Hash::Merge(3)

📦
version 0.302 · 8 versions
, in opensuse-leap 16.0

okay bestie, welcome to the Hash::Merge(3) page from the User Contributed Perl Documentation 💖 this is the manual for a Perl module that merges arbitrarily deep hashes into a single hash, and we're gonna make it cute, clear, and fully detailed. no cap.

🌸 NAME 🌸

Hash::Merge — merges arbitrarily deep hashes into a single hash. that's the whole name, and honestly? iconic. 🫶

✨ SYNOPSIS ✨

okay so here's the whole vibe in code. this part is copied exactly, so don't come for me:

    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::add_behavior_spec(
        {   '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 ) };

💅 DESCRIPTION 💅

Hash::Merge takes two arbitrarily deep hashes and smooshes them into one single hash 💖. at any level, it adds non-conflicting key-value pairs from one hash to the other, and it follows a set of specific rules when there are key value conflicts (all laid out below, don't worry). the hash is followed recursively, so deeply nested hashes that are at the same level will be merged when the parent hashes are merged.

⚠️ important caveat: self-referencing hashes, or recursive references, are not handled well by this method. like, it's giving "please don't" 😭.

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 below.)

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 your own. these are (currently):

🌸 Left Precedence 🌸

this is the default behavior. periodt. ✨

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_behavior('LEFT_PRECEDENT');
Hash::Merge::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_behavior('RIGHT_PRECEDENT');
Hash::Merge::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_behavior('STORAGE_PRECEDENT');
Hash::Merge::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_behavior('RETAINMENT_PRECEDENT');
Hash::Merge::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>] ) [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 isn't specified, the same default as add_behavior_spec is applied. if no such name is known referring to a behavior spec, nothing is returned. 🙈

🧠 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. 🦋

👑 AUTHOR 👑

this masterpiece was authored by Michael K. Neylon <mneylon-pm@masemware.com>, Daniel Muey <dmuey@cpan.org>, Jens Rehsack <rehsack@cpan.org>, and Stefan Hermes <hermes@cpan.org>. legends only. 👑

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. 🌈

and that's the whole Hash::Merge(3) page, bestie 💖 it's from perl v5.42.0, dated 2020-08-05, and the page itself is Hash::Merge(3). go merge some hashes responsibly, mwah 💋🌈

📦 where this page comes from