#clojure – 2008-09-22

2008-09-22

09:50	rhickey	I've added print-method multimethod and switched printing to use it
09:50	rhickey	built on contribution from Erik Soehnel
09:55	wwmorgan	rhickey: that's a lot cleaner than the old way of doing it
09:56	wwmorgan	also I think it should recognize Patterns and print them using the #"foo" form
09:56	rhickey	wwmorgan: right, it needed the isa multimethod enhancements I added this summer
09:56	rhickey	wwmorgan: ah yes, was waiting for this to do patterns...
10:02	rhickey	wwmorgan: I've added print-method for java.util.regex.Pattern
10:02	wwmorgan	rhickey: sweet
11:26	Chouser	excellent, now all I need to do is implement MultiFn in JS, and printing should work.
11:27	abrooks	Chouser: Great work!
13:42	jgracin	rhickey: I see that 1040 revision has fixed print-method exception. However, I'm still get the following when compiling code wlth failures, from Slime: ...(incoming paste)
13:42	lisppaste8	jgracin pasted "Compiler exception" at http://paste.lisp.org/display/67278
13:47	rhickey	jgracin: there's too much going on there for me to suss this out - can you narrow it down at all?
13:47	jgracin	rhickey: I'll try.
13:57	jgracin	rhickey: it seems to me that during compilation, LazyCons.first is called and it throws RuntimeException, and this exception is not encapsulated in CompilerException, which is what Slime expects. Any ideas? I'll keep digging...
13:58	rhickey	seems to me Slime shouldn't hardwire CompilerException
14:03	leafw	I have a conceptual problem. I am far too used to looping; suppose I have a list of objects, of which I have to find 3, given 3 desired names. I.e. getTitle has to be called on each object. What I would like to get back is a table of name - object. What is a good strategy in clojure to do that?
14:05	leafw	somwhow, is not obvious to me. I could do it perhaps with a recursive function that returns a table with a new entry, that table being given as argument ad thus extended, or nothing when reaching the end of the list.
14:09	ozzilee	leafw: You could do it with reduce: (reduce (fn [a x] (if (is-something-we-want (.getTitle x)) (assoc a x (.getTitle x) a) {})
14:09	cemerick	leafw: given a seq of those objects, this would give you a map of name ==> object: (apply hash-map (mapcat #(list (.getTitle %) %) seq-of-objects))
14:10	ozzilee	leafw: Where is-something-we-want is whatever determines whether that title is the one you want.
14:11	wwmorgan	leafw: here is one way to do it: http://paste.lisp.org/display/67282
14:11	leafw	thanks people, let me digest it
14:12	cemerick	I often end up using filter and map instead of reduce for stuff like that -- I've never liked the position of the first value in the 3-arg version of reduce.
14:13	ozzilee	cemerick: I prefer fold myself, as in (fold fn initial coll). But meh :-)
14:18	leafw	ozzilee: your pasted solution is missing parentheses...
14:19	leafw	cemerick: your solution looks really cool -- will it accept any list, like a java.util.ArrayList ? Or does it need a clojure list?
14:19	ozzilee	leafw: That's possible.
14:19	leafw	wwmorgan: thanks for that post, just got nice insight into memfn
14:20	leafw	It's amazing there are so many solutions
14:20	cemerick	leafw: seq-of-objects can be anything that's seq-able -- all collections can be treated as seqs
14:20	ozzilee	(reduce (fn [a x] (if (is-something-we-want (.getTitle x))) (assoc a x (.getTitle x) a) {}) ; Better.
14:22	leafw	thank you
14:23	leafw	hum, ozzilee : the 'if' has no body as you declared it.
14:26	ozzilee	Yeesh, writing lisp without emacs doesn't work out so well. This should be correct, sorry about that: (reduce (fn [a x] (if (is-something-we-want (.getTitle x)) (assoc a x (.getTitle x)) a)) coll {})
14:28	leafw	ozzilee: so you are doing: reduce fn list1 set1 , is that right?
14:29	ozzilee	(reduce function seq initial-value), yeah.
14:29	leafw	aha
14:29	leafw	intial-value ... got it, got it.
14:29	ozzilee	We start with an empty hash-map, and then assoc in x if it's something we want, otherwise we return what we've reduced so far (a).
14:30	leafw	I understand it. Thanks.
14:31	ozzilee	Ok. Next time I'll check my parens before I post :-)
14:31	rhickey	(into {} (for [x coll :when (we-like x)] [(key-for x) (val-for x)]))
14:32	cemerick	hrm, forgot about into...
15:48	leafw	for all clojure's power, debugging is one of its weaknessess: the stack traces never mean much to me ...
15:48	abrooks	leafw: There's been several threads about this on the Clojure group.
15:49	abrooks	leafw: And I have to agree.
15:49	TreeRex	I found it just takes practice.
15:49	abrooks	If I get things right, it just works. If I screw up I get to stare blankly at the stack trace and flail around until I figure out what's wrong.
15:50	leafw	problem is, when an exception is thrown on code pasted to an interpreter, it doesn't give back anything useful such as the names of the function where it failed, and its parent stack of caller functions.
15:50	abrooks	It seems that with all the introspective abilities, we should be able to provide much more useful information.
15:51	leafw	that alone would help a lot, much more than (defn pdatas (reduce (fn [project ptable] (assoc ptable project (create-pdata project)) ptable) (ControlWindow/getProjects) {}))
15:51	abrooks	leafw: By interpreter you mean REPL?
15:51	leafw	oops
15:51	leafw	bad paste (we luve linux pasting)
15:51	leafw	yes, REPL
15:56	leafw	at least, functional-style makes things trivial to debug manually
15:56	leafw	but I can't imageine what it will be to debug a large project.
15:56	leafw	even making on-site test functions is trivial and almost natural.
15:57	abrooks	If C compilers can print out the offending line with a "------^ " pointing to the offending expression / token it seems that Clojure should be able to as well.
15:58	leafw	I wish I knew how. I may dig in one of this days, into Clojure's exception handling system. With all the metadata, there must be a way to just print the stack of functions.
15:59	ozzilee	leafw: I actually hacked that in the other day on my lunch break, it shouldn't be too tough to do it nicely.
16:01	lisppaste8	ozzilee pasted "chicken's errors" at http://paste.lisp.org/display/67284
16:01	ozzilee	fwiw that's how chicken scheme shows errors, which I always appreciated. Other lisps may do something similar.
16:03	abrooks	ozzilee: That's exactly what I'd like to se.
16:03	abrooks	see
16:04	leafw	yeah, that's the right direction IMO
16:09	ozzilee	Should be do-able, but... "not it!" :-)
16:09	abrooks	ozzilee: :)
16:22	TreeRex	While I haven't used it, the screenshots of the Enclojure debugger look promising: perhaps there is some useful functionality in that IDE which could be moved into the REPL.
17:03	leafw	my problem with enclojure and the like is that its not an editor (aka it's not vim, it's not emacs)
17:03	leafw	I would use it only for debugging, if they make that worthwhile.
17:11	ozzilee	parth_m started a repl written in clojure, might be something worth looking at: http://code.google.com/p/iclj/
17:41	leafw	is there any built-in way to do a .toArray() to a list? Aka put all elements of a list into an array of a certain type. I can do that with a reduce, but I wonder if there are easier ways.
17:45	ozzilee	(into-array seq) ?
17:47	leafw	nice
17:47	leafw	thanks
18:36	leafw	and using reduce/map and the like, how can one add all values of a half-matrix? I.e. all values of an array NxM, where only values under the diagonal should be added.
18:44	rhickey_	leafw: try for
18:46	leafw	still te problem of a mutable var that accumulates the sum
18:47	rhickey_	the problem is in wanting one - just pick the values you care about first, then add them with reduce +
18:48	leafw	that means a huge memory increase -- which is fine in most occasions, but not in this one.
18:49	leafw	thanks anyway
18:49	rhickey_	um, no, since seqs can be lazy
18:49	leafw	if anything, there are integer indices to the arrays right?
18:49	rhickey_	for yields a lazy seq, for instance
18:49	rhickey_	range yields a lazy sequence
18:49	rhickey_	etc
18:50	leafw	I think I don't understand lazy seqs them
18:50	leafw	s/them/then
18:53	rhickey_	try (reduce + (range 100000000)) and watch memory use, it doesn't budge
18:54	leafw	meaning, each element of the sequence is returned without the sequence existing in memory?
18:55	rhickey_	right
18:56	rhickey_	a lazy seq is a recipe for a seq, the whole seq need never exist. As you call first/rest you get bits of it created on demand, as long as you don't hold onto them, they are discarded after you use them
18:56	rhickey_	cycle returns an infinite seq, for instance
18:56	rhickey_	you'll never use all of it
18:57	leafw	very nice. Now it's my task to figure out how to use it for the half-matrix sum.
18:58	rhickey_	try for
18:58	rhickey_	:)
19:04	leafw	so I need a lazy seq of lazy seqs right ?
19:04	leafw	but then I need to flatten it ... man, thinking this way is totally new
19:05	rhickey_	(for [ x (range 3) y (range 3)] [x y])
19:06	leafw	(for [x (range 10) y (range x)] [x y])
19:06	leafw	interesting
19:09	rhickey_	there's also :when and :while options
19:11	leafw	for a NxN matrix, the half-matrix is (for [x (range 0 N) y (range 0 x)] [x y])
19:11	leafw	the lower one, at least, without the diagonal of zeros.
19:48	ericthor	Has anything changed with arglists recently? If I say (meta #'clojure/*) I'm getting an IndexOutOfBoundsException when the REPL attempts to print the arglist
19:51	aking	ericthor: works here
19:52	ericthor	aking: what svn revision are u using?
19:52	aking	ericthor: 1039 - trying with 1040
19:53	ericthor	aking: could be time for a full rebuild on my end here...
19:53	aking	hmm.. doesn't work with 1040
19:54	ericthor	phew
19:54	ericthor	thanks
19:54	ericthor	It appears to be the 0th element
19:55	ericthor	is said to be out of bounds
19:55	ericthor	1 2 3 all ok of 4
19:55	ericthor	I'll back up to 1039
19:56	aking	ericthor: it might have been 1038 - I had updated last night, but there looks to be 2 updates today
19:56	ericthor	aking: ok thanks...I'll try both
20:07	aking	ericthor: looks like I was at a slightly older version then I thought - anything after 1037 breaks
20:07	ericthor	aking: 1037 is oko?
20:07	ericthor	ok?
20:07	aking	yup
20:08	ericthor	aking: I was just doing the same thing...saved me the trouble. Thanks!
20:34	rhickey_	fixed print-method on empty vector
20:35	rhickey_	rev 1041
21:30	ericthor	rhickey: Thanks!
21:30	rhickey_	sure, new print multimethods getting shaken out...
23:32	yangsx	what is the best way to use clojure.contrib? I'm trying clojure with the svn repo