MirBSD manpage: Filter::Simple(3p)

Filter::Simple(3pPerl Programmers Reference GuiFilter::Simple(3p)


     Filter::Simple - Simplified source filtering


      # in MyFilter.pm:

          package MyFilter;

          use Filter::Simple;

          FILTER { ... };

          # or just:
          # 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, recur-
     sively 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

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

     1.  Download, build, and install the Filter::Util::Call
         module. (If you have Perl 5.7.1 or later, this is
         already done for you.)

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

     3.  Within that module, create an "import" subroutine.

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     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;" state-
     ment (until the next "no BANG;" statement, if any):

         package BANG;

         use Filter::Util::Call ;

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         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

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

     1.  Download and install the Filter::Simple module. (If you
         have Perl 5.7.1 or later, this is already done for you.)

     2.  Set up a module that does a "use Filter::Simple" and
         then calls "FILTER { ... }".

     3.  Within the anonymous subroutine or block that is passed
         to "FILTER", 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;

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         FILTER {
             s/BANG\s+BANG/die 'BANG' if \$BANG/g;

         1 ;

     Note that the source code is passed as a single string, so
     any regex that uses "^" or "$" to detect line boundaries
     will need the "/m" flag.

     Disabling or changing <no> behaviour

     By default, the installed filter only filters up to a line
     consisting of one of the three standard source "termina-

         no ModuleName;  # optional comment





     but this can be altered by passing a second argument to "use
     Filter::Simple" or "FILTER" (just remember: there's no comma
     after the initial block when you use "FILTER").

     That second argument may be either a "qr"'d regular expres-
     sion (which is then used to match the terminator line), or a
     defined false value (which indicates that no terminator line
     should be looked for), or a reference to a hash (in which
     case the terminator is the value associated with the key

     For example, to cause the previous filter to filter only up
     to a line of the form:

         GNAB esu;

     you would write:

         package BANG;
         use Filter::Simple;

         FILTER {
             s/BANG\s+BANG/die 'BANG' if \$BANG/g;

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         FILTER {
             s/BANG\s+BANG/die 'BANG' if \$BANG/g;
         { terminator => qr/^\s*GNAB\s+esu\s*;\s*?$/ };

     and to prevent the filter's being turned off in any way:

         package BANG;
         use Filter::Simple;

         FILTER {
             s/BANG\s+BANG/die 'BANG' if \$BANG/g;
         "";    # or: 0


         FILTER {
             s/BANG\s+BANG/die 'BANG' if \$BANG/g;
         { terminator => "" };

     Note that, no matter what you set the terminator pattern to,
     the actual terminator itself must be contained on a single
     source line.

     All-in-one interface

     Separating the loading of Filter::Simple:

         use Filter::Simple;

     from the setting up of the filtering:

         FILTER { ... };

     is useful because it allows other code (typically parser
     support code or caching variables) to be defined before the
     filter is invoked. However, there is often no need for such
     a separation.

     In those cases, it is easier to just append the filtering
     subroutine and any terminator specification directly to the
     "use" statement that loads Filter::Simple, like so:

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

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     This is exactly the same as:

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

     except that the "FILTER" subroutine is not exported by

     Filtering only specific components of source code

     One of the problems with a filter like:

         use Filter::Simple;

         FILTER { s/BANG\s+BANG/die 'BANG' if \$BANG/g };

     is that it indiscriminately applies the specified transfor-
     mation to the entire text of your source program. So some-
     thing like:

         warn 'BANG BANG, YOU'RE DEAD';
         BANG BANG;

     will become:

         warn 'die 'BANG' if $BANG, YOU'RE DEAD';
         die 'BANG' if $BANG;

     It is very common when filtering source to only want to
     apply the filter to the non-character-string parts of the
     code, or alternatively to only the character strings.

     Filter::Simple supports this type of filtering by automati-
     cally exporting the "FILTER_ONLY" subroutine.

     "FILTER_ONLY" takes a sequence of specifiers that install
     separate (and possibly multiple) filters that act on only
     parts of the source code. For example:

         use Filter::Simple;

             code      => sub { s/BANG\s+BANG/die 'BANG' if \$BANG/g },
             quotelike => sub { s/BANG\s+BANG/CHITTY CHITTY/g };

     The "code" subroutine will only be used to filter parts of
     the source code that are not quotelikes, POD, or "__DATA__".
     The "quotelike" subroutine only filters Perl quotelikes

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     (including here documents).

     The full list of alternatives is:

         Filters only those sections of the source code that are
         not quotelikes, POD, or "__DATA__".

         Filters only those sections of the source code that are
         not quotelikes, POD, comments, or "__DATA__".

         Filters only those sections of the source code that are
         not POD or "__DATA__".

         Filters only those sections of the source code that are
         not POD, comments, or "__DATA__".

         Filters only Perl quotelikes (as interpreted by

         Filters only the string literal parts of a Perl quote-
         like (i.e. the contents of a string literal, either half
         of a "tr///", the second half of an "s///").

         Filters only the pattern literal parts of a Perl quote-
         like (i.e. the contents of a "qr//" or an "m//", the
         first half of an "s///").

         Filters everything. Identical in effect to "FILTER".

     Except for "FILTER_ONLY code => sub {...}", each of the com-
     ponent filters is called repeatedly, once for each component
     found in the source code.

     Note that you can also apply two or more of the same type of
     filter in a single "FILTER_ONLY". For example, here's a sim-
     ple macro-preprocessor that is only applied within regexes,
     with a final debugging pass that prints the resulting source

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         use Regexp::Common;
             regex => sub { s/!\[/[^/g },
             regex => sub { s/%d/$RE{num}{int}/g },
             regex => sub { s/%f/$RE{num}{real}/g },
             all   => sub { print if $::DEBUG };

     Filtering only the code parts of source code

     Most source code ceases to be grammatically correct when it
     is broken up into the pieces between string literals and
     regexes. So the 'code' and 'code_no_comments' component
     filter behave slightly differently from the other partial
     filters described in the previous section.

     Rather than calling the specified processor on each indivi-
     dual piece of code (i.e. on the bits between quotelikes),
     the 'code...' partial filters operate on the entire source
     code, but with the quotelike bits (and, in the case of
     'code_no_comments', the comments) "blanked out".

     That is, a 'code...' filter replaces each quoted string,
     quotelike, regex, POD, and __DATA__ section with a place-
     holder. The delimiters of this placeholder are the contents
     of the $; variable at the time the filter is applied (nor-
     mally "\034"). The remaining four bytes are a unique iden-
     tifier for the component being replaced.

     This approach makes it comparatively easy to write code
     preprocessors without worrying about the form or contents of
     strings, regexes, etc.

     For convenience, during a 'code...' filtering operation,
     Filter::Simple provides a package variable
     ($Filter::Simple::placeholder) that contains a pre-compiled
     regex that matches any placeholder...and captures the iden-
     tifier within the placeholder. Placeholders can be moved and
     re-ordered within the source code as needed.

     In addition, a second package variable
     (@Filter::Simple::components) contains a list of the various
     pieces of $_, as they were originally split up to allow
     placeholders to be inserted.

     Once the filtering has been applied, the original strings,
     regexes, POD, etc. are re-inserted into the code, by replac-
     ing each placeholder with the corresponding original com-
     ponent (from @components). Note that this means that the
     @components variable must be treated with extreme care
     within the filter. The @components array stores the "back-
     translations" of each placeholder inserted into $_, as well
     as the interstitial source code between placeholders. If the

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     placeholder backtranslations are altered in @components,
     they will be similarly changed when the placeholders are
     removed from $_ after the filter is complete.

     For example, the following filter detects concatentated
     pairs of strings/quotelikes and reverses the order in which
     they are concatenated:

         package DemoRevCat;
         use Filter::Simple;

         FILTER_ONLY code => sub {
             my $ph = $Filter::Simple::placeholder;
             s{ ($ph) \s* [.] \s* ($ph) }{ $2.$1 }gx

     Thus, the following code:

         use DemoRevCat;

         my $str = "abc" . q(def);

         print "$str\n";

     would become:

         my $str = q(def)."abc";

         print "$str\n";

     and hence print:


     Using Filter::Simple with an explicit "import" subroutine

     Filter::Simple generates a special "import" subroutine for
     your module (see "How it works") which would normally
     replace any "import" subroutine you might have explicitly

     However, Filter::Simple is smart enough to notice your
     existing "import" and Do The Right Thing with it. That is,
     if you explicitly define an "import" subroutine in a package
     that's using Filter::Simple, that "import" subroutine will
     still be invoked immediately after any filter you install.

     The only thing you have to remember is that the "import"
     subroutine must be declared before the filter is installed.
     If you use "FILTER" to install the filter:

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         package Filter::TurnItUpTo11;

         use Filter::Simple;

         FILTER { s/(\w+)/\U$1/ };

     that will almost never be a problem, but if you install a
     filtering subroutine by passing it directly to the "use
     Filter::Simple" statement:

         package Filter::TurnItUpTo11;

         use Filter::Simple sub{ s/(\w+)/\U$1/ };

     then you must make sure that your "import" subroutine
     appears before that "use" statement.

     Using Filter::Simple and Exporter together

     Likewise, Filter::Simple is also smart enough to Do The
     Right Thing if you use Exporter:

         package Switch;
         use base Exporter;
         use Filter::Simple;

         @EXPORT    = qw(switch case);
         @EXPORT_OK = qw(given  when);

         FILTER { $_ = magic_Perl_filter($_) }

     Immediately after the filter has been applied to the source,
     Filter::Simple will pass control to Exporter, so it can do
     its magic too.

     Of course, here too, Filter::Simple has to know you're using
     Exporter before it applies the filter. That's almost never a
     problem, but if you're nervous about it, you can guarantee
     that things will work correctly by ensuring that your "use
     base Exporter" always precedes your "use Filter::Simple".

     How it works

     The Filter::Simple module exports into the package that
     calls "FILTER" (or "use"s it directly) -- such as 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:

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         package BANG;

         use Filter::Simple;

         FILTER {
             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;" (or
     whatever terminator you've set) or the end of the source
     file, whichever occurs first. By default, any "no BANG;"
     call must appear by itself on a separate line, or it is


     Damian Conway (damian@conway.org)


         Copyright (c) 2000-2001, Damian Conway. All Rights Reserved.
         This module is free software. It may be used, redistributed
         and/or modified under the same terms as Perl itself.

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