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[ASYNC-6] alts! assumes nth works on ports argument Created: 15/Jul/13  Updated: 27/Sep/13  Resolved: 27/Sep/13

Status: Closed
Project: core.async
Component/s: None
Affects Version/s: None
Fix Version/s: None

Type: Defect Priority: Minor
Reporter: Brandon Bloom Assignee: Timothy Baldridge
Resolution: Completed Votes: 0
Labels: None


 Description   

The docstring for alts! reads:

ports is a set of channel endpoints, which
can be either a channel to take from or a vector of
[channel-to-put-to val-to-put], in any combination.

However, trying to use an actual set yields the following exception:

java.lang.UnsupportedOperationException: nth not supported on this type: PersistentHashSet



 Comments   
Comment by Brandon Bloom [ 07/Aug/13 2:32 PM ]

Discussed at length in IRC on Aug 7th: http://www.raynes.me/logs/irc.freenode.net/clojure/2013-08-07.txt

Comment by Rich Hickey [ 07/Aug/13 9:28 PM ]

The documentation should be changed. alts requires an efficiently nth-able collection (e.g. a vector), else it will be slow.

https://github.com/clojure/core.async/blob/master/src/main/clojure/clojure/core/async.clj#L183

If CLJS supports nth on sets it is broken.

Comment by Brandon Bloom [ 07/Aug/13 9:52 PM ]

> alts requires an efficiently nth-able collection

Is that the case? Or does alts actually only require efficiently sampling a single element from a collection?

Efficient nth is a reasonable proxy, since the only choice we have for randomness is rand-int. However, one could imagine a CollSample protocol that would allow efficient random choice leveraging data-structure-specific knowledge, much as CollReduce works. Given the world as it exists now, I agree that this is a doc bug.

> If CLJS supports nth on sets it is broken.

That's the conclusion we came to in IRC. David quickly fixed this:
https://github.com/clojure/clojurescript/commit/a113b08a8c2811b0590cc6a36b2e9e5adc1c4c1e

Comment by Timothy Baldridge [ 27/Sep/13 8:47 AM ]

fixed in master

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