#clojure – 2011-04-24

2011-04-24

00:00	KirinDave	The idea is 2 weeks arcs. Kids are signing up for it as opposed to a hs class where they get funneled in. Most have at least dicked around with javascript.
00:01	amalloy	seancorfield__: go try it out. we luvs new users
00:01	technomancy	huh; taking a peek
00:02	technomancy	KirinDave: heh; have you seen the Clojure peepcode?
00:02	technomancy	it's a lot like that.
00:02	KirinDave	Yes.
00:02	KirinDave	Actually
00:02	KirinDave	There are 3 ongoing projects
00:02	KirinDave	In this class
00:02	KirinDave	I hope that this class could get this project to a state where multiple people could log in
00:03	technomancy	yeah, that's a simple way to get people thinking about consistency and concurrency
00:03	KirinDave	In this case, we're covering file i/o, basic data structures, and data driven programming.
00:05	technomancy	sounds like fun
00:05	technomancy	I have a hard time gauging what kids can pull off, but provided the right level of hints I'm sure it would work.
00:05	KirinDave	Yeah
00:06	KirinDave	I will just be in to help out every 2 weeks.
00:17	brunoalfirevic	help /bar
00:17	brunoalfirevic	exit
00:53	seancorfield__	amalloy: i just did the first 40 4clojure problems :)
00:54	amalloy	seancorfield__: nice! glad you found them entertaining. any suggestions for new problems, or improvements in general? also, we welcome contributors at https://github.com/dbyrne/4clojure/
00:56	seancorfield__	the first one was so easy i got it wrong several times thinking it was asking something harder :)
00:56	amalloy	haha
00:57	seancorfield__	then it was a pretty easy run for a while and then suddenly things seemed to get harder than they looked
00:57	seancorfield__	when i got to 41 my first few ideas at a solution were horribly complex
00:58	amalloy	seancorfield__: yeah, i was really thrown by how hard some of them wound up being
00:59	amalloy	i think it may have been 41 that got me as well. i was the one who wrote problem 43, and i was blown away by a solution, much cleverer than mine, that got tweeted
01:00	amalloy	oh no, 44 was the hard one
01:00	seancorfield__	so i stopped to take a break... i'm sure i can figure out a simpler solution soon :)
02:43	joshua__	Whats the Clojure equivalent of writing a nested list comprehension?
02:43	amalloy	joshua__: for supports nesting
02:43	amalloy	&(for [x (range 5) y (range x)] [x y])
02:43	sexpbot	⟹ ([1 0] [2 0] [2 1] [3 0] [3 1] [3 2] [4 0] [4 1] [4 2] [4 3])
02:44	joshua__	amalloy, Nice. Thanks.
02:44	amalloy	note also the :when, :while, and :let keywords in (for...)
03:03	joshua__	amalloy, thanks amalloy, got it working =) https://gist.github.com/939383
03:08	joshua__	Reading up on for has been interesting. I didn't realize how powerful it was: http://www.bestinclass.dk/index.clj/2010/02/clojure-list-comprehension.html
03:09	amalloy	it's pretty massive
03:09	amalloy	not quite as featured as CL's loop, but it doesn't have to be in a functional language
03:09	joshua__	I find it a little funny that I've never had to use it before now.. =p
03:11	amalloy	yeah you have. you've just used something else instead
03:13	amalloy	eg, ##(into {} (map (fn [[k v]] [v k]) {1 2 3 4 5 6})) should really be ##(into {} (for [[k v] {1 2 3 4 5 6}] [v k]))
03:13	sexpbot	(into {} (map (fn [[k v]] [v k]) {1 2, 3... ⟹ {2 1, 4 3, 6 5}
03:13	sexpbot	(into {} (for [[k v] {1 2, 3 4, 5 6}] [v... ⟹ {2 1, 4 3, 6 5}
03:18	amalloy	joshua__: ^
03:21	joshua__	amalloy, Thats what I meant, I think. I've been able to use other things to ape the functionality with some ease.
03:52	amalloy	joshua__: ##(class #{1 2 3})
03:52	sexpbot	⟹ clojure.lang.PersistentHashSet
03:55	matthias__	amalloy, were you the guy who made 4clojure.com?
03:55	amalloy	matthias__: i was the second person to join the 4clojure team
03:55	matthias__	ah
03:55	amalloy	credit for the idea and the first draft goes to dbyrne
03:56	matthias__	i just finished one of the problems and it seems to have disappeared D:
03:56	matthias__	it was the one were interpose was forbidden. or was it interspose?
03:57	matthias__	also, one of the ones i had already solved is now unsolved (the reverse one). although i might have messed up the loging in
03:57	amalloy	matthias__: the problem disappeared, or your solution?
03:57	matthias__	the problem. i cant see it in the list anymore
03:57	amalloy	matthias__: try typing "inter" into the search bar at the top
03:57	matthias__	oh
03:57	matthias__	its on page two
03:58	matthias__	wtf, im sure i did the problems in order, but now it seems like i skipped a bunch. have they been reordered>
03:58	matthias__	?
03:58	amalloy	matthias__: kinda-sorta. we were mis-sorting earlier, for a while
03:58	matthias__	ah, ok
03:59	matthias__	damn, its also unsolved. good thing i have my solution copied D:
04:00	amalloy	matthias__: you don't have an account named matias, do you?
04:00	amalloy	we have users named matthias and matias
04:02	matthias_	i'm matthias
04:02	matthias_	why?
04:02	clojurebot	why not?
04:03	amalloy	i mean, if you accidentally signed in as matias as well as matthias, that would explain why you see as unsolved some problems you thought you solved
04:04	matthias_	yeah, i think its just because i solved the problem while not logged in. but i was pretty sure i logged in then and saw that it was marked as solved.
04:06	matthias_	i think maybe the site should have good example solutions you see after you solve a problem so you can see what proper code would have looked like ;)
04:07	mids	and automatically classify users based on their kind of submissions; 'macro lover', 'java renegate', 'ruby monkeypatcher'
04:07	amalloy	mids: we welcome suggestions that won't result in users hating us :)
04:08	mids	darn :)
04:08	mids	sweet site though, /me signs up
04:15	matthias_	(doc nil)
04:15	clojurebot	java.lang.NullPointerException
04:29	matthias_	i sometimes switch from a list to a vector just to change the order of the result. kinda feels like im doin it wrong
04:32	amalloy	matthias_: that's basically why both exist
04:32	matthias_	heh
04:34	matthias_	almost all my solution are basically like: make a function, make a new function in it with an extra parameter, put and if in there and recur, call the inner function
09:57	kzar	Is there something like reduce that you can use when you've got a list of functions instead of just one?
09:58	shachaf	kzar: What do you mean?
10:01	kzar	shachaf: Well somthing like (reduce [#(+ 5 %) #(+ 3 %)] 0) that returns 8
10:01	shachaf	Oh. Just apply the list of functions in turn?
10:02	matthias_	you could turn those into one function
10:02	shachaf	Reduce the list with composition and id, I guess.
10:02	kzar	Yea it was a dumb example sorry
10:02	shachaf	Or with application and x.
10:03	opqdonut_	it's kinda dumb that #(%1 %2) doesn't have a name
10:04	opqdonut_	or wait, is it (partial partial) or something like that?
10:04	kzar	I'm just hacking about trying to write a function that replaces characters in a string, I've got a list of functions (map (fn [a b] #(.replace % a b)) (seq "_-") (seq "/+")) and now I'm trying to figure out how to reduce the string with them
10:05	Chousuke	kzar: those seq calls are redundant, but hm
10:05	Chousuke	apply comp?
10:06	kzar	oh yea!
10:06	Chousuke	,((apply comp [(partial + 5) -]) 10)
10:06	clojurebot	-5
10:09	kzar	Sweet it actually works, thanks :D
11:17	ev4l	mornin` everyone
11:17	ev4l	anyone knows if there's a problem with mapping a side effecty fn over a seq inside a loop?
11:17	ev4l	https://gist.github.com/939616
11:20	TimMc	ev4l: doall
11:21	TimMc	ev4l: map returns a lazy seq -- so you need to force it
11:22	TimMc	In short... (doall (map ...))
11:23	ev4l	TimMc: hmm nice! I haven't seen that form before =]
11:23	ev4l	gotta remember about the lazyness
11:24	ev4l	TimMc: Thanks alot. It worked!
11:25	TimMc	ev4l: Read up on doall, dorun, and doseq
11:25	TimMc	ev4l: I think you actually want dorun here
11:25	ev4l	TimMc: Already doing it =D
11:28	ev4l	TimMc: it seems that dorun is the one
11:28	TimMc	Yep, you don't care about the return value.
11:29	ev4l	TimMc: I actually don't care about the results, because all i do is to save each element of the seq in the DB
11:29	ev4l	TimMc: :D
11:42	ev4l	TimMc: thanks a bunch!
11:44	TimMc	np
11:44	TimMc	ev4l: And now you know another debugging heuristic: If it looks like a side-effecty piece of your code isn't getting run when it should, look for lazy seqs.
11:49	ev4l	ev4l: Totally :) Initially i thought the problem was with the looping construct (it was the first time i actually used it.haha) , but after digging i came to the conclusion that it had something to do with using map+side-effecty fn
11:50	ev4l	s/:\beval:/TimMc
11:54	robonobo	hi, is there a function that 'repeats' a seq into a new seq? so (repeat '(1 2 3) 3) => (1 2 3 1 2 3 1 2 3) ?
11:55	robonobo	you could do it with for and flatten, but I know in python you can just multiply a list
11:57	opqdonut_	you can use cycle and take, too
11:58	robonobo	opqdonut_: nice, that's less verbose than the for-flatten thing
11:58	opqdonut_	IMO repeat + flatten is the best
11:59	opqdonut_	,(flatten (repeat 3 '(1 2 3)))
11:59	clojurebot	(1 2 3 1 2 3 1 2 3)
11:59	opqdonut_	but I guess there's no standard name for (comp flatten repeat)
12:00	opqdonut_	:)
12:00	robonobo	thanks
12:01	robonobo	wait, is cycle the same as repeat?
12:01	robonobo	ah, no, i see
12:01	robonobo	nvm
12:15	devn	java.lang.NoClassDefFoundError: com/mongodb/DB$WriteConcern (utils.clj:1)
12:15	devn	woops
12:29	rien	seancorfield: cool, thanks!
12:29	rien	technomancy: yes, I know you from #emacs :)
13:03	adamt	hi
13:05	adamt	i'm new with clojure and trying to get composure and com.ashafa.clutch working. https://gist.github.com/939700 is my project file and source. i used lein to install deps, but i'm getting the following error: java.io.FileNotFoundException: Could not locate com/ashafa/clutch__init.class or com/ashafa/clutch.clj on classpath: (core.clj:1)
13:06	adamt	Could anybody perhaps have a look and beat me with a bat while telling me what i did wrong? :)
14:38	ekoontz	so i'm getting runtime errors on my "lein ring" process:
14:38	ekoontz	http://pastebin.com/NuXDbey3
14:38	ekoontz	stack is impossible to debug :P
14:39	ekoontz	it happens randomly
14:39	ekoontz	not every time
16:07	thorwil	my view expects an IFn, but my model delivers a lazyseq
16:18	fliebel	thorwil: Oh, bad. What about #(seq the-lazy-seq) :) joking
16:18	thorwil	even with a cheat-sheet, i can't figure out how to bridge this
16:19	fliebel	thorwil: Depends on the context. What is in the lazy seq, and what kind of fn does the view need?
16:19	thorwil	and now i have to log off :/
16:19	fliebel	me too actually. :/
17:24	joshua__	$findfn (list 1 (list 1 2) 3) [1 [1 2] 3]
17:24	sexpbot	[clojure.set/union clojure.set/intersection clojure.set/difference clojure.core/list* clojure.core/distinct clojure.core/sequence clojure.core/vec clojure.core/concat clojure.core/seque clojure.core/seq clojure.core/doall clojure.core/macroexpand-1 clojure.core/comp ... http://gist.github.com/939892
17:54	joshua__	This is probably a silly question, but the documentation is confusing me. I understand a vector to have O(1) random access, but when I'm reading the docs it says that vectors in Clojure are functions of their indexes which use nth internally. When I read about nth it says that nth is O(n) for sequences.
17:54	joshua__	So would that imply that vectors are O(n) access in Clojure!?!
17:57	joshua__	is reading blogs about vectors now, because that just doesn't sound right
18:03	dnolen	,(supers PersistentVector)
18:03	clojurebot	java.lang.Exception: Unable to resolve symbol: PersistentVector in this context
18:03	dnolen	,(supers (class []))
18:03	clojurebot	#{clojure.lang.IObj clojure.lang.Sequential clojure.lang.Reversible clojure.lang.Counted java.lang.Comparable java.lang.Runnable clojure.lang.ILookup clojure.lang.Seqable clojure.lang.AFn clojure.lang.Indexed ...}
18:03	dnolen	(seq? [])
18:03	dnolen	,(seq? [])
18:03	clojurebot	false
18:03	dnolen	joshua__: ^
18:05	joshua__	Reading about vectors seems to imply that they are stored as a tree format with a branching factor of 32 which leads to a lookup factor of log_32(n). So now I'm thinking they have a log lookup time, which is better than the O(n) I was worried about before.
18:05	joshua__	dnolen, thanks. I get the documentation now.
18:58	rosejn	is it really this empty in here?
19:22	joshua__	rosejn, It is a holiday atm.
19:22	rosejn	I figured most geeks were atheists :-)
19:24	joshua__	rosejn, I'm not. I'm still not looking forward to hunting eggs >.<
19:30	rosejn	joshua, haha, and that's the only fun part. You know the eggs are from the true origins of easter, older pagan religions...
19:33	joshua__	rosejn, I do.
19:37	joshua__	rosejn, also the days of the week are named after pagan gods etc.
20:49	mreynolds	I'm building a protocol parser and I'm trying to do it more lazily than I'm currently doing it. The difficulty I'm running into is that the protocol has bits where I can read an entire "packet" because I know it's size, but also has variable length packets. I can think of a way to handle the known size packets (by reading off the entire thing into a buffer, then parsing the buffer when needed), but i'm not sure how to handle the
20:49	mreynolds	variable bits. Any suggestions or reading material I should look at?
20:50	gregh	does your variable part have a fixed size header? That's a common way to build a protocol. Then you can read the fixed size header and find out how much more to read
20:51	mreynolds	gregh: No, it's not a very well written protocol, and I can't change it :/
20:51	TimMc	joshua__: I celebrated Easter by feeding a Peep to a swan.
20:51	gregh	what determines the size of a variable length packet then?
20:52	mreynolds	gregh: it's something like this : int, int, string, short (short determines next packet, if short is -255, then terminate)
20:52	mreynolds	gregh: It's actually this : http://mc.kev009.com/Protocol
20:53	TimMc	haha
20:53	gregh	at first glance it looks like each variable length part is prefixed by a length
20:54	mreynolds	gregh: Search for "metadata"
20:54	mreynolds	0x28 doesn't have a length, at least the way I'm reading it
20:55	gregh	but http://mc.kev009.com/Protocol#Metadata describes the format of the metadata
20:57	mreynolds	gregh: Right, there's no size. Maybe I'm misunderstanding you? I can read the protocol. But right now I have to parse some packets completely, instead of just reading their length in and parsing it as needed. I'm trying to design something that, by default, just reads the bytes in for later parsing (true for most of the packets), but also can "degrade" to reading in the bytes and parsing them in a non-lazy fashion
20:58	gregh	yeah, it looks like you can't tell how big a metadata packet is without reading at least the structure
20:59	mreynolds	gregh: Yeah. Is there a known pattern for this style of parsing? I've been sort of beating my head against this problem for a bit without much success.
20:59	gregh	you could have a reader that just reads the bytes of a full packet, with knowledge of the internal structure
21:00	gregh	then have something else that takes a packet and extracts all the data items (ie. knows the meaning of the bytes)
21:00	mreynolds	gregh: Yeah, that's the path I went down, but got stuck when trying to do the optimized route of reading a full packet if none of the pieces are of unknown size
21:01	gregh	quit trying to optimize at this point :)
21:01	mreynolds	heh, yeah, agreed
21:01	mreynolds	I'll do the easy thing first
21:02	gregh	make it work first, because you can't get very far if it doesn't work
21:03	mreynolds	Yeah, I have the version based on a stream working first, but not as a sequence applying the operations against a stream. I'll do that first.
23:18	seancorfield__	amalloy_: hey! no fair! i'd got just one problem left to solve on 4clojure when i went out to dinner - when i came back you'd added another one!
23:18	seancorfield__	done it tho'... just #53 to do...
23:49	amalloy	seancorfield__: hey, i'm in the same boat here. i go out for the day, and dbyrne keeps dragging me back with new problems i have to solve while on my android
23:50	dbyrne	amalloy, seancorfield: at least now we have an rss feed you can monitor so you know when new problems have been posted
23:55	dbyrne	seancorfield__: #53 is a tricky one...I don't think I've seen an elegant solution to it yet.