Another style, using channels for local state, but could been plain old 
iterators, slight golf warning:

(require '[clojure.core.async :refer [to-chan <!!]])

(defn uniquify [s formatter]
  (let [g (memoize #(to-chan (cons % (map (partial formatter %) (next 
(range))))))]
    (map (fn f [x] ((some-fn #{x} f) (<!! (g x)))) s)))

(uniquify ["a" "a" "a" "a" "b" "a_2" "a_3" "a_3_1" "a_3_1" "a"] #(str %1 
"_" %2))
;=> ["a" "a_1" "a_2" "a_3" "b" "a_2_1" "a_3_1" "a_3_1_1" "a_3_1_2" "a_4"]


On Friday, 10 January 2014 14:59:10 UTC, Colin Yates wrote:
>
> I have a sequence of file names and I want to make them unique.  (uniquify 
> ["a" "b" "c" "a"]) => ["a" "b" "c" "a_1"])
>
> This is what I have come up with, but surely there is a better way?
>
> What would you all do?  Feedback welcome (including the word 'muppet' as I 
> am sure I have missed something simple) :)
>
> (defn uniquify
>   "Return a sequence, in the same order as s containing every element
>   of s. If s (which is presumed to be a string) occurs more than once
>   then every subsequent occurrence will be made unique.
>
>   Items will be updated to include an incrementing numeric count using
>   the specified formatter function. The formatter function will be
>   given the name and the number and should return a combination of the
>   two.
>
>   The set of unique s's in the returned sequence will be the count of
>   s's in s."  
>   ([s] (uniquify s (fn [item duplicates] (str item "_" duplicates))))
>   ([s formatter]
>      (let [occurrences (atom {})
>            register-occurrence (fn [item]
>                                  (if (get @occurrences item)
>                                    (swap! (get @occurrences item) inc)
>                                    (swap! occurrences assoc item (atom 1)))
>                                  @(get @occurrences item))
>            process (fn [item]
>                      (let [duplicates (dec (register-occurrence item))]
>                        (if (> duplicates 0)
>                          (formatter item duplicates)
>                          item)))
>            unique-s (map process s)]
>        unique-s)))
>

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