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

Travis CI Coverage Status


    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 ) };
    # This is the same as above
        {   '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 $merger = Hash::Merge->new('LEFT_PRECEDENT');
    my %c = %{ $merger->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.

    # re-use globally specified behavior
    my $merger = Hash::Merge->new();
    $merger->add_behavior_spec(Hash::Merge::get_behavior_spec("My Behavior"), "My Behavior");
    my %c = %{ $merger->merge( \%a, \%b ) };

    # re-use externally specified behavior
    use Hash::Merge::Extra ();
    my $merger = Hash::Merge->new();
    $merger->add_behavior_spec(Hash::Merge::Extra::L_REPLACE, "L_REPLACE");
    my %c = %{ $merger->merge( \%a, \%b ) };


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');
Right Precedence

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

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

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>] ) [deprecated]

Alias for add_behavior_spec.

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

Add a custom merge behavior spec 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.

get_behavior_spec( [<name>] )

Return a previously defined merge behavior spec. If name ism't specified, the same default as add_behavior_spec is applied.

If no such name is known referring to an behavior spec, nothing is returned.


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)

     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 )

     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.


Michael K. Neylon <>, Daniel Muey <>, Jens Rehsack <>, Stefan Hermes <>


Copyright (c) 2001,2002 Michael K. Neylon. All rights reserved. Copyright (c) 2013-2020 Jens Rehsack. All rights reserved. Copyright (c) 2017-2020 Stefan Hermes. All rights reserved.

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