Although I suppose this isn't too surprising: user> (second y) --> ConcurrentModificationException
--josh On Oct 30, 9:31 pm, Josh Daghlian <daghl...@gmail.com> wrote: > During the Boston Lisp Users meeting last November (?) I asked Rich > about whether seq's on mutable java.util.Collections were really > immutable if the underlying object (the Collection) was mutable. He > emphatically said that no, the seq is still immutable, and that it > caches values as it sees them. I just checked, and for ArrayList it > still works like Rich said: > > user> x > #<ArrayList [foo, bar, baz, quux] > > user> (def y (seq x)) > #'user/ > y > user> (first y) > "foo" > user> (.add x 0 "zzz") > nil > user> x > #<ArrayList [zzz, foo, bar, baz, quux] > > user> (first y) > "foo" > user> > > I am surprised that the same doesn't happen with java arrays. Sounds > like a bug, or at least a change request I'd second. > > After possibly improperly quoting rhickey, Josh slinks away... > --josh > > On Oct 30, 12:40 pm, John Harrop <jharrop...@gmail.com> wrote: > > > > > user=> (def x (int-array 3)) > > #'user/x > > user=> x > > [0, 0, 0] > > user=> (def y (seq x)) > > #'user/y > > user=> (first y) > > 0 > > user=> (aset x 1 3) > > 3 > > user=> x > > [0, 3, 0] > > user=> (second y) > > 3 > > user=> (aset x 0 2) > > 2 > > user=> x > > [2, 3, 0] > > user=> (first y) > > 2 > > > Here, (first y) returned first 0, then 2 without y being rebound in between. > > This is a bit disturbing; some seqs can't be counted on to remain constant. > > I'd expected (first y) once produced as a 0 to be carved in stone, while > > (second y) would be 3 because it would be lazily produced from the array > > only after the corresponding array cell was changed. For this, the seq > > implementation would be something analogous to > > > (defn lazy-array-seq > > ([arr] > > (lazy-array-seq arr 0)) > > ([arr from-idx] > > (lazy-array-seq arr 0 (count arr))) > > ([arr from-idx end-idx] > > (if-not (= from-idx end-idx) > > (lazy-seq (aget arr from-idx) (lazy-array-seq arr (inc from-idx) > > end-idx))))) > > > but apparently it's actually just a very thin wrapper around the array, > > something like a "Java Iterator with copy-on-next semantics". > > > Perhaps it should be changed to behave something more like the > > lazy-array-seq function above? > > > Probably the seq wrappers for java.util collections exhibit the same ... I > > hesitate to call it a "bug", but I'm not sure it's the best choice of > > semantics. My feeling is that once realized seq values should never change, > > however the seq was produced. --~--~---------~--~----~------------~-------~--~----~ 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 -~----------~----~----~----~------~----~------~--~---
