/ 
Hash-Merge-0.04
River stage three • 121 direct dependents • 806 total dependents
/ Hash::Merge

NAME

Hash::Merge - Merges arbitrarily deep hashes into a single hash

SYNOPSIS

  use Hash::Merge qw( merge );
  my %a = ( foo => 1,
            bar => [ a, b, e ],
		    querty => { bob => alice } );
  my %b = ( foo => 2, 
            bar => [ c, d ],
			querty => { ted => margeret } );

  my %c = merge( \%a, \%b );

  Hash::Merge::set_behavior( RIGHT_PRECEDENCE );

  # This is the same as above

  Hash::Merge::specify_behavior( {
  	SCALAR => {
		SCALAR => sub { $_[1] },
		ARRAY  => sub { [ $_[0], @{$_[1]} ] },
		HASH   => sub { $_[1] } },
	ARRAY => {
		SCALAR => sub { $_[1] },
		ARRAY  => sub { [ @{$_[0]}, @{$_[1]} ] },
		HASH   => sub { $_[1] } },
	HASH => {
		SCALAR => sub { $_[1] },
		ARRAY  => sub { [ values %{$_[0]}, @{$_[1]} ] },
		HASH   => sub { Hash::Merge::_merge_hashes( $_[0], $_[1] ) } }
  }, "My Behavior" );

DESCRIPTION

Hash::Merge merges two arbitrarily deep hashes into a single hash. That is, at any level, it will add non-conflicting key-value pairs from one hash to the other, and follows a set of specific rules when there are key value conflicts (as outlined below). The hash is followed recursively, so that deeply nested hashes that are at the same level will be merged when the parent hashes are merged. Please note that self-referencing hashes, or recursive references, are not handled well by this method.

Values in hashes are considered to be either ARRAY references, HASH references, or otherwise are treated as SCALARs.

Because there are a number of possible ways that one may want to merge values when keys are conflicting, Hash::Merge provides several preset methods for your convenience, as well as a way to define you own. These are (currently):

  • Left Precedence - The values buried in the left hash will never be lost; any values that can be added from the right hash will be attempted.

  • Right Precedence - Same as Left Precedence, but with the right hash values never being lost

  • Storage Precedence - If conflicting keys have two different storage mediums, the 'bigger' medium will win; arrays are preferred over scalars, hashes over either. The other medium will try to be fitted in the other, but if this isn't possible, the data is dropped.

  • Retainment Precedence - No data will be lost; scalars will be joined with arrays, and scalars and arrays will be 'hashified' to fit them into a hash.

Specific descriptions of how these work are detailed below.

merge ( <hashref>, <hashref> )

Merges two hashes given the rules specified. Returns a reference to the new hash.

_hashify( <scalar>|<arrayref> ) -- INTERNAL FUNCTION

Returns a reference to a hash created from the scalar or array reference, where, for the scalar value, or each item in the array, there is a key and it's value equal to that specific value. Example, if you pass scalar '3', the hash will be { 3 => 3 }.

_merge_hashes( <hashref>, <hashref> ) -- INTERNAL FUNCTION

Actually does the key-by-key evaluation of two hashes and returns the new merged hash. Note that this recursively calls merge.

set_behavior( <scalar> )

Specify which built-in behavior for merging that is desired. The scalar must be one of those given below.

get_behavior( )

Returns the behavior that is currently in use by Hash::Merge.

specify_behavior( <hashref>, [<name>] )

Specify a custom merge behavior for Hash::Merge. This must be a hashref defined with (at least) 3 keys, SCALAR, ARRAY, and HASH; each of those keys must have another hashref with (at least) the same 3 keys defined. Furthermore, the values in those hashes must be coderefs. These will be called with two arguments, the left and right values for the merge. Your coderef should return either a scalar or an array or hash reference as per your planned behavior. If necessary, use the functions _hashify and _merge_hashes as helper functions for these. For example, if you want to add the left SCALAR to the right ARRAY, you can have your behavior specification include:

%spec = ( ...SCALAR => { ARRAY => sub { [ $_[0], @$_[1] ] }, ... } } );

BUILT-IN BEHAVIORS

Here is the specifics on how the current internal behaviors are called, and what each does. Assume that the left value is given as $a, and the right as $b (these are either scalars or appropriate references)

LEFT TYPE RIGHT TYPE LEFT_PRECEDENT RIGHT_PRECEDENT SCALAR SCALAR $a $b SCALAR ARRAY $a ( $a, @$b ) SCALAR HASH $a %$b ARRAY SCALAR ( @$a, $b ) $b ARRAY ARRAY ( @$a, @$b ) ( @$a, @$b ) ARRAY HASH ( @$a, values %$b ) %$b HASH SCALAR %$a $b HASH ARRAY %$a ( values %$a, @$b ) HASH HASH merge( %$a, %$b ) merge( %$a, %$b )

LEFT TYPE RIGHT TYPE STORAGE_PRECEDENT RETAINMENT_PRECEDENT SCALAR SCALAR $a ( $a ,$b ) SCALAR ARRAY ( $a, @$b ) ( $a, @$b ) SCALAR HASH %$b merge( hashify( $a ), %$b ) ARRAY SCALAR ( @$a, $b ) ( @$a, $b ) ARRAY ARRAY ( @$a, @$b ) ( @$a, @$b ) ARRAY HASH %$b merge( hashify( @$a ), %$b ) HASH SCALAR %$a merge( %$a, hashify( $b ) ) HASH ARRAY %$a merge( %$a, hashify( @$b ) ) HASH HASH merge( %$a, %$b ) merge( %$a, %$b )

(*) note that merge calls _merge_hashes, hashify calls _hashify.

CAVEATS

This will not handle self-referencing/recursion within hashes well. Plans for a future version include incorporate deep recursion protection.

AUTHOR

Michael K. Neylon <mneylon-pm@masemware.com>

COPYRIGHT

Copyright (c) 2001 Michael K. Neylon. All rights reserved.

This library is free software. You can redistribute it and/or modify it under the same terms as Perl itself.

HISTORY

$Log: Merge.pm,v $ Revision 0.04 2001/10/31 03:59:03 mneylon Forced Test::More requirement in makefile Fixed problems with pod documentation

Revision 0.03 2001/10/28 23:36:12 mneylon CPAN Release with CVS fixes

Revision 0.02 2001/10/28 23:05:03 mneylon CPAN release