You don't need mutability to represent a stopwatch.
(defn start [stopwatch]
(assoc stopwatch ::start-time (System/currentTimeMillis)))
(defn elapsed-since-started [stopwatch]
(- (System/currentTimeMillis) (::start-time stopwatch)))
(defn stop [stopwatch]
(-> stopwatch
(dissoc ::start-time)
(update ::elapsed (fnil + 0)
(elapsed-since-started stopwatch))))
(defn elapsed [stopwatch]
(::elapsed (stop stopwatch) 0)
(defn reset [stopwatch]
(dissoc stopwatch ::start-time ::elapsed))
Mutability is typically only necessary for two reasons:
1. Communication across threads
2. Performance
- James
On 10 December 2016 at 07:47, Didier <[email protected]> wrote:
> I'm wondering what everyone thinks of using closures to mimic a simplistic
> object system in Clojure? I'm not sure what to think of it yet, but the
> idea is that you wrap object fields inside a closed function, and it
> returns a map of methods that operates over those fields.
>
> Here's an example of using this pattern to implement a StopWatch:
>
> (import [java.lang System])
> (defn new-stopwatch []
> (let [start-time (atom nil)
> elapsed (atom 0)]
> {:start (fn []
> (when (nil? @start-time)
> (reset! start-time (System/currentTimeMillis))))
> :stop (fn []
> (when-not (nil? @start-time)
> (reset! elapsed
> (+ @elapsed
> (- (System/currentTimeMillis) @start-time)))
> (reset! start-time nil))
> @elapsed)
> :reset (fn []
> (reset! start-time nil)
> (reset! elapsed 0)
> @elapsed)
> :elapsed (fn []
> (if-not (nil? @start-time)
> (- (System/currentTimeMillis) @start-time)
> @elapsed))}))
>
> (let [sw1 (new-stopwatch)
> sw2 (new-stopwatch)]
> ((:start sw1))
> ((:start sw2))
> (Thread/sleep 100)
> ((:reset sw1))
> ((:start sw1))
> (println (str "Elapsed for SW1: " ((:elapsed sw1))))
> (println (str "Elapsed for SW2: " ((:elapsed sw2))))
> (Thread/sleep 100)
> (println (str "SW1: " ((:stop sw1))))
> (println (str "SW2: " ((:stop sw2)))))
>
> I find for certain things, like a stopwatch, this pattern is actually
> pretty nice. I can't think of any alternative way to do this in Clojure
> that I'd like better actually.
>
> What are your thoughts?
>
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