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Filter::Simple(3pm)                   Perl Programmers Reference Guide                   Filter::Simple(3pm)



NAME
       Filter::Simple - Simplified source filtering

SYNOPSIS
        # 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

DESCRIPTION
       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 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 "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+$caller\s*;\s*?$/) {
                       $no_seen=1;
                       last;
                   }
                   $data .= $_;
                   $_ = "";
               }
               $_ = $data;
               s/BANG\s+BANG/die 'BANG' if \$BANG/g
                   unless $status < 0;
               $_ .= "no $class;\n" if $no_seen;
               return 1;
               })
           }

           sub unimport {
               filter_del();
           }

           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 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;

           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 "terminators":

           no ModuleName;  # optional comment

       or:

           __END__

       or:

           __DATA__

       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;
           }
           qr/^\s*GNAB\s+esu\s*;\s*?$/;

       or:

           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

       or:

           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 ms 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;
           };

       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 Filter::Simple.

       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 transformation to the entire text of your source
       program. So something 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 automatically 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;

           FILTER_ONLY
               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:

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

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

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

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

       "quotelike"
           Filters only Perl quotelikes (as interpreted by &Text::Balanced::extract_quotelike).

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

       "regex"
           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///").

       "all"
           Filters everything. Identical in effect to "FILTER".

       Except for "FILTER_ONLY code => sub {...}", each of the component 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 simple macro-preprocessor that is only applied within regexes, with a final
       debugging pass that prints the resulting source code:

           use Regexp::Common;
           FILTER_ONLY
               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 individual 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 placeholder. The delimiters of this placeholder are the contents 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 contains a pre-compiled regex that matches any placeholder...and
       captures the identifier 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 replacing each placeholder with the corresponding original component (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 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 concatenated 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:

           defabc

       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 declared.

       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:

           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 -such
       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:

           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 ignored.

AUTHOR
       Damian Conway ([email protected])

COPYRIGHT
           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.



perl v5.10.0                                     2007-12-18                              Filter::Simple(3pm)