Ubuntu Manpage: Tie::ShadowHash - Merge multiple data sources into a hash

Provided by: libtie-shadowhash-perl_1.00-1_all

NAME

       Tie::ShadowHash - Merge multiple data sources into a hash

SYNOPSIS

           use Tie::ShadowHash;
           use DB_File;
           tie (%db, 'DB_File', 'file.db');
           $obj = tie (%hash, 'Tie::ShadowHash', \%db, "otherdata.txt");

           # Accesses search %db first, then the hashed "otherdata.txt".
           print "$hash{key}\n";

           # Changes override data sources, but don't change them.
           $hash{key} = 'foo';
           delete $hash{bar};

           # Add more data sources on the fly.
           %extra = (fee => 'fi', foe => 'fum');
           $obj->add (\%extra);

           # Add a text file as a data source, taking the first "word" up
           # to whitespace on each line as the key and the rest of the line
           # as the value.
           $split = sub { split (' ', $_[0], 2) };
           $obj->add ([text => "pairs.txt", $split]);

           # Add a text file as a data source, splitting each line on
           # whitespace and taking the first "word" to be the key and an
           # anonymous array consisting of the remaining words to be the
           # data.
           $split = sub { split (' ', $_[0]) };
           $obj->add ([text => "triples.txt", $split]);

DESCRIPTION

This module merges together multiple sets of data in the form of hashes into a data
structure that looks to Perl like a single simple hash. When that hash is accessed, the
data structures managed by that shadow hash are searched in order they were added for that
key. This allows the rest of a program simple and convenient access to a disparate set of
data sources.

Tie::ShadowHash can handle anything that looks like a hash; just give it a reference as
one of the additional arguments to tie(). This includes other tied hashes, so you can
include DB and DBM files as data sources for a shadow hash. If given a plain file name
instead of a reference, it will build a hash to use internally, with each chomped line of
the file being the key and the number of times that line is seen in the file being the
value.

Tie::Shadowhash also supports special tagged data sources that can take options specifying
their behavior. The only tagged data source currently supported is "text", which takes a
file name of a text file and a reference to a sub. The sub is called for every line of
the file, with that line as an argument, and is expected to return a list. The first
element of the list will be the key, and the second and subsequent elements will be the
value or values. If there is more than one value, the value stored in the hash and
associated with that key is an anonymous array containing all of them.

Tagged data sources are distinguished from normal data sources by passing them to tie()
(or to add() -- see below) as an anonymous array. The first element is the data source
tag and the remaining elements are arguments for that data source. For a text data
source, see the usage summary above for examples.

The shadow hash can be modified, and the modifications override the data sources, but
modifications aren't propagated back to the data sources. In other words, the shadow hash
treats all data sources as read-only and saves your modifications only in internal memory.
This lets you make changes to the shadow hash for the rest of your program without
affecting the underlying data in any way (and this behavior is the main reason why this is
called a shadow hash).

If the shadow hash is cleared, by assigning the empty list to it, by explicitly calling
CLEAR(), or by some other method, all data sources are dropped from the shadow hash.
There is no other way of removing a data source from a shadow hash after it's been added
(you can, of course, always untie the shadow hash and dispose of the underlying object if
you saved it to destroy the shadow hash completely).

INSTANCE METHODS

       add(SOURCE [, SOURCE ...])
           Adds the given sources to an existing shadow hash.  This method can be called on the
           object returned by the initial tie() call.  It takes the same arguments as the initial
           tie() and interprets them the same way.

DIAGNOSTICS

       Can't open file %s: %s
           Tie::ShadowHash was given a file name to use as a source, but when it tried to open
           that file, the open failed with that system error message.

       Invalid source type %s
           Tie::Shadowhash was given a tagged data source of an unknown type.  The only currently
           supported tagged data source is "text".

CAVEATS

It's worth paying very careful attention to "The untie Gotcha" in perltie when using this
module. It's also important to be careful about what you do with tied hashes that are
included in a shadow hash. Tie::ShadowHash stores a reference to such arrays; if you
untie them out from under a shadow hash, you may not get the results you expect. Remember
that if you put something in a shadow hash, you'll need to clean out the shadow hash as
well as everything else that references a variable if you want to free it completely.

Not all tied hashes implement EXISTS; in particular, ODBM_File, NDBM_File, and some old
versions of GDBM_File don't, and therefore AnyDBM_File doesn't either. Calling exists on
a shadow hash that includes one of those tied hashes as a data source may therefore result
in an exception. Tie::ShadowHash doesn't use exists except to implement the EXISTS method
because of this.

Because it can't use EXISTS due to the above problem, Tie::ShadowHash cannot correctly
distinguish between a non-existent key and an existing key associated with an undefined
value. This isn't a large problem, since many tied hashes can't store undefined values
anyway, but it means that if one of your data sources contains a given key associated with
an undefined value and one of your later data sources contains the same key but with a
defined value, when the shadow hash is accessed using that key, it will return the first
defined value it finds. This is an exception to the normal rule that all data sources are
searched in order and the value returned by an access is the first value found.
(Tie::ShadowHash does correctly handle undefined values stored directly in the shadow
hash.)

AUTHOR

       Russ Allbery <rra@stanford.edu>

COPYRIGHT AND LICENSE

       Copyright 1999, 2002, 2010 by Russ Allbery <rra@stanford.edu>

       This program is free software; you may redistribute it and/or modify it under the same
       terms as Perl itself.