How to use realized? on a lazy sequence?
Here's a REPL session, wherein I try to use realized? on a lazy-seq. Clojure 1.3.0 Define the lazy-seq: user= (def naturals (iterate inc 0)) #'user/naturals Force realization of the first 1 + 123456 elements: user= (time (nth naturals 123456)) Elapsed time: 481.349 msecs 123456 Due to previous realization, the same expression now eval's quickly: user= (time (nth naturals 123456)) Elapsed time: 15.571 msecs 123456 Now I try to use realized? on 123456th element: user= (realized? (nth naturals 123456)) ClassCastException java.lang.Long cannot be cast to clojure.lang.IPending clojure.core/realized? (core.clj:6505) Ouch! I guess realized? isn't a macro. Next try: user= (realized? (drop 123456 naturals)) false Hmmm... could I be off by one? Let's leave lots of room for error: user= (realized? (drop 12345 naturals)) false user= (realized? (drop 0 naturals)) false Huh? How do I get realized? to tell me what I want to know? What I want to know, is wether the element at the nth index has been computed and cached. Maybe something like realized- length, just for lazy-seq's, which would report how far along the sequence has realization occurred. Thanks, George Kangas -- 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
Re: How to use realized? on a lazy sequence?
George Kangas gwkan...@gmail.com writes: Now I try to use realized? on 123456th element: user= (realized? (nth naturals 123456)) ClassCastException java.lang.Long cannot be cast to clojure.lang.IPending clojure.core/realized? (core.clj:6505) Hm, that's strange indeed. I also get user (realized? (iterate int 0)) ;clojure.lang.Cons cannot be cast to clojure.lang.IPending ; Evaluation aborted. where I'd expect it to return false. However, it works just fine with `range' in place of iterate: user (realized? (range 0 10)) false user (realized? (doall (range 0 10))) true Bye, Tassilo Ouch! I guess realized? isn't a macro. Next try: No, it's a function. But why should that be important. Ok, before a funcall all args are evaluated, but the result of evaluating a lazy seq is still the same lazy seq. Evaluation doesn't realize anything, only calling first/rest on the seq does. Huh? How do I get realized? to tell me what I want to know? What I want to know, is wether the element at the nth index has been computed and cached. Ditto. Bye, Tassilo -- 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
Re: How to use realized? on a lazy sequence?
Can't you write that function yourself? (defn realized-length [xs] (loop [n 0 xs xs] (if (realized? xs) (recur (inc n) (rest xs)) n))) drop returns a new lazy sequence, with no realized elements, so naturally you can't ask if the sequence under it is realized. If you want to work at such a low level you can't build a new lazy sequence; the above avoids that problem by working with the sequence primitives. user= (def naturals (rest (iterate inc 0))) #'user/naturals user= (realized-length naturals) 0 user= (take 10 naturals) (1 2 3 4 5 6 7 8 9 10) user= (realized-length naturals) 10 Note that this does not work for the base case of an iterated sequence, because that is not a lazy-seq but a cons. Seems a bit weird to me, but then realized? itself is a bit weird... On Oct 7, 1:46 pm, George Kangas gwkan...@gmail.com wrote: Here's a REPL session, wherein I try to use realized? on a lazy-seq. Clojure 1.3.0 Define the lazy-seq: user= (def naturals (iterate inc 0)) #'user/naturals Force realization of the first 1 + 123456 elements: user= (time (nth naturals 123456)) Elapsed time: 481.349 msecs 123456 Due to previous realization, the same expression now eval's quickly: user= (time (nth naturals 123456)) Elapsed time: 15.571 msecs 123456 Now I try to use realized? on 123456th element: user= (realized? (nth naturals 123456)) ClassCastException java.lang.Long cannot be cast to clojure.lang.IPending clojure.core/realized? (core.clj:6505) Ouch! I guess realized? isn't a macro. Next try: user= (realized? (drop 123456 naturals)) false Hmmm... could I be off by one? Let's leave lots of room for error: user= (realized? (drop 12345 naturals)) false user= (realized? (drop 0 naturals)) false Huh? How do I get realized? to tell me what I want to know? What I want to know, is wether the element at the nth index has been computed and cached. Maybe something like realized- length, just for lazy-seq's, which would report how far along the sequence has realization occurred. Thanks, George Kangas -- 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
Re: How to use realized? on a lazy sequence?
On Oct 7, 5:02 pm, Alan Malloy a...@malloys.org wrote: Can't you write that function yourself? (defn realized-length [xs] (loop [n 0 xs xs] (if (realized? xs) (recur (inc n) (rest xs)) n))) Thanks, Alan! drop returns a new lazy sequence, with no realized elements, I didn't realize that! I expected that drop would call rest recursively, like your realized-length does. But apparently, it's lazier than that: user= (def c (drop 12345678 naturals)) #'user/c user= (time (first c)) Exception in thread main java.lang.OutOfMemoryError: Java heap space (more java.lang.barfing) user= (def naturals (rest (iterate inc 0))) #'user/naturals You introduce another subtlety here: realized? won't work on the result of iterate (as Tassilo Horn found), but it will work on (rest (iterate ...)). And this is because: Note that this does not work for the base case of an iterated sequence, because that is not a lazy-seq but a cons. Seems a bit weird to me, but then realized? itself is a bit weird... realized? doesn't seem so weird, when it deals with promises, delays and futures. Those objects have explicit API's for their creation and realization, and realized? has a natural and predictable role in that API. Lazy sequences are more magic, and less explicit about creation and realization (or so it seems to me). Implementors of functions such as iterate, drop, rest, range, etc., have some freedom to decide times of creation/realization. Then, when you try to use realized? on lazy sequences, those decisions (which you may not have expected) come to the surface. Maybe realized? doesn't really belong in the lazy sequence API, but it is fun to play around with it. Thanks for showing me how! regards, George -- 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
