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Tie::Filter - Tie a facade around a scalar, array, or hash


  use Tie::Filter;

  my $wrapped;
  tie $scalar, 'Tie::Filter', \$wrapped,
      FETCH => sub { $_ = lc },
      STORE => sub { $_ = uc };

  my @wrapped;
  tie @array, 'Tie::Filter', \@wrapped,
      FETCH => sub { $_ = uc },
      STORE => sub { $_ = lc };

  my %wrapped;
  tie %hash, 'Tie::Filter', \%wrapped,
      FETCHKEY   => sub { $_ = lc },
      STOREKEY   => sub { $_ = uc },
      FETCHVALUE => sub { $_ = uc },
      STOREVALUE => sub { $_ = lc };


This module ties a facade around a scalar, array, or hash. The facade then filters data as it is being fetched from or stored to the internal object. For scalars, it provides an API for filtering values stored to and fetched from an internal scalar. For arrays, it provides an API for filtering elements stored in an internal array. For hashes, it provides an API for filtering the keys and values stored in the internal hash.

This is meant to provide an easy form of syntactic sugar to be built upon other objects. The original purpose of this library was to provide a drop in replacement for DBM filters for general hashes. It is capable of much more.


The only work that needs to be done is to provide the methods for fetching and storing keys and values. Fetching occurs when taking internal data and using it externally and storing occurs when taking external data and using it internally. (These terms are not directly related to the FETCH and STORE methods of a tied object.)

Each fetch and store method will have the filtered variable set in $_. The method should then modify this variable in place to perform filtering. The return value of the method is ignored.


Be careful that your FETCH/STORE methods will properly handle all possible inputs; be especially careful of undef. For example, this might seem safe:

  use strict;
  tie %hash, 'Tie::Filter', %wrapped,
      STOREVALUE => sub { $_ = join ':', @{ $_ } },
          FETCHVALUE => sub { $_ = [ split /:/, $_ ] };
  $hash{foo} = undef; # Can't use an undefined value as an ARRAY reference

This is also dangerous when using undef as elements of an array which will be lost or changed to an empty string upon retrieval depending on the contents of the rest of the array.


perltie, Tie::Filter


I would like to add support for creating filters around file handles with a similar interface, but this is a much more complicated problem then the creation of a facade around scalars, arrays, and hashes.

Anyone who is interested is welcome to contribute a Tie::Filter::Handle package for consideration. Send any ideas or comments to my email address below.


  Andrew Sterling Hanenkamp, <>


Copyright 2003 Andrew Sterling Hanenkamp. All Rights Reserved. This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.