Yes, there were some changes relating to tuples in the 1.8 alphas that made
2-element vectors valid map entries. However, this was done by having
IPersistentVector extend IMapEntry which meant that (from a type
perspective) all vectors were valid map entries. In practice, this led to
some tricky conditional code both inside and outside Clojure itself to
account for the separate cases of generic vectors and 2-element vectors
acting as map entries (this comes up when doing generic tree walk
traversals on heterogeneous data). This caused some pretty subtle breakage.
In the end, nearly all of that was backed out prior to release of 1.8. It
may be considered again in the future - as we found, there are tradeoffs.
Re performance, I think you'll find that key and val are many orders of
magnitude faster than first and second as they will take a sequence view of
entry.
By no means rigorous, but a quick test (just reporting last timing):
(def me (first {:a 1}))
(dotimes [_ 20] (time (dotimes [_ 1000000] (key me))))
"Elapsed time: 0.86 msecs"
(dotimes [_ 20] (time (dotimes [_ 1000000] (val me))))
"Elapsed time: 0.959 msecs"
(dotimes [_ 20] (time (dotimes [_ 1000000] (first me))))
"Elapsed time: 52.712 msecs"
(dotimes [_ 20] (time (dotimes [_ 1000000] (second me))))
"Elapsed time: 56.786 msecs"
If you want something portable between map entries and vectors and also
fast, I would prefer nth:
(dotimes [_ 20] (time (dotimes [_ 1000000] (nth me 0))))
"Elapsed time: 0.573 msecs"
(dotimes [_ 20] (time (dotimes [_ 1000000] (nth me 1))))
"Elapsed time: 0.573 msecs"
(def v [:a 1])
(dotimes [_ 20] (time (dotimes [_ 1000000] (first v))))
"Elapsed time: 48.3 msecs"
(dotimes [_ 20] (time (dotimes [_ 1000000] (second v))))
"Elapsed time: 54.236 msecs"
(dotimes [_ 20] (time (dotimes [_ 1000000] (nth v 0))))
"Elapsed time: 2.011 msecs"
(dotimes [_ 20] (time (dotimes [_ 1000000] (nth v 1))))
"Elapsed time: 2.093 msecs"
On Wednesday, May 4, 2016 at 11:52:40 AM UTC-5, adrian.med...@mail.yu.edu
wrote:
>
> I may be misremembering here, but I think this was briefly implemented for
> an alpha release of 1.8.0. I think the entire feature in question is being
> reworked for the future, but I'm uncertain of the details. Perhaps others
> can chime in with the background motivating those changes and reversions?
>
> On Wednesday, May 4, 2016 at 12:44:19 PM UTC-4, JvJ wrote:
>>
>>
>> I've noticed that, in Clojure, there is a MapEntry type that supports the
>> operations "key" and "val", but in Clojurescript, MapEntry is replaced by
>> simple 2-element vectors.
>>
>> Furthermore, 2-element vectors can be used in almost all other cases
>> where a MapEntry is expected, and MapEntries support all vector
>> operations,but "key" and "val" are
>> not supported on these vectors.
>>
>> Is there a particular design reason for this? It doesn't seem like much
>> more than an inconvenience
>>
>
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