I spent a long time debugging some Clojure code yesterday. The essence of it looked similar to this:
(defn items [] (mapcat expensive-function (range 0 4000 100))) . . . (take 5 (items)) . . . expensive-function is a function that issues an HTTP GET to retrieve a vector of data. Since range's docstring says it returns a lazy sequence, and since mapcat is defined in terms of map and concat, which are both supposed to return lazy sequences, I expected (take 5 (items)) to cause only one HTTP GET. In reality, it causes 32 GETs. That's kind of costly in time and space, considering I merely wanted the first 5 of the 100 items returned in the response to the first GET. This behavior was baffling to me at first, but after some research I found section 12.3 of _The Joy of Clojure_, which mentions that ever since Clojure 1.1 some functions (such as range) which are advertised as lazy are actually moderately eager, realizing chunks of up to 32 elements at a time. I retrieved the release notes for Clojure 1.1. In the section about chunking, it says, "Please share any use cases where chunked processing has resulted in behavioral differences that matter to you on the Clojure Google group." That's why I posted this. -- 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
