1 package Filter::Simple;
3 use vars qw{ $VERSION };
7 use Filter::Util::Call;
12 my ($class, $filter) = @_;
13 croak "Usage: use Filter::Simple sub {...}" unless ref $filter eq CODE;
14 *{"${caller}::import"} = gen_filter_import($caller, $filter);
15 *{"${caller}::unimport"} = \*filter_unimport;
18 sub gen_filter_import {
19 my ($class, $filter) = @_;
21 my ($imported_class, @args) = @_;
26 while ($status = filter_read()) {
27 if (m/^\s*no\s+$class\s*;\s*$/) {
35 $filter->(@args) unless $status < 0;
36 $_ .= "no $class;\n" if $off;
53 Filter::Simple - Simplified source filtering
61 use Filter::Simple sub { ... };
68 # this code is filtered
79 Source filtering is an immensely powerful feature of recent versions of Perl.
80 It allows one to extend the language itself (e.g. the Switch module), to
81 simplify the language (e.g. Language::Pythonesque), or to completely recast the
82 language (e.g. Lingua::Romana::Perligata). Effectively, it allows one to use
83 the full power of Perl as its own, recursively applied, macro language.
85 The excellent Filter::Util::Call module (by Paul Marquess) provides a
86 usable Perl interface to source filtering, but it is often too powerful
87 and not nearly as simple as it could be.
89 To use the module it is necessary to do the following:
95 Download, build, and install the Filter::Util::Call module.
99 Set up a module that does a C<use Filter::Util::Call>.
103 Within that module, create an C<import> subroutine.
107 Within the C<import> subroutine do a call to C<filter_add>, passing
108 it either a subroutine reference.
112 Within the subroutine reference, call C<filter_read> or C<filter_read_exact>
113 to "prime" $_ with source code data from the source file that will
114 C<use> your module. Check the status value returned to see if any
115 source code was actually read in.
119 Process the contents of $_ to change the source code in the desired manner.
123 Return the status value.
127 If the act of unimporting your module (via a C<no>) should cause source
128 code filtering to cease, create an C<unimport> subroutine, and have it call
129 C<filter_del>. Make sure that the call to C<filter_read> or
130 C<filter_read_exact> in step 5 will not accidentally read past the
131 C<no>. Effectively this limits source code filters to line-by-line
132 operation, unless the C<import> subroutine does some fancy
133 pre-pre-parsing of the source code it's filtering.
137 For example, here is a minimal source code filter in a module named
138 BANG.pm. It simply converts every occurrence of the sequence C<BANG\s+BANG>
139 to the sequence C<die 'BANG' if $BANG> in any piece of code following a
140 C<use BANG;> statement (until the next C<no BANG;> statement, if any):
144 use Filter::Util::Call ;
149 my ($status, $no_seen, $data);
150 while ($status = filter_read()) {
151 if (/^\s*no\s+$caller\s*;\s*$/) {
159 s/BANG\s+BANG/die 'BANG' if \$BANG/g
161 $_ .= "no $class;\n" if $no_seen;
172 This level of sophistication puts filtering out of the reach of
178 The Filter::Simple module provides a simplified interface to
179 Filter::Util::Call; one that is sufficient for most common cases.
181 Instead of the above process, with Filter::Simple the task of setting up
182 a source code filter is reduced to:
188 Set up a module that does a C<use Filter::Simple sub { ... }>.
192 Within the anonymous subroutine passed to C<use Filter::Simple>, process the
193 contents of $_ to change the source code in the desired manner.
197 In other words, the previous example, would become:
201 use Filter::Simple sub {
202 s/BANG\s+BANG/die 'BANG' if \$BANG/g;
210 The Filter::Simple module exports into the package that C<use>s it (e.g.
211 package "BANG" in the above example) two automagically constructed
212 subroutines -- C<import> and C<unimport> -- which take care of all the
215 In addition, the generated C<import> subroutine passes its own argument
216 list to the filtering subroutine, so the BANG.pm filter could easily
221 use Filter::Simple sub {
222 my ($die_msg, $var_name) = @_;
223 s/BANG\s+BANG/die '$die_msg' if \${$var_name}/g;
226 # and in some user code:
228 use BANG "BOOM", "BAM; # "BANG BANG" becomes: die 'BOOM' if $BAM
231 The specified filtering subroutine is called every time a C<use BANG>
232 is encountered, and passed all the source code following that call,
233 up to either the next C<no BANG;> call or the end of the source file
234 (whichever occurs first). Currently, any C<no BANG;> call must appear
235 by itself on a separate line, or it is ignored.
240 Damian Conway (damian@conway.org)
244 Copyright (c) 2000, Damian Conway. All Rights Reserved.
245 This module is free software; you can redistribute it and/or
246 modify it under the same terms as Perl itself.