bionic (3) Tie::ShadowHash.3pm.gz
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.
SEE ALSO
perltie
The current version of this module is always available from its web site at
<http://www.eyrie.org/~eagle/software/shadowhash/>.