The following Crystal code attempts to implement custom YAML tag processing

custom_tag_handler.cr

require "yaml"

# The default representation of null values in Crystal is an empty value which while valid
# (see http://yaml.org/type/null.html) but is less familiar and clear to me, use the string
# "null" instead
struct Nil
  def to_yaml(yaml : YAML::Nodes::Builder)
    yaml.scalar "null"
  end
end

class FotsiesYAMLParser < YAML::Schema::Core::Parser
  protected def parse_join
    # TODO: what does anchor actually do? is it needed? e.g. anchor new_mapping
    data = parse_sequence

    # TODO: how do we add a value to the sequence and return that
    # Error: undefined method '<<' for Bool (compile-time type is (Array(YAML::Any) | Bool | Float64 | Hash(YAML::Any, YAML::Any) | Int64 | Set(YAML::Any) | Slice(UInt8) | String | Time | Nil))
    # value = data.raw
    # value << YAML::Any.new("string")

    # Scalars
    value = "my string"
    # value = 123.to_i64  # Integers must be 64-bit
    # value = true
    # value = 1234.46
    # value = nil

    # Arrays
    # value = [] of YAML::Any
    # value << YAML::Any.new("string")
    # value << YAML::Any.new(123.to_i64)
    # value << YAML::Any.new(123)
    # value << YAML::Any.new(true)
    # value << YAML::Any.new(123.5)
    # value << YAML::Any.new(nil)

    # Mappings
    # value = {} of YAML::Any => YAML::Any
    # value[YAML::Any.new("hi1")] = YAML::Any.new("string")
    # value[YAML::Any.new("hi2")] = YAML::Any.new(123.to_i64)
    # value[YAML::Any.new("hi3")] = YAML::Any.new(123)
    # value[YAML::Any.new("hi4")] = YAML::Any.new(true)
    # value[YAML::Any.new("hi5")] = YAML::Any.new(123.5)
    # value[YAML::Any.new("hi6")] = YAML::Any.new(nil)

    YAML::Any.new value
  end

  protected def parse_manipulate
    data = parse_mapping
    value = data.raw
    YAML::Any.new value
  end

  protected def parse_upper
    data = parse_scalar
    YAML::Any.new(data.to_s.upcase)
  end

  private def process_fots_tag(pull_parser, tag)
    # Checking the type of data beneath the tag
    # puts "scalar: #{pull_parser.kind.scalar?}"
    # puts "alias: #{pull_parser.kind.alias?}"
    # puts "sequence_start: #{pull_parser.kind.sequence_start?}"
    # puts "mapping_start: #{pull_parser.kind.mapping_start?}"

    case tag
    when "!join"  # sequence
      parse_join
    when "!manipulate"  # mapping
      parse_manipulate
    when "!upper"  # scalar
      parse_upper
    else
      nil
    end
  end

  def process_tag(tag)
    # TODO: how do we call the parent method here
    # super
    if value = process_fots_tag(@pull_parser, tag)
      yield value
    end
  end
end

data = <<-END
         ---
         types: [1, 1.2, abc, true, null]
         joining: !join [a, b, c, d]
         thingo: !manipulate
          name: fots
          age: 30
         wow: !upper make me uppercase
         END
data = FotsiesYAMLParser.new(data).parse
puts data.to_yaml