NAME

Filter::Handle - Apply filters to output filehandles

SYNOPSIS

use Filter::Handle;
my $f = Filter::Handle->new(\*STDOUT);
$f->print(...);

use Filter::Handle qw/subs/;
Filter \*STDOUT;
...
UnFilter \*STDOUT;

tie *STDOUT, 'Filter::Handle', \*HANDLE;
...
untie *STDOUT;

DESCRIPTION

Filter::Handle allows you to apply arbitrary filters to output filehandles. You can perform any sorts of transformations on the outgoing text: you can prepend it with some data, you can replace all instances of one word with another, etc.

You can even filter all of your output to one filehandle and send it to another; for example, you can filter everything written to STDOUT and write it instead to another filehandle. To do this, you need to explicitly use the tie interface (see below).

Calling Interfaces

There are three interfaces to filtering a handle:

• Functional

  use Filter::Handle qw/subs/;
  
  Filter \*STDOUT;
  print "I am filtered text";
  UnFilter \*STDOUT;
  
  print "I am normal text";

  The functional interface works by exporting two functions into the caller's namespace: Filter and UnFilter. To start filtering a filehandle, call the Filter function; to stop, call UnFilter on that same filehandle.

  Any writes between the time you start and stop filtering will be filtered.

• Object-Oriented

  use Filter::Handle;
  
  {
      my $f = Filter::Handle->new(\*STDOUT);
      $f->print("I am filtered text");
  }
  
  print "I am normal text";

  The object-oriented interface works differently than the other two interfaces (Functional and Tie); while the others use Perl's tie mechanism to provide the filtering, the OO interface expects you to explicitly call methods on your Filter::Handle object. This is really just a difference of approach; you should get the same results, either way. The filter is in scope as long as your Filter::Handle object is in scope. But in order to write to the filtered filehandle, you must explicitly use either print or printf methods.

• Tie

  use Filter::Handle;
  
  local *HANDLE;
  tie *STDOUT, 'Filter::Handle', \*HANDLE;
  
  print "I am filtered text written to HANDLE";
  
  untie *STDOUT;

  The tie interface will filter your filehandle until you explicitly untie it. This is the only interface that allows you to filter one filehandle through another. The above example will filter all writes to STDOUT through the output filter, then write it out on HANDLE. Note that this is different behavior than that of the first two interfaces; if you want your output written to the same handle that you're filtering, you could use:

  tie *STDOUT, 'Filter::Handle', \*STDOUT;

Customized Filters

The default filter is relatively boring: it simply prepends any text you print with the filename and line of the invoking caller. You'll probably want to do something more interesting.

To do so, pass an anonymous subroutine as a second argument to either the new method, if you're using the OO interface, or to the Filter function, if you're using the functional interface. Your subroutine will be passed the list originally passed to print, and it should return another list, suitable for passing to your (unfiltered) output filehandle.

For example, say that we want to replace all instances of "blue" with "red". We could say:

use Filter::Handle qw/subs/;

Filter \*STDOUT,
    sub { local $_ = "@_"; s/blue/red/g; $_ };

print "My house is blue.\n";
print "So is my cat, whose nose is blue.\n";

UnFilter \*STDOUT;

print "And the plane is also blue.\n";

This prints:

My house is red.
So is my cat, whose nose is red.
And the plane is also blue.

As expected.

Tips, Tricks, Samples

• Capturing Output

  Normally, output is passed through your filtering function, then printed on the output filehandle that you're filtering. Suppose that, instead of writing the filtered output to the filehandle, you just want to capture that filtered output. In other words, you want to store the output and not have it written to the filehandle. Here's an example that does just that:

  my($out, $i);
  Filter \*STDOUT, sub {
      $out .= sprintf "%d: %s\n", $i++, "@_";
      ()
  };
  print "Foo";
  print "Bar";
  UnFilter \*STDOUT;

  $out now contains:

  0: Foo
  1: Bar

  And nothing has been written to STDOUT.

CAVEATS

Note that this won't work correctly with output from XSUBs or system calls. This is due to a limitation of Perl's tie mechanism when tying filehandles.

AUTHOR

Benjamin Trott, ben@rhumba.pair.com

CREDITS

Thanks to tilly, chromatic, and merlyn at PerlMonks.org for suggestions, critiques, and code samples.