filter::simple.3

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.\" ========================================================================
.\"
.IX Title "Filter::Simple 3"
.TH Filter::Simple 3 "2007-12-18" "perl v5.10.0" "Perl Programmers Reference Guide"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
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.SH "NAME"
Filter::Simple \- Simplified source filtering
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\& # in MyFilter.pm:
\&
\&     package MyFilter;
\&
\&     use Filter::Simple;
\&     
\&     FILTER { ... };
\&
\&     # or just:
\&     #
\&     # use Filter::Simple sub { ... };
\&
\& # in user\*(Aqs code:
\&
\&     use MyFilter;
\&
\&     # this code is filtered
\&
\&     no MyFilter;
\&
\&     # this code is not
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
.Sh "The Problem"
.IX Subsection "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.
.PP
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.
.PP
To use the module it is necessary to do the following:
.IP "1." 4
Download, build, and install the Filter::Util::Call module.
(If you have Perl 5.7.1 or later, this is already done for you.)
.IP "2." 4
Set up a module that does a \f(CW\*(C`use Filter::Util::Call\*(C'\fR.
.IP "3." 4
Within that module, create an \f(CW\*(C`import\*(C'\fR subroutine.
.IP "4." 4
Within the \f(CW\*(C`import\*(C'\fR subroutine do a call to \f(CW\*(C`filter_add\*(C'\fR, passing
it either a subroutine reference.
.IP "5." 4
Within the subroutine reference, call \f(CW\*(C`filter_read\*(C'\fR or \f(CW\*(C`filter_read_exact\*(C'\fR
to \*(L"prime\*(R" \f(CW$_\fR with source code data from the source file that will
\&\f(CW\*(C`use\*(C'\fR your module. Check the status value returned to see if any
source code was actually read in.
.IP "6." 4
Process the contents of \f(CW$_\fR to change the source code in the desired manner.
.IP "7." 4
Return the status value.
.IP "8." 4
If the act of unimporting your module (via a \f(CW\*(C`no\*(C'\fR) should cause source
code filtering to cease, create an \f(CW\*(C`unimport\*(C'\fR subroutine, and have it call
\&\f(CW\*(C`filter_del\*(C'\fR. Make sure that the call to \f(CW\*(C`filter_read\*(C'\fR or
\&\f(CW\*(C`filter_read_exact\*(C'\fR in step 5 will not accidentally read past the
\&\f(CW\*(C`no\*(C'\fR. Effectively this limits source code filters to line-by-line
operation, unless the \f(CW\*(C`import\*(C'\fR subroutine does some fancy
pre-pre-parsing of the source code it's filtering.
.PP
For example, here is a minimal source code filter in a module named
\&\s-1BANG\s0.pm. It simply converts every occurrence of the sequence \f(CW\*(C`BANG\es+BANG\*(C'\fR
to the sequence \f(CW\*(C`die \*(AqBANG\*(Aq if $BANG\*(C'\fR in any piece of code following a
\&\f(CW\*(C`use BANG;\*(C'\fR statement (until the next \f(CW\*(C`no BANG;\*(C'\fR statement, if any):
.PP
.Vb 1
\&    package BANG;
\& 
\&    use Filter::Util::Call ;
\&
\&    sub import {
\&        filter_add( sub {
\&        my $caller = caller;
\&        my ($status, $no_seen, $data);
\&        while ($status = filter_read()) {
\&            if (/^\es*no\es+$caller\es*;\es*?$/) {
\&                $no_seen=1;
\&                last;
\&            }
\&            $data .= $_;
\&            $_ = "";
\&        }
\&        $_ = $data;
\&        s/BANG\es+BANG/die \*(AqBANG\*(Aq if \e$BANG/g
\&            unless $status < 0;
\&        $_ .= "no $class;\en" if $no_seen;
\&        return 1;
\&        })
\&    }
\&
\&    sub unimport {
\&        filter_del();
\&    }
\&
\&    1 ;
.Ve
.PP
This level of sophistication puts filtering out of the reach of
many programmers.
.Sh "A Solution"
.IX Subsection "A Solution"
The Filter::Simple module provides a simplified interface to
Filter::Util::Call; one that is sufficient for most common cases.
.PP
Instead of the above process, with Filter::Simple the task of setting up
a source code filter is reduced to:
.IP "1." 4
Download and install the Filter::Simple module.
(If you have Perl 5.7.1 or later, this is already done for you.)
.IP "2." 4
Set up a module that does a \f(CW\*(C`use Filter::Simple\*(C'\fR and then
calls \f(CW\*(C`FILTER { ... }\*(C'\fR.
.IP "3." 4
Within the anonymous subroutine or block that is passed to
\&\f(CW\*(C`FILTER\*(C'\fR, process the contents of \f(CW$_\fR to change the source code in
the desired manner.
.PP
In other words, the previous example, would become:
.PP
.Vb 2
\&    package BANG;
\&    use Filter::Simple;
\&    
\&    FILTER {
\&        s/BANG\es+BANG/die \*(AqBANG\*(Aq if \e$BANG/g;
\&    };
\&
\&    1 ;
.Ve
.PP
Note that the source code is passed as a single string, so any regex that
uses \f(CW\*(C`^\*(C'\fR or \f(CW\*(C`$\*(C'\fR to detect line boundaries will need the \f(CW\*(C`/m\*(C'\fR flag.
.Sh "Disabling or changing <no> behaviour"
.IX Subsection "Disabling or changing <no> behaviour"
By default, the installed filter only filters up to a line consisting of one of
the three standard source \*(L"terminators\*(R":
.PP
.Vb 1
\&    no ModuleName;  # optional comment
.Ve
.PP
or:
.PP
.Vb 1
\&    _\|_END_\|_
.Ve
.PP
or:
.PP
.Vb 1
\&    _\|_DATA_\|_
.Ve
.PP
but this can be altered by passing a second argument to \f(CW\*(C`use Filter::Simple\*(C'\fR
or \f(CW\*(C`FILTER\*(C'\fR (just remember: there's \fIno\fR comma after the initial block when
you use \f(CW\*(C`FILTER\*(C'\fR).
.PP
That second argument may be either a \f(CW\*(C`qr\*(C'\fR'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
\&\f(CW\*(Aqterminator\*(Aq\fR.
.PP
For example, to cause the previous filter to filter only up to a line of the
form:
.PP
.Vb 1
\&    GNAB esu;
.Ve
.PP
you would write:
.PP
.Vb 2
\&    package BANG;
\&    use Filter::Simple;
\&    
\&    FILTER {
\&        s/BANG\es+BANG/die \*(AqBANG\*(Aq if \e$BANG/g;
\&    }
\&    qr/^\es*GNAB\es+esu\es*;\es*?$/;
.Ve
.PP
or:
.PP
.Vb 4
\&    FILTER {
\&        s/BANG\es+BANG/die \*(AqBANG\*(Aq if \e$BANG/g;
\&    }
\&    { terminator => qr/^\es*GNAB\es+esu\es*;\es*?$/ };
.Ve
.PP
and to prevent the filter's being turned off in any way:
.PP
.Vb 2
\&    package BANG;
\&    use Filter::Simple;
\&