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Jarkko Hietaniemi


Filter::Simple - Simplified source filtering


 # in MyFilter.pm:

         package MyFilter;

         use Filter::Simple sub { ... };

 # in user's code:

         use MyFilter;

         # this code is filtered

         no MyFilter;

         # this code is not


The Problem

Source filtering is an immensely powerful feature of recent versions of Perl. It allows one to extend the language itself (e.g. the Switch module), to simplify the language (e.g. Language::Pythonesque), or to completely recast the language (e.g. Lingua::Romana::Perligata). Effectively, it allows one to use the full power of Perl as its own, recursively applied, macro language.

The excellent Filter::Util::Call module (by Paul Marquess) provides a usable Perl interface to source filtering, but it is often too powerful and not nearly as simple as it could be.

To use the module it is necessary to do the following:

  1. Download, build, and install the Filter::Util::Call module. (If you are using Perl 5.7.1 or later, you already have Filter::Util::Call.)

  2. Set up a module that does a use Filter::Util::Call.

  3. Within that module, create an import subroutine.

  4. Within the import subroutine do a call to filter_add, passing it either a subroutine reference.

  5. Within the subroutine reference, call filter_read or filter_read_exact to "prime" $_ with source code data from the source file that will use your module. Check the status value returned to see if any source code was actually read in.

  6. Process the contents of $_ to change the source code in the desired manner.

  7. Return the status value.

  8. If the act of unimporting your module (via a no) should cause source code filtering to cease, create an unimport subroutine, and have it call filter_del. Make sure that the call to filter_read or filter_read_exact in step 5 will not accidentally read past the no. Effectively this limits source code filters to line-by-line operation, unless the import subroutine does some fancy pre-pre-parsing of the source code it's filtering.

For example, here is a minimal source code filter in a module named BANG.pm. It simply converts every occurrence of the sequence BANG\s+BANG to the sequence die 'BANG' if $BANG in any piece of code following a use BANG; statement (until the next no BANG; statement, if any):

        package BANG;

        use Filter::Util::Call ;

        sub import {
            filter_add( sub {
                my $caller = caller;
                my ($status, $no_seen, $data);
                while ($status = filter_read()) {
                        if (/^\s*no\s+$caller\s*;\s*$/) {
                        $data .= $_;
                        $_ = "";
                $_ = $data;
                s/BANG\s+BANG/die 'BANG' if \$BANG/g
                        unless $status < 0;
                $_ .= "no $class;\n" if $no_seen;
                return 1;

        sub unimport {

        1 ;

This level of sophistication puts filtering out of the reach of many programmers.

A Solution

The Filter::Simple module provides a simplified interface to Filter::Util::Call; one that is sufficient for most common cases.

Instead of the above process, with Filter::Simple the task of setting up a source code filter is reduced to:

  1. Set up a module that does a use Filter::Simple sub { ... }.

  2. Within the anonymous subroutine passed to use Filter::Simple, process the contents of $_ to change the source code in the desired manner.

In other words, the previous example, would become:

        package BANG;

        use Filter::Simple sub {
            s/BANG\s+BANG/die 'BANG' if \$BANG/g;

        1 ;

How it works

The Filter::Simple module exports into the package that uses it (e.g. package "BANG" in the above example) two automagically constructed subroutines -- import and unimport -- which take care of all the nasty details.

In addition, the generated import subroutine passes its own argument list to the filtering subroutine, so the BANG.pm filter could easily be made parametric:

        package BANG;

        use Filter::Simple sub {
            my ($die_msg, $var_name) = @_;
            s/BANG\s+BANG/die '$die_msg' if \${$var_name}/g;

        # and in some user code:

        use BANG "BOOM", "BAM;  # "BANG BANG" becomes: die 'BOOM' if $BAM

The specified filtering subroutine is called every time a use BANG is encountered, and passed all the source code following that call, up to either the next no BANG; call or the end of the source file (whichever occurs first). Currently, any no BANG; call must appear by itself on a separate line, or it is ignored.


Damian Conway (damian@conway.org)


 Copyright (c) 2000, Damian Conway. All Rights Reserved.
This module is free software; you can redistribute it and/or
modify it under the same terms as Perl itself.