Merge(3)						User Contributed Perl Documentation						  Merge(3)

NAME

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

SYNOPSIS

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

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

	   Hash::Merge::set_behavior( 'RIGHT_PRECEDENT' );

	   # 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',
	       );

	       # Also there is OO interface.

	       my $merge = Hash::Merge->new( 'LEFT_PRECEDENT' );
	       my %c = %{ $merge->merge( \%a, \%b ) };

	       # All behavioral changes (e.g. $merge->set_behavior(...)), called on an object remain specific to that object
	       # The legacy "Global Setting" behavior is respected only when new called as a non-OO function.

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.  By default, the data
       passed to the merge function will be cloned using the Clone module; however, if necessary, this behavior can be changed to use as many of
       the original values as possible.  (See "set_clone_behavior").

       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
	   This is the default behavior.

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

	      my $merge = Hash::Merge->new();
	      my $merge = Hash::Merge->new('LEFT_PRECEDENT');
	      $merge->set_set_behavior('LEFT_PRECEDENT')
	      Hash::Merge::set_set_behavior('LEFT_PRECEDENT')

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

	      my $merge = Hash::Merge->new('RIGHT_PRECEDENT');
	      $merge->set_set_behavior('RIGHT_PRECEDENT')
	      Hash::Merge::set_set_behavior('RIGHT_PRECEDENT')

       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.

	      my $merge = Hash::Merge->new('STORAGE_PRECEDENT');
	      $merge->set_set_behavior('STORAGE_PRECEDENT')
	      Hash::Merge::set_set_behavior('STORAGE_PRECEDENT')

       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.

	      my $merge = Hash::Merge->new('RETAINMENT_PRECEDENT');
	      $merge->set_set_behavior('RETAINMENT_PRECEDENT')
	      Hash::Merge::set_set_behavior('RETAINMENT_PRECEDENT')

       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_clone_behavior( <scalar> )
	   Sets how the data cloning is handled by Hash::Merge.  If this is true, then data will be cloned; if false, then original data will be
	   used whenever possible.  By default, cloning is on (set to true).

       get_clone_behavior( )
	   Returns the current behavior for data cloning.

       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] ] }, ... } } );

	   Note that you can import _hashify and _merge_hashes into your program's namespace with the 'custom' tag.

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.

       As of Feb 16, 2002, ActiveState Perl's PPM of Clone.pm is only at 0.09.	This version does not support the cloning of scalars if passed to
       the function.  This is fixed by 0.10 (and currently, Clone.pm is at 0.13).  So while most other users can upgrade their Clone.pm
       appropriately (and I could put this as a requirement into the Makefile.PL), those using ActiveState would lose out on the ability to use
       this module.  (Clone.pm is not pure perl, so it's not simply a matter of moving the newer file into place).  Thus, for the time being, a
       check is done at the start of loading of this module to see if a newer version of clone is around.  Then, all cloning calls have been
       wrapped in the internal _my_clone function to block any scalar clones if Clone.pm is too old.  However, this also prevents the cloning of
       anything that isn't a hash or array under the same conditions.  Once ActiveState updates their Clone, I'll remove this wrapper.

AUTHOR

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

COPYRIGHT

       Copyright (c) 2001,2002 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.

perl v5.16.2							    2010-02-15								  Merge(3)