If your data elements are essentially map-entries (key-value pairs), I'd
use reduce. Maybe something like this would work for you...
(defn mapping-group-by [grouping-fn mapping-fn coll]
(reduce (fn [res [k v]] (update res k grouping-fn (mapping-fn v)))
{}
coll))
(def foos [[:a "foo"] [:b "bar"] [:a "baz"]])
(mapping-group-by conj clojure.string/upper-case foos)
;;=> {:a ("BAZ" "FOO"), :b ("BAR")}
If you need vector values, you can wrap `conj` with `fnil` to get a vector.
(mapping-group-by (fnil conj []) clojure.string/upper-case foos)
;;=> {:a ["FOO" "BAZ"] :b ["BAR"]}
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