#clojure – 2008-05-22

2008-05-22

11:07	leafw	Hi. I'm having trouble iterating lists of elements
11:07	leafw	if I use a loop and call (.get sequence i), it's ugly.
11:08	leafw	also, the loop requires an (if (< ....) this that) where 'this' can't be more than one statemnt (no progn?)
11:08	leafw	I'd appreciate suggestions on how to loop properly a sequence
11:08	asbjxrn	(doseq item list ....) ?
11:08	rhickey	is there some reason why map/reduce/doseq etc are unsuitable?
11:09	leafw	never tried
11:09	leafw	yes: because I don't know how to use them. Will soon.
11:09	ozzilee	(doseq x [1 2 3 4 5] (println x))
11:09	rhickey	ok :)
11:09	leafw	thanks for the keywords.
11:13	leafw	and, is there a way to do a multiline comment in a text file?
11:14	asbjxrn	(comment ...) is the one I use
11:14	leafw	thanks.
11:15	rhickey	comment is pretty much it right now, its main limitation being that the contents must be read-able
11:15	leafw	read-able meaning, parsable
11:15	rhickey	kind-of
11:15	leafw	ok
11:15	leafw	a triple ;;; .... ;;; could imitate python's """ .... """
11:16	leafw	but I know that other lisps don't do it.
11:21	leafw	in a map call, how can one give arguments to the mapped function?
11:21	leafw	can't get the syntax right.
11:22	asbjxrn	(map (lambda (x) (println x)) '(1 2 3)) ?
11:22	asbjxrn	Not sure I understand the question, map by definition calls the mapped function with an argument.
11:23	asbjxrn	(eh, lambda should be (fn [x] (println x)) .... )
11:24	asbjxrn	Is there a function? function? (How to test if a symbol is a function?)
11:24	rhickey	leafw: the first thing you must determine is, are you iterating for side-effects or to calculate a result?
11:24	leafw	suppose I have a function named 'find-all', that takes one arg, and a list of 'children': (map (list-all [sb]) children) ... fails
11:24	leafw	I am iterating for side effects.
11:24	leafw	I accumulate thousands of strings in a StringBuffer.
11:24	rhickey	then you should be using something starting with do
11:24	leafw	doseq works
11:25	leafw	I am just trying to see how I could use map as well.
11:25	rhickey	accumulating is not really side effects
11:25	leafw	kind of. I could also return the string and just append it, recursively.
11:26	rhickey	you could just reduce - see the implementation of str in boot.clj
11:26	leafw	thanks.
11:27	leafw	by the way, is there any effort to make error reporting better?
11:27	leafw	like: Class java.lang.StringBuffer does not implement the requested interface clojure.lang.IFn ---> means: don't try to evaluate an instance of a java object, you can't.
11:27	rhickey	that's kind of general, any specific case?
11:29	rhickey	what was the code?
11:29	leafw	also, the jdbc for postgresql uses a system of double exception: there is an SQLException that carries a second error within it, which is clear and specific to the case. For clojure, the second one would be the one I'd like to see all the time, not the first (the java one).
11:30	leafw	the code was my silly effort to make map accept a function with an argument: (map (find-all (sb)) children) .... where 'sb' is a StringBuffer instance.
11:32	rhickey	yeah, the operator position just gets a direct IFn call, so will just give IncompatibleClassChangeError, or ClassCastException
11:33	rhickey	to do otherwise would seriously compromise the speed of Clojure
11:33	leafw	which doesn't help much ... until one gets it.
11:34	rhickey	I've thought about a debug mode, which would be slower but more verbose - no time for that right now
11:34	leafw	I'm looking forward to it. Exceptions have been quite puzzling so far.
11:34	ozzilee	Could you could have the REPL catch exceptions and examine the call stack to provide more helpful errors in certain cases?
11:35	ozzilee	Not sure if that would be easier to implement than a debug mode.
11:36	rhickey	One of the problems is that ClassCastException says only what you had, not what it was trying to cast to - if it said:
11:37	rhickey	java.lang.ClassCastException: java.lang.Integer is not java.lang.IFn
11:37	rhickey	that would be a lot better
11:37	leafw	indeed.
11:39	leafw	guys, I still don't see it: can a non-lambda function that takes arguments be mapped to a sequence?
11:39	leafw	I'd appreciate help
11:39	rhickey	user=> (map inc [1 2 3])
11:39	rhickey	(2 3 4)
11:39	ozzilee	leafw: You need to create a lambda so that the fn only takes one argument
11:40	ozzilee	(map (fn [x] (find-all sb x)) [...])
11:40	rhickey	or, if it takes multiple args, pass multiple seqs
11:40	leafw	I see
11:40	leafw	then it's definitely not the proper construct, I'd be misusing it.
11:40	leafw	thank you.
11:40	ozzilee	More concretely: (map (fn [x] (+ x 1)) [1 2 3 4])
11:41	rhickey	(map (partial find-all sb) ...)
11:41	leafw	partial -now that's a new one
11:42	ozzilee	I'm not sure what you're trying to do, but reduce will most likely do it for you.
11:42	ozzilee	If you'd like to read up: http://www.cs.nott.ac.uk/~gmh/fold.pdf
11:42	ozzilee	(reduce is pretty much fold with a different name)
11:42	ozzilee	(warning: pdf)
11:43	asbjxrn	In case everyone missed it earlier: Is there a way to test if something is a function?
11:43	leafw	I understand. Thanks ozzilee
11:43	rhickey	asbjxrn: (instance? clojure.lang.IFn x)
11:44	asbjxrn	Thanks.
11:45	asbjxrn	I thought about instance?, but with clojure.fns
11:45	ozzilee	So, on the subject of predicates... doesn't make sense to me to call out to java to test things internal to clojure. Shouldn't there be a (fn?) predicate?
11:45	leafw	that's an ugly one ... ready for a (defn is-fn [x] ...)
11:47	rhickey	ozzilee: it's a slippery slope. If you ar egoing to do type testing you need to understand the type system
11:48	rhickey	for instance (fn? :fred) would be true
11:48	rhickey	also (fn? {}) and (fn? [])
11:48	ozzilee	(shrug) so call it (callable?). That would be accurate.
11:49	rhickey	There are an indefinite number of types, I don't want a corresponding set of ?s
11:50	ozzilee	I just find it ugly to need to think about Java to do things that don't otherwise call out to Java code.
11:50	rhickey	If you import some clojure.lang it would just be (instance? IFn x)
11:51	ozzilee	Yeah, idk. instance == Java for me.
11:51	ozzilee	nit-picking :-)
11:51	rhickey	oh, well, even if there were no Java I'd still prefer (instance? type) to (this-type?, (that-type? etc
11:52	leafw	on anothe topic: is it possible to have more than one clojure repl running in the same JVM? The init part looks static to me
11:53	ozzilee	Well, there's only so many types... it's not like Clojure itself gives the user power to create arbitrary new types, unless I'm mistaken.
11:53	rhickey	leafw: there's been a lot of discussion on the google group re: hosting Clojure
11:53	leafw	thanks
11:53	rhickey	ozzilee: with genclass there is
11:54	ozzilee	Hmm. Ok, I'll have to look into that, never heard of it.
11:54	asbjxrn	To me one of the main features of clojure is that it's so closely tied to Java. So embracing java comes with the territory in a way.
11:55	ozzilee	Yeah, but if I wanted to think about Java in every line of code... I'd write in Java :-)
11:56	ozzilee	No biggie, I'll write my own callable?, list?, etc, as I suspect many others will.
12:00	leafw	rhickey : can't find "Re: hosting Clojure" on google groups ... ?
12:01	asbjxrn	Maybe this thread is what you're looking for?
12:01	asbjxrn	http://groups.google.com/group/clojure/browse_thread/thread/d98cedb860f16a34/136ca458a73dc643?q=hosting&lnk=ol&
12:02	leafw	apparently I signed up too late. Thanks.
12:06	leafw	sentry out.
12:33	dudleyf	Wow.
12:33	dudleyf	That was an incredibly enlightening thread.
12:38	rhickey	ozzilee: encapsulating Java is a non-goal of Clojure. But even heavy use of Java is still 1/3-1/10th the size in Clojure
12:46	rhickey	http://sourceforge.net/community/cca08
12:49	drewr	Ugh, requires login.
12:50	dudleyf	rhickey: What do you think about nested namespaces? Are they possible? feasible? desirable?
12:51	rhickey	I think they are complex and not needed
12:52	rhickey	namespace aliases would be useful
13:03	dudleyf	Are namespaces the mechanism for grouping together related functions?
13:04	dudleyf	Like modules in Haskell?
13:04	dudleyf	So that a given project would probably have multiple namespaces?
13:48	leafw	hi all. I have another very basic question that keeps scaping me
13:48	leafw	how to set a valuble to a clojure-defined variable, for instance from a (let [sum 0] ...)
13:49	leafw	I can't use reduce in this case because it's an array of byte[]; with reduce, the second element is bit-and 255 ok, but can't do so for the very first element.
13:49	leafw	so I (doseq pixel pix ...) to sum, but I can't figure out how to set the value of sum once it's declared!
13:49	leafw	any help appreciated.
13:50	ozzilee	leafw: Try with-local-vars and var-set/var-get perhaps.
13:51	ozzilee	leafw: You can supply reduce with a starting value, as well.
13:51	leafw	nice!
13:51	ozzilee	(reduce + 10 [1 2 3 4 5]) ;; gives 25
13:51	leafw	meaning, then it will start at starting-value + first-element ?
13:52	leafw	thanks ozzilee, will try al.
13:54	ozzilee	You can do some neat things with reduce. Your starting value doesn't need to be the same type as the rest of the values.
13:54	ozzilee	(reduce (fn [a x] (str a x)) "" [1 2 3 4 5])
13:54	ozzilee	Gives you "12345". A bit contrived, maybe.
13:54	leafw	xD
13:54	leafw	indeed
13:54	leafw	but makes sense
13:54	leafw	thank you
13:55	ozzilee	(reduce (fn [a x] (+ a (count x))) 0 ["alice" "bob" "carol"]) ;; Sum the number of chars.
13:56	ozzilee	It'll do pretty much anything, might get hard to read if you do too much though :-)
14:03	leafw	:)
14:03	leafw	will be careful.
14:09	ozzilee	pastie question: "manipulating vars in loaded file" http://paste.lisp.org/display/61148
14:09	ozzilee	Any help with the above would be much appreciated.
14:10	ozzilee	I've got to head out for a bit, back in a couple hours.
14:18	rhickey	more like packages in Java
14:19	dudleyf	rhickey: But flat
14:20	rhickey	Java packages are also flat, dots in names are a convention for hierarchy
14:21	ozzilee	rhickey: Naming quesiton: would it be more canonical to have http/get, or http/http-get?
14:22	dudleyf	I'm not a Java programmer, so forgive me, but if you have some.package.Thing, can't you import some.* and then reference package.Thing?
14:23	dudleyf	ozzilee: I hope the answer is http/get
14:25	dudleyf	otherwise the namespace is fairly irrelevant
14:30	ozzilee	dudleyf: Right, but in that case it would shadow the clojure/get on (refer 'http). Is that something to be avoided? I do not know.
14:31	dudleyf	Is there away to rename imports?
14:32	rhickey	dudleyf: imagine clojure.lang., java.lang. - lang.Thing means which?
14:33	dudleyf	A compile error?
14:33	dudleyf	But point taken
14:35	rhickey	It's a compile error in any case
14:36	rhickey	ozzilee: http/get
14:36	ozzilee	rhickey: Thanks.
22:06	ozzilee	pmap
22:07	ozzilee	Hey go figure, that doesn't work in this buffer (grumble)
22:10	Chouser	heh
23:08	abrooks	Heh. http://www.gadgetell.com/tech/comment/firefox-usage-spreads-its-now-found-even-in-outer-space/
23:10	abrooks	Ooops.
23:11	abrooks	Wrong window.
23:17	Chouser	I wondered.