So, inspired by this post on SO: http://stackoverflow.com/questions/10942607/clojure-multi-maps
I've been trying to write some multimap functions, in a transient version: https://gist.github.com/anonymous/5660467 This was my first experience with transients, and it's been problematic. First problem: transients do not support or preserve metadata, or so it seems. I wanted my multimaps to function also as regular maps until the multi- functionality is needed, so I needed a way to distinguish "multi" entries from regular ones. At first blush, you'd just test to see if the entry is a set; but if you think about it, you might want to make a regular map have set entries too. No problem, that's what metadata's for, right? But of course this code doesn't work: (defn plural? [item] (get (meta item) :multi)) (defn pluralize! [item] (if (plural? item) item (transient (with-meta (set (list item)) {:multi true})))) Since transients do not preserve metadata. Is this an oversight in Clojure, or something that's intentionally missing? The following thread throws a little light on the matter but is not conclusive (and I couldn't find anything more recent): https://groups.google.com/group/clojure-dev/browse_thread/thread/acb8e6c1ace4613f/0bd110fd226fe547 In the meantime, how would I best accomplish what I am trying to do with this use of metadata? And here's still another issue: (defn persist-all! [mmap] (let [persisted-map (persistent! mmap) mkeys (keys persisted-map)] (loop [item (first mkeys) restof (rest mkeys) wmap (transient persisted-map)] ....... You can see here that in order to get the keys from the transient multimap, I have to persist it first (since you can't seq on transients, and calling keys requires seq-ability); but then to persist all the sets, I re-convert the persisted transient multimap to a transient (since I'm about to do a bunch of changes on the multimap at once, which would seem to be what transients are for in the first place). That seems redundant, but I haven't found any way around it: I don't see how to get the keys otherwise, and once I've called persist! I can't re-use the underlying transient. I'm sure all these problems are eminently solvable by approaching the problem in a different way; but I thought this was a good opportunity to seed some discussion about using transients (and metadata). -- -- 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 --- You received this message because you are subscribed to the Google Groups "Clojure" group. To unsubscribe from this group and stop receiving emails from it, send an email to clojure+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/groups/opt_out.
