Change ordered-map assoc behavior to be like that of Clojure 1.5.1

[jafingerhut]

If a key is assoc'd multiple times to the same map, the first key,
along with any metadata it might have, is the one that stays in the
map. The last value assoc'd to the key is the one that is in the map,
along with any metadata it might have.

Similar behavior was added to Clojure with the commit at this link,
released with 1.5.0:

clojure/clojure@7bc871f

Refer to CLJ-1065: http://dev.clojure.org/jira/browse/CLJ-1065

[jafingerhut]

Feel free to ask questions about this one, or just close it if it isn't of interest. It is a somewhat esoteric change in behavior to make ordered-maps more similar to Clojure maps in terms of how assoc works.

[amalloy]

The behavior you describe for keys sounds weird to me. Why is it desirable to keep the first key, rather than the last key, among multiple that compare as equal?

[jafingerhut]

That's a fair question. I'm not wedded to the current Clojure behavior, just an interested observer because of the changes I made with CLJ-1065 that brought the detailed behavior to my attention.

Note one thing: That is the way your ordered-sets work right now: If you try to add an item equal to an existing one, the first one stays in there with its metadata, rather than being replaced with the new equal item with the new metadata.

Doing the same for map keys, at least, seems consistent with the set behavior.

If Clojure sets and maps always replaced an existing item with a newly added equal item (and its new metadata), that would seem like reasonable behavior to me, too, but it isn't how they work today.

[slipset]

@jafingerhut, do you remember the status here?

[jafingerhut]

I believe there is no change in status here -- everything about Clojure's detailed behavior, and the behavior of ordered sets is likely the same now as it was when I created the PR. I don't consider this a high priority change, since probably most people don't use metadata, and even if they do, they might not care so much about the detailed behavior here in Clojure's sets and maps vs. the behavior for ordered.