EDIT: Transducers are actually not safe in `fold` contexts as I thought:
(let [f (fn [i x] (println (str "i " i " " (Thread/currentThread))) (flush)
x)
r-map-indexed #(r/folder %2 (map-indexed %1))]
(->> [6 7 8 9 10]
(r-map-indexed f)
(r/fold 1 (fn ([] (vector)) ([x] x) ([a b] (into a b))) conj)))
Produces:
i 0 Thread[ForkJoinPool-1-worker-2,5,main]
i 2 Thread[ForkJoinPool-1-worker-1,5,main]
i 3 Thread[ForkJoinPool-1-worker-1,5,main]
i 4 Thread[ForkJoinPool-1-worker-1,5,main]
i 1 Thread[ForkJoinPool-1-worker-3,5,main]
So you would have to be careful to e.g. create different `map-indexed`
transducers for single-threaded (e.g. `unsynchronized-mutable` box) and
multi-threaded (e.g. `atom` box) contexts.
On Sunday, April 9, 2017 at 2:10:06 AM UTC-4, Alexander Gunnarson wrote:
>
> I was wondering the same thing, shintotomoe. This thread
> <https://groups.google.com/forum/#!topic/clojure/CjxK7xEsOKQ> talks about
> it as well. I think it's safe to assume that since `ArrayList` uses
> unsynchronized mutability internally (a quick review of the GrepCode
> entry for `ArrayList` confirms this
> <http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/8u40-b25/java/util/ArrayList.java>),
>
> then we can rest assured that a `volatile` box as opposed to a totally
> unsynchronized mutable variable is unnecessary, even in the context of
> `fold`. After all, `reduce` (and by extension, `transduce`) is only ever
> going to be single-threaded unless the data structure in question
> unexpectedly implements a multithreaded reduce, which should never happen
> (and if it does, you likely have bigger problems). To be honest, I'm not
> sure why `volatile` is used in transducers instead of e.g. an
> `unsynchronized-mutable` box. There may be a good reason, but I'm not
> seeing it immediately. I'd love to learn.
>
> On Thursday, January 1, 2015 at 10:36:13 PM UTC-5, shintotomoe wrote:
>>
>> Thank you for the superfast response. I take it implementing your own
>> transducing process is not something you would usually do unless you have a
>> unique use case (my own use case being already implemented by chan taking a
>> transducer).
>>
>> Still, I was wondering about the use of ArrayList in partition-all, and
>> the recommendation to use volatiles inside transducers, which seem at odds.
>> It seems we don't need to implement transducers in a thread-safe way. Is
>> that correct?
>>
>> On Friday, January 2, 2015 12:58:51 PM UTC+11, tbc++ wrote:
>>>
>>> Core.async already has pipeline, pipeline-blocking and pipeline-async.
>>> In addition you can use a transducer inside a channel. Use those instead.
>>>
>>> Timothy
>>>
>>> On Thu, Jan 1, 2015 at 6:55 PM, shintotomoe <[email protected]>
>>> wrote:
>>>
>>>> I was wondering how to apply a transducer inside a go process. What
>>>> I've so far is the following
>>>>
>>>> (defn pipe-xform [in-ch out-ch xform]
>>>> (let [tr
>>>> (let [tr (xform (fn
>>>> ([result] result)
>>>> ([result input] (conj! result input))))]
>>>> (fn
>>>> ([] (locking tr (persistent! (tr (transient [])))))
>>>> ([input] (locking tr (persistent! (tr (transient []) input
>>>> ))))))]
>>>> (go-loop []
>>>> (if-some [value (<! in-ch)]
>>>> (do (doseq [v (tr value)]
>>>> (>! out-ch v))
>>>> (recur))
>>>> (do (doseq [v (tr)]
>>>> (>! out-ch v))
>>>> (close! out-ch))))))
>>>>
>>>> Now, I could just do
>>>>
>>>> (let [xf-ch (chan 1 xform)]
>>>> (pipe in-ch xf-ch)
>>>> (pipe xf-ch out-ch)
>>>>
>>>>
>>>> Or just redesign my code so that I can create in-ch or out-ch with the
>>>> transducer directly, but I was wondering whether there are any obvious
>>>> flaws with the pipe-xform implementation.
>>>>
>>>> In particular, I was wondering about the locking. At first I was under
>>>> the impression that transducers are thread-safe due to the use of
>>>> volatiles, but looking at the partition-all transducer, which uses an
>>>> ArrayList for its state, It appears that's not the case.
>>>>
>>>> Any feedback greatly appreciated.
>>>>
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>>>
>>>
>>>
>>> --
>>> “One of the main causes of the fall of the Roman Empire was that–lacking
>>> zero–they had no way to indicate successful termination of their C
>>> programs.”
>>> (Robert Firth)
>>>
>>
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