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


    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
    my $data = $yp->load_string($yaml);
    # $data->[4] == $data->[5] == $data->[6] == $data->[7]


See for the specification.


"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.



Called by YAML::PP::Schema