On Sat, Jun 25, 2011 at 4:18 PM, Laurent PETIT <laurent.pe...@gmail.com> wrote: > Beware the devil hidden in the details: > " > Note that an array map will only maintain sort order when > un-'modified'. Subsequent assoc-ing will eventually cause it to > 'become' a hash-map. > "
Besides a LinkedHashMap-alike, Clojure could probably use at least these other map variations: * WeakHashMap (keys weakly bound) * Map with weak values (useful for caching) * ConcurrentHashMap and possibly combinations of the above. Even Java seems to lack weak-value maps; you can fudge it though by putting a WeakReference(val) in in a wrapper layer and using a ReferenceQueue and a thread that drains the queue every so often to prune dead Map.Entries from the map. Note that a map of refs doesn't cut it as a substitute for a ConcurrentHashMap if keys will be added/removed by multiple threads, and sticking the entire map in an atom doesn't if you want to support more concurrency than a plain old Collections.synchronizedMap. Sticking the entire map in a ref and using commute doesn't quite work, if two concurrent assoces of the same key should force a retry, and sticking the entire map in a ref and using alter allows no more concurrency than synchronizedMap. We really could use an STM-map that supports normal Clojure map get semantics, but not assoc or dissoc, and supports alter-key and remove-key which have to be called in a transaction and act like commuted assoc and dissoc except that one transaction retries if there are concurrent attempts to change the *same* key. Perhaps we should also have a way to simply specify, separately and combinably, in both stm-map and hash-map, options to make the keys weak, the values weak, or the seq/keys/vals views use the insertion order. With both weakable keys and weakable vals you can also throw in a weak-hash-set which can be used to "intern" or "canonicalize" any desired immutable objects, without packratting them and eventually blowing the heap, and is implemented under the hood as a weak-keyed, weak-valued map of objects to themselves. -- Protege: What is this seething mass of parentheses?! Master: Your father's Lisp REPL. This is the language of a true hacker. Not as clumsy or random as C++; a language for a more civilized age. -- You received this message because you are subscribed to the Google Groups "Clojure" group. To post to this group, send email to clojure@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en
