multiple_fetch.rb

  def fetch(hash, *keys, &block)
    index = 0
    keys.reduce(hash) do |coll, key|
      index += 1
      v = coll[key]
      if v
        v
      else
        if !block.nil? && index == keys.size
          block.call
        else
          coll.fetch(key)
        end
      end
    end
  end
  
fetch({a: {}}, :b)# exception for missing b
fetch({a: {}}, :a) # {}
fetch({a: {}}, :b) { "default"} # default
fetch({a: {}}, :b, :c) { "default"} # exception for missing b