text which includes expressions to be evaluated.
Interpretation of these expressions generates text fragments which are
substituted in place of the expressions. In the case of
Parse::Template the expressions to be evaluated are placed within
two %%.
Evaluation takes place within an environment in which, for example, you can place data structures which will serve to generate the parts to be completed.
TEMPLATE
Text + Perl Expression
|
+-----> Evaluation ----> Text(document or program)
|
Subs + Data structures
ENVIRONMENT
The Parse::Template class permits decomposing a template into
parts. These parts are defined by a hash passed as an argument to the
class constructor:
Parse::Template->new('someKey', '... text with expressions to
evaluate ...'). Within a part, a sub-part can beincluded by means of
an expression of the form:
$self->eval('SUB_PART_NAME')
$self designates the instance of the Parse::Template class.
In an expression you can also use the $part which contains the
part of the template where the expression is found.
Within an expression it is possible to specify only the name of a part
to be inserted. In this case a subroutine with the name of this part
is generated dynamically. In the example given in the synopsis, the
insertion of the TOP part can thus be rewritten as follows:
'TOP' => q!Text before %%DATA()%% text after!
DATA() is placed within %% and is in effect treated as an
expression to be evaluated.
The subroutines take arguments. In the following example, the argument is used to control the depth of recursive calls of a template:
print Parse::Template->new(
'TOP' => q!%%$_[0] < 10 ? '[' . TOP($_[0] + 1) . ']' : ''%%!
)->eval('TOP', 0);
$_[0] initially contains 0. TOP is included as long as the
argument is less than 10. For each inclusion, 1 is added to the argument.
The env() method permits constructing the environment required for
evaluation of a template. Each entry to be defined within this
environment must be specified using a key consisting of the name of
the symbol to be created, associated with a reference whose type is
that of the entry to be created within this environment (for example,
a reference to an array to create an array). A scalar variable is
defined by associating the name of the variable with its value. A
scalar variable containing a reference is defined by writing
'var'=>\$variable, where $variable is a lexical variable
that contains the reference.
Each instance of Parse::Template is defined within a specific class,
a subclass of Parse::Template. The subclass contains the environment
specific to the template and inherits methods from the Parse::Template
class.
In case of a syntax error in the evalutaion of an expression,
Parse::Template tries to indicate the template part and the
expression that is ``incriminated''. If the variable
$Parse::Template::CONFESS contains the value TRUE, the stack
of evaluations is printed.
HASH is a hash which defines the
template text.
Example:
use Parse::Template;
$t = new Parse::Template('key' => 'associated text');
env(SYMBOL) returns the reference associated with the symbol, or
undef if the symbol is not defined. The reference that is returned
is of the type indicated by the character (&, $, %, @, *) that
prefixes the symbol.
Examples:
$tmplt->env('LIST' => [1, 2, 3])} Defines a list
@{$tmplt->env('*LIST')} Returns the list
@{$tmplt->env('@LIST')} Ditto
PART_NAME. Returns the
string resulting from this evaluation.
setPart() permits defining a new entry in the hash that defines the
contents of the template.
The Parse::Template class can be used in all sorts of amusing
ways. Here are a few illustrations.
The first example shows how to generate an HTML document by using a
data structure placed within the evaluation environment. The template
consists of two parts, DOC and SECTION. The SECTION part is
called within the DOC part to generate as many sections as there are
elements in the array section_content.
my %template = ('DOC' => <<'END_OF_DOC;', 'SECTION' => <<'END_OF_SECTION;');
<html>
<head></head>
<body>
%%
my $content;
for (my $i = 0; $i <= $#section_content; $i++) {
$content .= SECTION($i);
}
$content;
%%
</body>
</html>
END_OF_DOC;
%%
$section_content[$_[0]]->{Content} =~ s/^/<p>/mg;
join '', '<H1>', $section_content[$_[0]]->{Title}, '</H1>', $section_content[$_[0]]->{Content};
%%
END_OF_SECTION;
my $tmplt = new Parse::Template (%template);
$tmplt->env('section_content' => [
{
Title => 'First Section',
Content => 'Nothing to declare'
},
{
Title => 'Second section',
Content => 'Nothing else to declare'
}
]
);
print $tmplt->eval('DOC'), "\n";
The second example shows how to generate an HTML document using a functional notation, in other words, obtaining the text:
<P><B>text in bold</B><I>text in italic</I></P>
from
P(B("text in bold"), I("text in italic"))
The Perl expression that permits producing the content of an element is very simple, and reduces to:
join '', @_
The content to be evaluated is the same regardless of the tag and can therefore be placed within a variable. We therefore obtain the following template:
my $ELT_CONTENT = q!%%join '', @_%%!;
my $HTML_T1 = new Parse::Template(
'DOC' => '%%P(B("text in bold"), I("text in italic"))%%',
'P' => qq!<P>$ELT_CONTENT</P>!,
'B' => qq!<B>$ELT_CONTENT</B>!,
'I' => qq!<I>$ELT_CONTENT</I>!,
);
print $HTML_T1->eval('DOC'), "\n";
We can go further by making use of the $part variable, which
is defined by default in the environment of evaluation of the template:
$ELT_CONTENT = q!%%"<$part>" . join('', @_) . "</$part>"%%!;
$HTML_T2 = new Parse::Template(
'DOC' => '%%P(B("text in bold"), I("text in italic"))%%',
'P' => qq!$ELT_CONTENT!,
'B' => qq!$ELT_CONTENT!,
'I' => qq!$ELT_CONTENT!,
);
print $HTML_T2->eval('DOC'), "\n";
Let's look at another step which automates the production of expressions from the list of HTML tags which are of interest to us:
$DOC = q!P(B("text in bold"), I("text in italic"))!;
$ELT_CONTENT = q!%%"<$part>" . join('', @_) . "</$part>"%%!;
$HTML_T3 = new Parse::Template(
'DOC' => qq!%%$DOC%%!,
map { $_ => $ELT_CONTENT } qw(P B I)
);
print $HTML_T3->eval('DOC'), "\n";
With a slight transformation it is possible to use a method-invocation notation:
$ELT_CONTENT = q!%%shift(@_); "<$part>" . join('', @_) . "</$part>"%%!;
$HTML_T4 = new Parse::Template(
map { $_ => $ELT_CONTENT } qw(P B I)
);
print $HTML_T4->P(
$HTML_T4->B("text in bold"),
$HTML_T4->I("text in italic")
), "\n";
The shift(@_) permits getting rid of the template object, which
we don't need within the expression.
Parse::Template was initially created to serve as a code generator
for the Parse::Lex class. You will find other examples of its use
in the classes Parse::Lex, Parse::CLex and Parse::Token.
I would be very interested to receive your comments and suggestions. English documentation isn't up to date.
Instances are not destroyed. Therefore, do not use this class to create a large number of instances.
Philippe Verdret (with translation of documentation into English by Ocrat).
Copyright (c) 1995-2000 Philippe Verdret. All rights reserved. This module is free software; you can redistribute it and/or modify it under the same terms as Perl itself.
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Parse::Template - Processor for templates containing Perl expressions |