_original.rb

helper = lambda do |responses, ids, val|
  if responses.empty?
    ids.push(val)
  elsif val.is_a?(Integer)
    helper.call(responses, ids.push(val), boutique_curator.plan)
  else
    first, *rest = responses
    helper.call(rest, ids, val[first])
  end
end
collection_ids = helper.call(responses, [], boutique_curator.plan)

functional_equivalent.rb

node = boutique_curator.plan
collection_ids = responses.each_with_object([]) do |response, collection_ids|
  next_node = node[response]
  if next_node.is_a?(Integer)
    collection_ids << next_node
    node = boutique_curator.plan
  else
    node = next_node
  end
end