send_to = -> (arr, target) { arr.map { |v| target << v } }

def flatten_this(array, lambda)
  raise NoMethodError unless array.is_a?(Array)
  result = []
  array.each do |element|
    element.is_a?(Array) ? lambda.call(element, result) : result << element
  end
  result
end

flatten_this([1, 2, 3, [4, 5, [6], [ ] ] ], send_to)