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Filter::Template(3) User Contributed Perl Documentation Filter::Template(3)

Filter::Template - a source filter for inline code templates (macros)

version 1.043 

        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

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.

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.

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

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.

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.

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 }

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.

https://rt.cpan.org/Dist/Display.html?Status=Active&Queue=Filter-Template

http://github.com/rcaputo/filter-template http://gitorious.org/filter-template

http://search.cpan.org/dist/Filter-Template/

Text::Trie, PAR, Filter::Template::UseBytes.

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.

2013-08-12 perl v5.32.1
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