rollup.clj

(require '[net.cgrand.xforms :as x])

(defn rollup [dimensions valfn]
  (let [[dim & dims] (reverse dimensions)]
    (reduce
      (fn [xform dim]
        (comp
          (x/by-key dim xform)
          (x/transjuxt
            {:detail (x/into {})
             :total (comp x/vals (map :total) (x/reduce +))})))
      (comp (x/by-key dim (map valfn))
        (x/transjuxt
          {:detail (x/into {})
           :total (comp x/vals (x/reduce +))}))
      dims)))

=> (into {} (rollup [:continent :country] :population)
     [{:continent "Europe" :country "France" :population 66}
      {:continent "Europe" :country "Germany" :population 80}
      {:continent "Europe" :country "Belarus" :population 9}
      {:continent "North-America" :country "USA" :population 319}
      {:continent "North-America" :country "Canada" :population 35}])
{:detail
 {"Europe"
  {:detail {"France" 66, "Germany" 80, "Belarus" 9}, :total 155},
  "North-America" {:detail {"USA" 319, "Canada" 35}, :total 354}},
 :total 509}