NAME¶
Filter::Template - a source filter for inline code templates (macros)
VERSION¶
version 1.043
SYNOPSIS¶
use Filter::Template;
# use Filter::Template ( isa => 'SomeModule' );
template max (one,two) {
((one) > (two) ? (one) : (two))
}
print {% max $one, $two %}, "\n";
const PI 3.14159265359
print "PI\n"; # Constants are expanded inside strings.
print "HAPPINESS\n"; # Also expanded due to naive parser.
enum ZERO ONE TWO
enum 12 TWELVE THIRTEEN FOURTEEN
enum + FIFTEEN SIXTEEN SEVENTEEN
# Prints numbers, due to naive parser.
print "ZERO ONE TWO TWELVE THIRTEEN FOURTEEN FIFTEEN SIXTEEN SEVENTEEN\n";
if ($expression) { # include
... lines of code ...
} # include
unless ($expression) { # include
... lines of code ...
} elsif ($expression) { # include
... lines of code ...
} else { # include
... lines of code ...
} # include
DESCRIPTION¶
Filter::Template is a Perl source filter that provides simple inline source code
templates. Inlined source code can be significantly faster than subroutines,
especially for small-scale functions like accessors and mutators. On the other
hand, they are more difficult to maintain and use. Choose your trade-offs
wisely.
Templates¶
Code templates are defined with the "template" statement, which looks
a lot like "sub". Because this is a naive source filter, however,
the open brace must be on the same line as the "template" keyword.
Furthermore, the first closing brace in column zero ends a macro body.
template oops {
die "Oops";
}
Templates are inserted into a program using a simple syntax that was adapted
from other template libraries. It was chosen to be compatible with the Perl
syntax highlighting of common text editors.
This inserts the body of "template oops".
{% oops %}
Templates can have parameters. The syntax for template parameters was based on
prototypes for Perl subroutines. The two main differences are that parameters
are named, and sigils are not used.
template sum_2 (parameter_0, parameter_1) {
print( parameter_0 + parameter_1, "\n" );
}
To insert a template with parameters, simply list the parameters after the
template name.
{% sum_2 $base, $increment %}
At expansion time, occurrences of the parameter names within the template are
replaced with the source code provided in the template invocation. In the
previous example, "sum_2" literally expands to
print( $base + $increment, "\n" );
and is then compiled by Perl.
Constants and Enumerations¶
Filter::Template also defines "const" and "enum" keywords.
They are essentially simplified templates without parameters.
"const" defines a constant that is replaced before compile time.
Unlike Perl's native constants, these are not demoted to function calls when
Perl is run in debugging or profiling mode.
const CONSTANT_NAME 'constant value'
const ANOTHER_CONSTANT 23
Enumerations are like constants but several sequential integers can be defined
in one statement. Enumerations start from zero by default:
enum ZEROTH FIRST SECOND
If the first parameter of an enumeration is a number, then the enumerated
constants will start with that value:
enum 10 TENTH ELEVENTH TWELFTH
Enumerations may not span lines, but they can be continued. If the first
enumeration parameter is the plus sign, then constants will start where the
previous enumeration left off.
enum 13 THIRTEENTH FOURTEENTH FIFTEENTH
enum + SIXTEENTH SEVENTEENTH EIGHTEENTH
Conditional Code Inclusion (#ifdef)¶
The preprocessor supports something like cpp's #if/#else/#endif by usurping a
bit of Perl's conditional syntax. The following conditional statements will be
evaluated at compile time if they are followed by the comment "#
include":
if (EXPRESSION) { # include
BLOCK;
} elsif (EXPRESSION) { # include
BLOCK;
} else { # include
BLOCK;
} # include
unless (EXPRESSION) { # include
BLOCK;
} # include
The code in each conditional statement's BLOCK will be included or excluded in
the compiled code depending on the outcome of its EXPRESSION.
Conditional includes are nestable, but else and elsif must be on the same line
as the previous block's closing brace, as they are in the previous example.
Filter::Template::UseBytes uses conditional code to define different versions of
a {% use_bytes %} macro depending whether the "bytes" pragma exists.
IMPORTING TEMPLATES¶
Filter::Template can import templates defined by another class. For example,
this invocation imports the "use_bytes" template:
use Filter::Template ( isa => 'Filter::Template::UseBytes' );
Imported templates can be redefined in the current namespace.
Note: If the imported templates require additional Perl modules, any code which
imports them must also "use" those modules.
DEBUGGING¶
Filter::Template has three debugging constants which will only take effect if
they are defined before the module is first used.
To trace source filtering in general, and to see the resulting code and
operations performed on each line, define:
sub Filter::Template::DEBUG () { 1 }
To trace template invocations as they happen, define:
sub Filter::Template::DEBUG_INVOKE () { 1 }
To see template, constant, and enum definitions, define:
sub Filter::Template::DEBUG_DEFINE () { 1 }
To see warnings when a template or constant is redefined, define:
sub Filter::Template::DEFINE () { 1 }
CAVEATS¶
Source filters are line-based, and so is the template language. The only
constructs that may span lines are template definitions, and those
must
span lines.
Filter::Template does not parse perl. The regular expressions that detect and
replace code are simplistic and may not do the right things when parsing
challenging Perl syntax. Constants are replaced within strings, for example.
The regexp optimizer makes silly subexpressions like /(?:|m)/. That could be
done better as /m?/ or /(?:jklm)?/ if the literal is longer than a single
character.
The regexp optimizer does not optimize (?:x|y|z) as character classes.
The regexp optimizer is based on code in Ilya Zakharevich's Text::Trie. Better
regexp optimizers were released afterwards, and Filter::Template should use
one of them.
LINKS¶
BUG TRACKER¶
https://rt.cpan.org/Dist/Display.html?Status=Active&Queue=Filter-Template
REPOSITORY¶
http://github.com/rcaputo/filter-template http://gitorious.org/filter-template
OTHER RESOURCES¶
http://search.cpan.org/dist/Filter-Template/
SEE ALSO¶
Text::Trie, PAR, Filter::Template::UseBytes.
AUTHOR & COPYRIGHT¶
Filter::Template is Copyright 2000-2013 Rocco Caputo. Some parts are Copyright
2001 Matt Cashner. All rights reserved. Filter::Template is free software; you
may redistribute it and/or modify it under the same terms as Perl itself.
Filter::Template was previously known as POE::Preprocessor.