I'm a real newbe to clojure and functionnal programming but reading your
answer raised a question :
Where do you "store" the stopwatch object which you are passing to all your
functions ?
I understand that mutability is not needed to develop the functions you
described but all of them take a stopwatch as as an argument. This
stopwatch must be stored somewhere to call these functions against ?
Le samedi 10 décembre 2016 21:51:27 UTC-5, James Reeves a écrit :
>
> 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 <did...@gmail.com <javascript:>>
> 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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