swap vs. reset

swap-vs-reset.clj

; First with reset! (don't do this!)
user=> (defn id-generator []
         (let [id (atom 0)]
           (fn []
             (reset! id (inc @id)))))
#'user/id-generator
user=> (def g (id-generator)) 
#'user/g
user=> (doseq [i (range 10)] 
        (future 
          (doseq [j (range 5000)] (g)) 
          (println i " done")))
nil
0  done
5  done
8  done
2  done
1  done
7  done
4  done
9  done
6  done
3  done

user=> (g)
9095       <<<----- WTFBBQ?!?!?

;; Now with swap!

user=> (defn id-generator []
         (let [id (atom 0)]
           (fn []
             (swap! id inc))))
#'user/id-generator
user=> (def g (id-generator))
#'user/g
user=> (doseq [i (range 10)] 
         (future 
           (doseq [j (range 5000)] (g)) 
           (println i " done")))
0  done
2  done
1  done
3  done
7  done
5  done
4  done
nil
6  done
9  done
8  done
user=> (g)
50001         <<<--- That's better.