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 $status after every
call to "filter_read" or "filter_read_exact".
- filter_del
- The function, "filter_del", is used to disable
the current filter. It does not affect the running of the filter. All it
does is tell Perl not to call filter any more.
See "Example 4: Using filter_del" for details.
EXAMPLES¶
Here are a few examples which illustrate the key concepts - as such most of them
are of little practical use.
The "examples" sub-directory has copies of all these filters
implemented both as
method filters and as
closure filters.
Example 1: A simple filter.¶
Below is a
method filter which is hard-wired to replace all occurrences
of the string "Joe" to "Jim". Not particularly Useful, but
it is the first example and I wanted to keep it simple.
package Joe2Jim ;
use Filter::Util::Call ;
sub import
{
my($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