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.