NAME

       YAML::PP::Schema::Merge - Enabling YAML merge keys for mappings

SYNOPSIS

           use YAML::PP;
           my $yp = YAML::PP->new( schema => [qw/ + Merge /] );

           my $yaml = <<'EOM';
           ---
           - &CENTER { x: 1, y: 2 }
           - &LEFT { x: 0, y: 2 }
           - &BIG { r: 10 }
           - &SMALL { r: 1 }

           # All the following maps are equal:

           - # Explicit keys
             x: 1
             y: 2
             r: 10
             label: center/big

           - # Merge one map
             << : *CENTER
             r: 10
             label: center/big

           - # Merge multiple maps
             << : [ *CENTER, *BIG ]
             label: center/big

           - # Override
             << : [ *BIG, *LEFT, *SMALL ]
             x: 1
             label: center/big
           EOM
           my $data = $yp->load_string($yaml);
           # $data->[4] == $data->[5] == $data->[6] == $data->[7]

DESCRIPTION

       See <https://yaml.org/type/merge.html> for the specification.

       Quote:

       "Specify one or more mappings to be merged with the current one.

       The "<<" merge key is used to indicate that all the keys of one or more specified maps
       should be inserted into the current map. If the value associated with the key is a single
       mapping node, each of its key/value pairs is inserted into the current mapping, unless the
       key already exists in it. If the value associated with the merge key is a sequence, then
       this sequence is expected to contain mapping nodes and each of these nodes is merged in
       turn according to its order in the sequence. Keys in mapping nodes earlier in the sequence
       override keys specified in later mapping nodes."

       The implementation of this in a generic way is not trivial, because we also have to handle
       duplicate keys, and YAML::PP allows you to write your own handler for processing mappings.

       So the inner API of that is not stable at this point.

       Note that if you enable this schema, a plain scalar `<<` will be seen as special anywhere
       in your document, so if you want a literal `<<`, you have to put it in quotes.

       Note that the performed merge is not a "deep merge". Only top-level keys are merged.

METHODS

       register
           Called by YAML::PP::Schema