Filter::Util::Call(3)  Perl Programmers Reference Guide  Filter::Util::Call(3)

       Filter::Util::Call - Perl Source Filter Utility Module

           use Filter::Util::Call ;

       This module provides you with the framework to write Source Filters in

       An alternate interface to Filter::Util::Call is now available. See Fil-
       ter::Simple for more details.

       A Perl Source Filter 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 method filter and the
       second as closure filter.

       Here is a skeleton for the method filter:

           package MyFilter ;

           use Filter::Util::Call ;

           sub import
               my($type, @arguments) = @_ ;
               filter_add([]) ;

           sub filter
               my($self) = @_ ;
               my($status) ;

               $status = filter_read() ;
               $status ;

           1 ;

       and this is the equivalent skeleton for the closure filter:

           package MyFilter ;

           use Filter::Util::Call ;

           sub import
               my($type, @arguments) = @_ ;

                       my($status) ;
                       $status = filter_read() ;
                       $status ;
                   } )

           1 ;

       To make use of either of the two filter modules above, place the line
       below in a Perl source file.

           use MyFilter;

       In fact, the skeleton modules shown above are fully functional Source
       Filters, albeit fairly useless ones. All they does is filter the source
       stream without modifying it at all.

       As you can see both modules have a broadly similar structure. They both
       make use of the "Filter::Util::Call" module and both have an "import"
       method. The difference between them is that the method filter requires
       a filter method, whereas the closure filter gets the equivalent of a
       filter method with the anonymous sub passed to filter_add.

       To make proper use of the closure filter shown above you need to have a
       good understanding of the concept of a closure. See perlref for more
       details on the mechanics of closures.

       use Filter::Util::Call

       The following functions are exported by "Filter::Util::Call":



       The "import" method is used to create an instance of the filter. It is
       called indirectly by Perl when it encounters the "use MyFilter" line in
       a source file (See "import" in perlfunc for more details on "import").

       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 "MyFilter".

       Apart from the first parameter, import can accept an optional list of
       parameters. These can be used to pass parameters to the filter. For

           use MyFilter qw(a b c) ;

       will result in the @_ array having the following values:

           @_ [0] => "MyFilter"
           @_ [1] => "a"
           @_ [2] => "b"
           @_ [3] => "c"

       Before terminating, the "import" function must explicitly install the
       filter by calling "filter_add".


       The function, "filter_add", 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.

       If a CODE reference is used then a closure filter will be assumed.

       If a CODE reference is not used, a method filter will be assumed.  In a
       method filter, the reference can be used to store context information.
       The reference will be blessed into the package by "filter_add".

       See the filters at the end of this documents for examples of using con-
       text information using both method filters and closure filters.

       filter() and anonymous sub

       Both the "filter" method used with a method filter and the anonymous
       sub used with a closure filter is where the main processing for the
       filter is done.

       The big difference between the two types of filter is that the method
       filter uses the object passed to the method to store any context data,
       whereas the closure filter uses the lexical variables that are main-
       tained by the closure.

       Note that the single parameter passed to the method filter, $self, is
       the same reference that was passed to "filter_add" blessed into the
       filter's package. See the example filters later on for details of using

       Here is a list of the common features of the anonymous sub and the
       "filter()" method.

       $_   Although $_ doesn't actually appear explicitly in the sample fil-
            ters above, it is implicitly used in a number of places.

            Firstly, when either "filter" or the anonymous sub are called, a
            local copy of $_ will automatically be created. It will always
            contain the empty string at this point.

            Next, both "filter_read" and "filter_read_exact" will append any
            source data that is read to the end of $_.

            Finally, when "filter" or the anonymous sub are finished process-
            ing, they are expected to return the filtered source using $_.

            This implicit use of $_ greatly simplifies the filter.

            The status value that is returned by the user's "filter" method or
            anonymous sub and the "filter_read" and "read_exact" functions
            take the same set of values, namely:

                < 0  Error
                = 0  EOF
                > 0  OK

       filter_read and filter_read_exact
            These functions are used by the filter to obtain either a line or
            block from the next filter in the chain or the actual source file
            if there aren't any other filters.

            The function "filter_read" takes two forms:

                $status = filter_read() ;
                $status = filter_read($size) ;

            The first form is used to request a line, the second requests a

            In line mode, "filter_read" will append the next source line to
            the end of the $_ scalar.

            In block mode, "filter_read" will append a block of data which is
            <= $size to the end of the $_ scalar. It is important to emphasise
            the that "filter_read" will not necessarily read a block which is
            precisely $size bytes.

            If you need to be able to read a block which has an exact size,
            you can use the function "filter_read_exact". It works identically
            to "filter_read" in block mode, except it will try to read a block
            which is exactly $size bytes in length. The only circumstances
            when it will not return a block which is $size bytes long is on
            EOF or error.

            It is very important to check the value of $status after every
            call to "filter_read" or "filter_read_exact".

            The function, "filter_del", is used to disable the current filter.
            It does not affect the running of the filter. All it does is tell
            Perl not to call filter any more.

            See "Example 4: Using filter_del" for details.

       Here are a few examples which illustrate the key concepts - as such
       most of them are of little practical use.

       The "examples" sub-directory has copies of all these filters imple-
       mented both as method filters and as closure filters.

       Example 1: A simple filter.

       Below is a method filter which is hard-wired to replace all occurrences
       of the string "Joe" to "Jim". Not particularly Useful, but it is the
       first example and I wanted to keep it simple.

           package Joe2Jim ;

           use Filter::Util::Call ;

           sub import
               my($type) = @_ ;

               filter_add(bless []) ;

           sub filter
               my($self) = @_ ;
               my($status) ;

                   if ($status = filter_read()) > 0 ;
               $status ;

           1 ;

       Here is an example of using the filter:

           use Joe2Jim ;
           print "Where is Joe?\n" ;

       And this is what the script above will print:

           Where is Jim?

       Example 2: Using the context

       The previous example was not particularly useful. To make it more
       general purpose we will make use of the context data and allow any
       arbitrary from and to strings to be used. This time we will use a clo-
       sure filter. To reflect its enhanced role, the filter is called

           package Subst ;

           use Filter::Util::Call ;
           use Carp ;

           sub import
               croak("usage: use Subst qw(from to)")
                   unless @_ == 3 ;
               my ($self, $from, $to) = @_ ;
                       my ($status) ;
                           if ($status = filter_read()) > 0 ;
                       $status ;
           1 ;

       and is used like this:

           use Subst qw(Joe Jim) ;
           print "Where is Joe?\n" ;

       Example 3: Using the context within the filter

       Here is a filter which a variation of the "Joe2Jim" filter. As well as
       substituting all occurrences of "Joe" to "Jim" it keeps a count of the
       number of substitutions made in the context object.

       Once EOF is detected ($status is zero) the filter will insert an extra
       line into the source stream. When this extra line is executed it will
       print a count of the number of substitutions actually made.  Note that
       $status is set to 1 in this case.

           package Count ;

           use Filter::Util::Call ;

           sub filter
               my ($self) = @_ ;
               my ($status) ;

               if (($status = filter_read()) > 0 ) {
                   s/Joe/Jim/g ;
                   ++ $$self ;
               elsif ($$self >= 0) { # EOF
                   $_ = "print q[Made ${$self} substitutions\n]" ;
                   $status = 1 ;
                   $$self = -1 ;

               $status ;

           sub import
               my ($self) = @_ ;
               my ($count) = 0 ;
               filter_add(\$count) ;

           1 ;

       Here is a script which uses it:

           use Count ;
           print "Hello Joe\n" ;
           print "Where is Joe\n" ;


           Hello Jim
           Where is Jim
           Made 2 substitutions

       Example 4: Using filter_del

       Another variation on a theme. This time we will modify the "Subst" fil-
       ter to allow a starting and stopping pattern to be specified as well as
       the from and to patterns. If you know the vi editor, it is the equiva-
       lent of this command:


       When used as a filter we want to invoke it like this:

           use NewSubst qw(start stop from to) ;

       Here is the module.

           package NewSubst ;

           use Filter::Util::Call ;
           use Carp ;

           sub import
               my ($self, $start, $stop, $from, $to) = @_ ;
               my ($found) = 0 ;
               croak("usage: use Subst qw(start stop from to)")
                   unless @_ == 5 ;

                       my ($status) ;

                       if (($status = filter_read()) > 0) {

                           $found = 1
                               if $found == 0 and /$start/ ;

                           if ($found) {
                               s/$from/$to/ ;
                               filter_del() if /$stop/ ;

                       $status ;
                   } )


           1 ;

       If you intend using the Filter::Call functionality, I would strongly
       recommend that you check out Damian Conway's excellent Filter::Simple
       module. Damian's module provides a much cleaner interface than Fil-
       ter::Util::Call. Although it doesn't allow the fine control that Fil-
       ter::Util::Call does, it should be adequate for the majority of appli-
       cations. It's available at

       Paul Marquess

       26th January 1996

perl v5.8.6                       2001-09-21             Filter::Util::Call(3)