code: def array(method = nil)
        -> (v) do
          if v
            v.split(",").map{|a| cast(a, method) }
          end
        end
      end
docs: optional :accronyms, array(string)
   => ['a', 'b']
   optional :numbers, array(int)
   => [1, 2]
   optional :notype, array
   => ['a', 'b']