(f (first items)) => nil ((f (first items)) (for-each f (rest items))) => (nil (for-each f (rest items))) => (.invoke nil (for-each f (rest items))) => calling a method on nil is a NPE
lists are function applications On Tue, Nov 3, 2009 at 9:33 AM, Dean Wampler <deanwamp...@gmail.com> wrote: > I'm working the exercises in SICP using Clojure and learning Clojure as I > go. I've stumbled across a mystifying NullPointerException. I'm implementing > exercise 2.23, where you're supposed to implement a "for-each" method that > takes a function and a list of items. It applies the function to teach item. > Here's what I tried. > (defn for-each [f items] > (if (not (empty? items)) > ((f (first items)) (for-each f (rest items))))) ; line #3 > > (for-each (fn [x] (println (* x x))) (list 1 2 3 4 5)) > Here's the output: > 1 > 4 > 9 > 16 > 25 > Exception in thread "main" java.lang.NullPointerException (ex2.23.clj:0) > at clojure.lang.Compiler.eval(Compiler.java:4620) > at clojure.lang.Compiler.load(Compiler.java:4934) > at clojure.lang.Compiler.loadFile(Compiler.java:4901) > at clojure.main$load_script__7261.invoke(main.clj:210) > at clojure.main$script_opt__7298.invoke(main.clj:262) > at clojure.main$main__7322.doInvoke(main.clj:337) > at clojure.lang.RestFn.invoke(RestFn.java:413) > at clojure.lang.Var.invoke(Var.java:359) > at clojure.lang.AFn.applyToHelper(AFn.java:173) > at clojure.lang.Var.applyTo(Var.java:476) > at clojure.main.main(main.java:37) > Caused by: java.lang.NullPointerException > at user$for_each__1.invoke(ex2.23.clj:3) > at user$for_each__1.invoke(ex2.23.clj:3) > at user$for_each__1.invoke(ex2.23.clj:3) > at user$for_each__1.invoke(ex2.23.clj:3) > at user$for_each__1.invoke(ex2.23.clj:3) > at user$eval__4.invoke(ex2.23.clj:5) > at clojure.lang.Compiler.eval(Compiler.java:4604) > ... 10 more > > I tried replacing the list in the last line with a vector. Same result. > Suggestions welcome! > dean > > -- > Dean Wampler > coauthor of "Programming Scala" (O'Reilly) > - http://programmingscala.com > > twitter: @deanwampler, @chicagoscala > Chicago-Area Scala Enthusiasts (CASE): > - http://groups.google.com/group/chicagoscala > - http://www.meetup.com/chicagoscala/ (Meetings) > http://www.linkedin.com/in/deanwampler > http://www.polyglotprogramming.com > http://aquarium.rubyforge.org > http://www.contract4j.org > > > > -- And what is good, Phaedrus, And what is not good— Need we ask anyone to tell us these things? --~--~---------~--~----~------------~-------~--~----~ 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 -~----------~----~----~----~------~----~------~--~---
