extract.rb

class Enumerator
  def extract(&blk)
    self.each do |obj|
      blk.call(*blk.parameters.map(&:last).map {|attr| obj.send(attr)})
    end
  end
end

Treasure = Struct.new(:coins, :gems)

treasures = [
  Treasure.new(100, %w[emerald ruby]),
  Treasure.new(70,  %w[amethyst], [])
]

treasures.each.extract do |coins|
  coins # => 100, 70
end

treasures.each.extract do |gems|
  gems # => ["emerald", "ruby"], ["amethyst"]
end

you're right, that empty array is a leftover from earlier code.

Thanks for the confirmation.