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NAME
Filter::Util::Call - Perl Source Filter Utility Module
SYNOPSIS
use Filter::Util::Call ;
DESCRIPTION
This module provides you with the framework to write Source Filters in
Perl.
An alternate interface to Filter::Util::Call is now available. See
Filter::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) = @_ ;
filter_add(
sub
{
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":
filter_add()
filter_read()
filter_read_exact()
filter_del()
import()
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
example:
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".
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
context 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
maintained 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
$self.
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
filters 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
processing, they are expected to return the filtered source using
$_.
This implicit use of $_ greatly simplifies the filter.
$status
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
block.
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 $status after every
call to "filter_read" or "filter_read_exact".
filter_del
The function, "filter_del", is used to disable the current filter.
It does not affect the running of the filter. All it does is tell
Perl not to call filter any more.
See "Example 4: Using filter_del" for details.
real_import
Internal function which adds the filter, based on the filter_add
argument type.
EXAMPLES
Here are a few examples which illustrate the key concepts - as such
most of them are of little practical use.
The "examples" sub-directory has copies of all these filters
implemented both as method filters and as closure filters.
Example 1: A simple filter.
Below is a method filter which is hard-wired to replace all occurrences
of the string "Joe" to "Jim". Not particularly Useful, but it is the
first example and I wanted to keep it simple.
package Joe2Jim ;
use Filter::Util::Call ;
sub import
{
my($type) = @_ ;
filter_add(bless []) ;
}
sub filter
{
my($self) = @_ ;
my($status) ;
s/Joe/Jim/g
if ($status = filter_read()) > 0 ;
$status ;
}
1 ;
Here is an example of using the filter:
use Joe2Jim ;
print "Where is Joe?\n" ;
And this is what the script above will print:
Where is Jim?
Example 2: Using the context
The previous example was not particularly useful. To make it more
general purpose we will make use of the context data and allow any
arbitrary from and to strings to be used. This time we will use a
closure filter. To reflect its enhanced role, the filter is called
"Subst".
package Subst ;
use Filter::Util::Call ;
use Carp ;
sub import
{
croak("usage: use Subst qw(from to)")
unless @_ == 3 ;
my ($self, $from, $to) = @_ ;
filter_add(
sub
{
my ($status) ;
s/$from/$to/
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 ($count) = 0 ;
filter_add(\$count) ;
}
1 ;
Here is a script which uses it:
use Count ;
print "Hello Joe\n" ;
print "Where is Joe\n" ;
Outputs:
Hello Jim
Where is Jim
Made 2 substitutions
Example 4: Using filter_del
Another variation on a theme. This time we will modify the "Subst"
filter to allow a starting and stopping pattern to be specified as well
as the from and to patterns. If you know the vi editor, it is the
equivalent of this command:
:/start/,/stop/s/from/to/
When used as a filter we want to invoke it like this:
use NewSubst qw(start stop from to) ;
Here is the module.
package NewSubst ;
use Filter::Util::Call ;
use Carp ;
sub import
{
my ($self, $start, $stop, $from, $to) = @_ ;
my ($found) = 0 ;
croak("usage: use Subst qw(start stop from to)")
unless @_ == 5 ;
filter_add(
sub
{
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 ;
Filter::Simple
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
Filter::Util::Call. Although it doesn't allow the fine control that
Filter::Util::Call does, it should be adequate for the majority of
applications. It's available at
http://search.cpan.org/dist/Filter-Simple/
AUTHOR
Paul Marquess
DATE
26th January 1996
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