filter::util::call.3

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.\" ========================================================================
.\"
.IX Title "Filter::Util::Call 3"
.TH Filter::Util::Call 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.
.if n .ad l
.nh
.SH "NAME"
Filter::Util::Call \- Perl Source Filter Utility Module
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\&    use Filter::Util::Call ;
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
This module provides you with the framework to write \fISource Filters\fR
in Perl.
.PP
An alternate interface to Filter::Util::Call is now available. See
Filter::Simple for more details.
.PP
A \fIPerl Source Filter\fR is implemented as a Perl module. The structure
of the module can take one of two broadly similar formats. To
distinguish between them, the first will be referred to as \fImethod
filter\fR and the second as \fIclosure filter\fR.
.PP
Here is a skeleton for the \fImethod filter\fR:
.PP
.Vb 1
\&    package MyFilter ;
\&
\&    use Filter::Util::Call ;
\&
\&    sub import
\&    {
\&        my($type, @arguments) = @_ ;
\&        filter_add([]) ;
\&    }
\&
\&    sub filter
\&    {
\&        my($self) = @_ ;
\&        my($status) ;
\&
\&        $status = filter_read() ;
\&        $status ;
\&    }
\&
\&    1 ;
.Ve
.PP
and this is the equivalent skeleton for the \fIclosure filter\fR:
.PP
.Vb 1
\&    package MyFilter ;
\&
\&    use Filter::Util::Call ;
\&
\&    sub import
\&    {
\&        my($type, @arguments) = @_ ;
\&
\&        filter_add(
\&            sub 
\&            {
\&                my($status) ;
\&                $status = filter_read() ;
\&                $status ;
\&            } )
\&    }
\&
\&    1 ;
.Ve
.PP
To make use of either of the two filter modules above, place the line
below in a Perl source file.
.PP
.Vb 1
\&    use MyFilter;
.Ve
.PP
In fact, the skeleton modules shown above are fully functional \fISource
Filters\fR, albeit fairly useless ones. All they does is filter the
source stream without modifying it at all.
.PP
As you can see both modules have a broadly similar structure. They both
make use of the \f(CW\*(C`Filter::Util::Call\*(C'\fR module and both have an \f(CW\*(C`import\*(C'\fR
method. The difference between them is that the \fImethod filter\fR
requires a \fIfilter\fR method, whereas the \fIclosure filter\fR gets the
equivalent of a \fIfilter\fR method with the anonymous sub passed to
\&\fIfilter_add\fR.
.PP
To make proper use of the \fIclosure filter\fR shown above you need to
have a good understanding of the concept of a \fIclosure\fR. See
perlref for more details on the mechanics of \fIclosures\fR.
.Sh "\fBuse Filter::Util::Call\fP"
.IX Subsection "use Filter::Util::Call"
The following functions are exported by \f(CW\*(C`Filter::Util::Call\*(C'\fR:
.PP
.Vb 4
\&    filter_add()
\&    filter_read()
\&    filter_read_exact()
\&    filter_del()
.Ve
.Sh "\fB\fP\f(BIimport()\fP\fB\fP"
.IX Subsection "import()"
The \f(CW\*(C`import\*(C'\fR method is used to create an instance of the filter. It is
called indirectly by Perl when it encounters the \f(CW\*(C`use MyFilter\*(C'\fR line
in a source file (See \*(L"import\*(R" in perlfunc for more details on
\&\f(CW\*(C`import\*(C'\fR).
.PP
It will always have at least one parameter automatically passed by Perl
\&\- this corresponds to the name of the package. In the example above it
will be \f(CW"MyFilter"\fR.
.PP
Apart from the first parameter, import can accept an optional list of
parameters. These can be used to pass parameters to the filter. For
example:
.PP
.Vb 1
\&    use MyFilter qw(a b c) ;
.Ve
.PP
will result in the \f(CW@_\fR array having the following values:
.PP
.Vb 4
\&    @_ [0] => "MyFilter"
\&    @_ [1] => "a"
\&    @_ [2] => "b"
\&    @_ [3] => "c"
.Ve
.PP
Before terminating, the \f(CW\*(C`import\*(C'\fR function must explicitly install the
filter by calling \f(CW\*(C`filter_add\*(C'\fR.
.PP
\&\fB\f(BIfilter_add()\fB\fR
.PP
The function, \f(CW\*(C`filter_add\*(C'\fR, actually installs the filter. It takes one
parameter which should be a reference. The kind of reference used will
dictate which of the two filter types will be used.
.PP
If a \s-1CODE\s0 reference is used then a \fIclosure filter\fR will be assumed.
.PP
If a \s-1CODE\s0 reference is not used, a \fImethod filter\fR will be assumed.
In a \fImethod filter\fR, the reference can be used to store context
information. The reference will be \fIblessed\fR into the package by
\&\f(CW\*(C`filter_add\*(C'\fR.
.PP
See the filters at the end of this documents for examples of using
context information using both \fImethod filters\fR and \fIclosure
filters\fR.
.Sh "\fB\fP\f(BIfilter()\fP\fB and anonymous sub\fP"
.IX Subsection "filter() and anonymous sub"
Both the \f(CW\*(C`filter\*(C'\fR method used with a \fImethod filter\fR and the
anonymous sub used with a \fIclosure filter\fR is where the main
processing for the filter is done.
.PP
The big difference between the two types of filter is that the \fImethod
filter\fR uses the object passed to the method to store any context data,
whereas the \fIclosure filter\fR uses the lexical variables that are
maintained by the closure.
.PP
Note that the single parameter passed to the \fImethod filter\fR,