Example code for https://groups.google.com/d/topic/clojure/pRla3_7vqvc/discussion

gistfile1.clj

(def animals [{:name "Betsy" :type :cow}
              {:name "Murmur" :type :cat}
              {:name "Lessie" :type :dog}
              {:name "Dingo" :type :dog}
              {:name "Rosie" :type :cat}
              {:name "Rex" :type :dog}
              {:name "Alf" :type :cat}])

(def group-by-uc-name (group-by-with #(conj (or % []) (.toUpperCase (:name %2)))))

(group-by-uc-name :type animals)

=> {:cow ["BETSY"]
    :cat ["MURMUR" "ROSIE" "ALF"]
    :dog ["LESSIE" "DINGO" "REX"]}

;; now we can use this custom grouping fn to build an indexing fn to handle more than one keyword for grouping.

(def animals [{:name "Betsy"  :type :cow :sex :female}
              {:name "Murmur" :type :cat :sex :male}
              {:name "Lessie" :type :dog :sex :female}
              {:name "Dingo"  :type :dog :sex :male}
              {:name "Rosie"  :type :cat :sex :female}
              {:name "Rex"    :type :dog :sex :male}
              {:name "Alf"    :type :cat :sex :male}])

(def indexed-by-uc-name (index-by-with group-by-uc-name))

(indexed-by-uc-name animals :type :sex)

=> {:cow {:female ["BETSY"]}
    :cat {:male   ["MURMUR" "ALF"]
          :female ["ROSIE"]}
    :dog {:male   ["DINGO" "REX"]
          :female ["LESSIE"]}}