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 -~----------~----~----~----~------~----~------~--~---
