Ah, of course. Thanks. This works:
(defn for-each [f items]
(if (not (empty? items))
(let [] (f (first items)) (for-each f (rest items)))))
(for-each (fn [x] (println (* x x))) (list 1 2 3 4 5))
On Tue, Nov 3, 2009 at 3:13 PM, Kevin Downey <redc...@gmail.com> wrote:
>
> (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?
>
> >
>
--
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
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