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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
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;
};
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 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
CONTACT
Filter::Simple is now maintained by the Perl5-Porters. Please submit bug via the
"perlbug" tool that comes with your perl. For usage instructions, read "perldoc perlbug"
or possibly "man perlbug". For mostly anything else, please contact
<perl5-porters@perl.org>.
Maintainer of the CPAN release is Steffen Mueller <smueller@cpan.org>. Contact him with
technical difficulties with respect to the packaging of the CPAN module.
Praise of the module, flowers, and presents still go to the author, Damian Conway
<damian@conway.org>.
COPYRIGHT AND LICENSE
Copyright (c) 2000-2014, 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.