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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 lan-
       guage 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 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.

       4.  Within the "import" subroutine do a call to "fil-
           ter_add", passing it either a subroutine reference.

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       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 "fil-
           ter_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 {

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

       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;

               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 <&lt;no>&gt; 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




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       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
       expression (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 'terminator'.

       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;


               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

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       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;

       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 trans-
       formation to the entire text of your source program. So
       something like:

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

       will become:

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               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 automat-
       ically 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 (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 POD 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
           quotelike (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
       component filters is called repeatedly, once for each

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       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
       simple macro-preprocessor that is only applied within
       regexes, with a final debugging pass that prints the
       resulting source code:

               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' component filter behaves
       slightly differently from the other partial filters
       described in the previous section.

       Rather than calling the specified processor on each indi-
       vidual piece of code (i.e. on the bits between quote-
       likes), the 'code' partial filter operates on the entire
       source code, but with the quotelike bits "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 con-
       tents of the $; variable at the time the filter is applied
       (normally "\034"). The remaining four bytes are a unique
       identifier 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 con-
       tains a pre-compiled regex that matches any placeholder.
       Placeholders can be moved and re-ordered within the source
       code as needed.

       Once the filtering has been applied, the original strings,
       regexes, POD, etc. are re-inserted into the code, by
       replacing each placeholder with the corresponding original

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

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               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" subrou-
       tine 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:

               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;

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               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 Fil-

       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 con-
       structed 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;

               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

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       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 (damianATconway.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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