#clojure – 2008-06-15

2008-06-15

08:48	rhickey	Chouser: a var root is an atomic thing (a single Object). For last-one-in wins, write-over behavior nothing special is needed, other than that it be volatile, which it is.
08:49	rhickey	read-modify-write is trickier, and that is why there are synchronized bindroot and commuteRoot, which you can use for safe ongoing multithreaded mutation of a var root (not recommended generally)
09:05	meredydd	What's the idiomatic way of dealing with Maps and Lists passed to you by Java code?
09:07	meredydd	A lot of the syntactic pretty (using assoc to add elements, invoking a map to look up elements) is understandably dependent on the object being a Cojure persistent map
09:08	meredydd	But is there a sensible way to do a conversion?
09:10	meredydd	(sorry, I mean "idiomatic" rather than "sensible". I'm aware it's possible to (reduce) across (keys x), using (assoc)...)
10:02	rhickey	meredydd: (into {} your-java-map)
10:03	meredydd	aha. Thankee, rhickey.
10:05	rhickey	but note that seq/get/contains?/count/destructuring etc work for Java maps without conversion
10:55	meredydd	rhickey: Is the website your patch? If so, the redirects from sourceforge.net are all broken after the redesign (which is a pain, as they tend to score high on google)
13:03	jcrites_	how does one partially apply a function in closure?
13:03	jcrites_	say I want a function that adds one to a number
13:03	jcrites_	I want something like ....(+ 1)
13:03	jcrites_	do I need to use the fn form?
13:03	jcrites_	or is it possible to partially apply functions to other functions?
13:03	jcrites_	(or values)
13:06	jcrites_	ah, I see..... (partial + 1) is the same as (fn [x] (+ 1 x))
13:09	hoeck	jcrites: you can use #(+ 1 %) too
13:09	jcrites_	oh? cool, thanks!
13:09	jcrites_	what form is that exactly?
13:10	hoeck	jcrites: its an abbrevation for fn
13:12	hoeck	#(...) => (fn [args] (...))
13:15	jcrites	is % special then?
13:16	Chouser	jcrites: inside #(), yes it is. Also: %, %1, %2, etc. and %&
13:16	Chouser	oh, % is apparently special all the time -- only allowed inside #()
13:18	jcrites	% vs %&?
13:19	jcrites	is %& "rest"?
13:19	Chouser	yup
13:19	jcrites	like %1 %&
13:19	jcrites	ah, k
13:19	Chouser	yup
13:19	jcrites	Clojure is pretty neat
13:19	Chouser	yup
13:19	jcrites	:)
13:19	Chouser	that little feature is documented at http://clojure.org/reader?responseToken=e9a25f9ed12c8e758646d63847044670 -- search for Anonymous
13:29	jcrites	hmmm
13:29	jcrites	how come closure lists print with ( ) ?
13:30	Chouser	classic lisp. what should it use?
13:30	jcrites	this confuses me
13:30	jcrites	> [1 2 3]
13:30	jcrites	[1 2 3]
13:30	jcrites	> (rest [1 2 3 4])
13:30	jcrites	(2 3 4)
13:31	jcrites	is it something to do with sequences?
13:31	jcrites	I would expect (rest [1 2 3 4]) to print [2 3 4] not (2 3 4)
13:31	jcrites	maybe there is some subtle property I am missing
13:31	Chouser	that's a great question.
13:32	Chouser	I think sequences also print with ()
13:32	jcrites	> (list 1 2 3)
13:32	jcrites	(1 2 3)
13:32	jcrites	so I guess [] is just for vectors
13:32	Chouser	(seq [1 2 3]) ==> (1 2 3)
13:32	Chouser	yes, [] is just for vectors
13:33	Chouser	"rest" and "first" work on sequences, and "rest" always returns a sequence.
13:33	jcrites	can a sequence also be a vector?
13:34	jcrites	i.e., let's say I do (rest [1 2 3]) and I get a sequence. can I get back to the vector 'type' and index on it?
13:34	jcrites	or is it hidden by the rest?
13:35	jcrites	well let me find out :)
13:35	jcrites	but even if it works it doesn't mean it's guaranteed to work right?
13:35	Chouser	sorry, watching kids too here...
13:35	Chouser	seqs are generally views of the underlying object
13:35	Chouser	I don't think you can get back to the underlying object from a seq
13:36	Chouser	but you can choose to use more specific functions.
13:36	Chouser	for example, cons works on seqs and returns a seq: (cons 4 [1 2 3]) => (4 1 2 3)
13:37	jcrites	how can cons take [ ]? is it treated like (cons 4 (seq [1 2 3])) ?
13:38	Chouser	exactly
13:38	Chouser	all the seq-based functions wrap a (seq ...) call around their collection parameter
13:38	Chouser	on the other hand, conj is implemented for each of the collections seperately
13:39	Chouser	(conj [1 2 3] 4) ==> [1 2 3 4]
13:39	Chouser	(conj {1 "one", 2 "two"} [3 "three"]) ==> {1 "one", 2 "two", 3 "three"}
13:48	meredydd	Anyone care to enlighten me as to why macros can be passed as function parameters without the compiler squawking?
13:49	meredydd	I just attempted to use (and) in a (reduce), and got a nasty surprise
13:49	meredydd	It executed just fine, but using the macro's code-transformation function
13:51	meredydd	so, if I try: (def a true) (def b false) (def c true)
13:52	meredydd	and then (reduce and [a b c])
13:52	meredydd	I get: (clojure/let [and__152 (clojure/let [and__152 true] (if and__152 (clojure/and false) and__152))] (if and__152 (clojure/and true) and__152))
13:52	meredydd	(where I'd normally be expecting 'false')
13:53	meredydd	I appreciate that it might be a good idea to allow people to access a macro as a function...but does it have to happen silently like that?
13:54	hoeck	meredydd: macros are functions that are handled specially by the compiler
13:55	hoeck	and its bad the compiler doesn't warn if you pass them around as functions
13:55	meredydd	I agree with you on both counts.
13:56	meredydd	(although there should be a way to pass a macro as a funarg if you assure the compiler you know what you're doing)
13:58	hoeck	i guess macros being functions is an implementation detail for clojure
14:02	jcrites	what's the idiomatic way to represent factorial recursively in Clojure?
14:05	jcrites	(defn fact [n] (* n (if (> n 1) (fact (- n 1)) 1))) is that good?
14:06	jcrites	I'm new to Lisp
14:12	Chouser	meredydd: you can't pass macros where something (like reduce) is expecting a function.
14:12	hoeck	jcrites: totally valid
14:12	Chouser	But frequently trivial to wrap a macro call in a function: (reduce #(and %1 %2) [a b c])
14:12	meredydd	Chouser: On the contrary; I have just spent some time learning that you can...
14:12	jcrites	so I'm writing this function and I need a way for it to call itself
14:12	Chouser	meredydd: sorry. "shouldn't"
14:12	jcrites	does that mean I need to put it inside a let expression?
14:13	meredydd	Chouser: Yeah. I'm taking that as a given ;)
14:13	jcrites	and can functions in let expressions call themselves through their name in the let expression?
14:15	Chouser	functions defined using defn can call themselves by name (although depending on how you use it you might overflow Java's call stack)
14:15	Chouser	functions defined using fn can be named for the purpose of calling themselves: (fn foo [x] (...can call foo here...))
14:16	jcrites	oh
14:16	jcrites	I didn't know fn worked that way
14:16	jcrites	I was trying to do some awkward (let [f (fn ...)]) thing :)
14:16	Chouser	yeah, there's an optional name parameter there. But that name is not valid outside the (fn) expression
14:16	jcrites	that's awesome!
14:16	jcrites	it makes sense
14:17	hoeck	this name us also useful for debugging
14:17	Chouser	you still might need to do that if you want a function defined locally: (let [f (fn ...)] (f ...))
14:17	hoeck	it pops up in the stack if the named fn throws an exception
14:17	Chouser	or worse if f is recursive: (let [f (fn f [] ...)] (f ...))
14:18	Chouser	...because in that case it's really two different f's: (let [f1 (fn f2 [] ...use f2 here...)] (f1 ...))
14:19	jcrites	right. I was just trying to use let to give it a name for itself :) but if it has that built-into fn already I'm set.
14:19	jcrites	so does Clojure have something like "letrec"?
14:19	jcrites	or is that just "def"
14:19	Chouser	but you should be a bit careful doing recursive calls like that, since there's no tail-call optimization. You may have to use recur or lazy calls.
14:19	jcrites	oic
14:20	jcrites	let me show you what I'm actually trying to do.... 1 sec
14:20	Chouser	jcrites: I don't think there's a letrec. I suppose you could write a macro that does (let [f (fn f [] ...)] ...) for you.
14:21	Chouser	def and defn are always for globals
14:36	jcrites	let's say I have a list.... I have a value... if the value is non-nil, I want to append it; if it's nil I want to do nothing
14:36	jcrites	should I write that logic myself or is there a built-in with that kind of meaning?
14:36	Chouser	vector, right? you don't append to lists generally.
14:37	jcrites	I was planning on building up a list with recursive calls and cons ....
14:37	Chouser	jcrites: out of curiosity, what's your language background?
14:37	jcrites	I'm trying to translate my little Scheme knowledge to Clojure :)
14:37	Chouser	cons "appends" at the beginning. Which is fine if that's what you want.
14:37	jcrites	most of my experience is with curly-bracket languages. I have some university experience in Scheme
14:38	Chouser	ok, very good. So you already know what I just said about lists. :-)
14:38	Chouser	ok, so cons-ing onto a list returns a new list, right? It never modifies anything. So by "do nothing" you mean return the original list unchanged?
14:38	jcrites	right
14:39	yrb	jcrites: Just out of interest what is your opinion on all the parens... ;)
14:39	jcrites	I'm trying to write some parser combinators in Clojure as an exercise, if you know what those are :)
14:39	jcrites	I don't like them :)
14:39	Chouser	I've heard them mentioned, but the words me nothing to me. :-)
14:39	jcrites	so a parser is a function that takes in a sequence and returns a sequence (possibly empty)
14:40	jcrites	it returns what it has "matched"
14:40	jcrites	so if your parser matches everything up to space " " it might take in the sequence "hello, world" and return the sequence "hello,"
14:40	jcrites	a parser combinator is a function that builds parsers
14:40	Chouser	afaik, the only builtin thing that does a bit like you want is concat (or mapcat): (concat nil '(1 2 3)) ==> (1 2 3)
14:41	jcrites	you could imagine a parser combinator const that takes in a string. what it does is build a parser that matches exactly that string or fails (matches nothing, returns nil)
14:41	Chouser	but of course that only works if the first thing is a seq of some sort, not a single value.
14:41	jcrites	so I was trying to write some of these combinators in Clojure...
14:42	jcrites	(defn success [] (fn [q] (list q)))
14:42	Chouser	yeah, ok, sounds like a good exercise.
14:42	jcrites	(defn failure [] (fn [q] nil))
14:42	Chouser	Hm, I may have read about those in On Lisp.
14:43	Chouser	I don't think clojure has a builtin that does: (if x (cons x lst) lst)
14:43	jcrites	yeah I am just trying to make sure I'm doing things the right way :-)
14:43	Chouser	yup. nobel. :-)
14:43	jcrites	sometimes the urge to make something yourself is actually misdirected and only exists because you're doing something wrong, hehe
14:44	jcrites	I've been bit by that before ;-)
14:44	Chouser	yep, especially in schemes and lisps.
14:45	jcrites	looks like cons may already do what I want kinda...
14:46	Chouser	really? maybe I misunderstood your question.
14:46	jcrites	here is my attempt at const:
14:46	jcrites	(defn const [cq] (fn p [q] (if (= (first cq) (first q)) (cons (first cq) (p (rest q))) nil))
14:46	jcrites	on entering it the repl is frozen so I must be doing something wrong :(
14:47	Chouser	missing a )
14:47	jcrites	basically, if the first of cq and q are equal then return a new list with that element, recur by calling myself
14:47	jcrites	otherwise return nil (empty)
14:47	jcrites	it looks like cons will, ... well, (cons 1 nil) => (1)
14:48	jcrites	if I remember Scheme that's the same way Scheme does it
14:48	Chouser	ah! yes, but (cons nil '(1)) => (nil 1)
14:48	Chouser	I had your question backwards.
14:48	Chouser	nil acts like an empty list for cons
14:48	jcrites	I actually think I was thinking the wrong way ;-) or not explaining that the "rest" would be returned by a recursion
14:49	jcrites	so you mentioned that this will be real Java recursion and that that could bite me
14:49	Chouser	yes
14:49	jcrites	if I were going to use a bunch of these all nested together, on files with thousands of characters, I'm guessing I would be screwed?
14:49	Chouser	but just replace (p (rest q)) with (recur (rest q)) and you're all set
14:49	Chouser	...and then you don't need to name p anymore
14:50	jcrites	oh, so recur is always a call to the self-fn ?
14:50	jcrites	that's neat
14:50	Chouser	yes
14:50	Chouser	or the surrounding (loop ...), whichever is closer.
14:50	Chouser	It's a pretty elegant hack to work around the JVM's lack of tail call optimization.
14:51	jcrites	interesting
14:51	jcrites	how does it work? I mean Clojure is compiled to jvm bytecode right?
14:51	Chouser	yes
14:52	Chouser	I don't really understand it that deeply, but I think regular clojure functions calls (including recursive ones) are compiled into real Java function calls.
14:52	Chouser	hense the stack overflow issue.
14:52	jcrites	yeah, it's complex... I wonder how the compiler avoids that with recur
14:53	Chouser	But recur is a special form that the Clojure compiler must turn into some kind of goto (with parameter munging).
14:53	jcrites	maybe it actually compiles to a JVM loop.... yeah, that makes sense
14:55	jcrites	link?
14:55	jcrites	I'm no JVM expert but I've worked on compilers before
14:55	Chouser	um, sure, just a sec...
14:55	jcrites	oh no
14:55	jcrites	I got an error
14:55	jcrites	java.lang.UnsupportedOperationException: Can only recur from tail position
14:55	jcrites	(defn const [cq] (fn [q] (if (= (first cq) (first q)) (cons (first cq) (recur (rest q))) nil)))
14:55	jcrites	it isn't in the tail position I guess :(
14:55	Chouser	oh! I'm sorry, I misread your code.
14:56	Chouser	no, of course the compiler's right -- you're passing the result of recur to cons
14:56	Chouser	so even real tail call optimization wouldn't help you.
14:58	jcrites	so I can't implement my algorithm recursively with ease
14:58	jcrites	perhaps I can get the cons inside the recursive call....
14:58	Chouser	well, you can as long as it's not too deep. :-)
14:59	Chouser	right, that's one route -- rewrite your function to be tail-call-optimizable (something else I read about in On Lisp)
14:59	Chouser	you actually might be able to make it lazy instead.
14:59	Chouser	replace cons with lazy-cons
15:00	Chouser	(...and put p back in)
15:00	meredydd	More questions: Why does (contains?) only work with maps, and what should I be using instead on (seq)s?
15:01	Chouser	contains? is constant time (or log(n)) on maps. for a seq you have to search linearly
15:01	Chouser	(some #{3} [1 2 3 4])
15:01	meredydd	And there are two separate functions just to keep me from accidentally doing so?)
15:02	meredydd	Aha. Ta.
15:02	jcrites	it sort of makes sense
15:02	Chouser	some is much more general than just finding a specific item in a seq
15:02	jcrites	contains? reads like a quick predicate, not a "search"
15:03	meredydd	Chouser: Yep, that's what I was planning on doing; just reckoned it ugly. However, the neat trick with the invokable set renders it quite passable :)
15:04	Chouser	meredydd: yeah. That's straight from rhickey -- I didn't think of it.
15:05	meredydd	Oh, plus, looks like (contains?) works exactly as advertised on vectors - if you feed it an index, it will tell you whether (get) will give you anything
15:05	Chouser	huh!
15:05	meredydd	So, it fulfils exactly the same contract. Clever.
15:06	meredydd	I do love this language. Whenever I poke into "this looks like a pain", the answer regularly comes back as "oh, that is clever"
15:06	jcrites	:)
15:06	jcrites	it has a lot of elegance and aesthetic appeal
15:07	jcrites	and consistency to the design of the APIs
15:07	Chouser	I would give up some performance for those characteristics, but apparently I don't have to.
15:08	yrb	:P the cooler bit is that you can invent the language you need as you need it
15:08	jcrites	so Clojure manages not to work with boxes objects often?
15:09	meredydd	That's the wonder of Lisp, yrb. The wonder of Clojure is that so far, I'm spending my time doing that for my own purposes, rather than trying to work around corner cases in the language design (cough Common Lisp cough)
15:10	jcrites	Common Lisp has some ugly parts of the language that I sort of don't understand
15:10	jcrites	like the fact that functions aren't values, and you need to do weird #' stuff
15:10	meredydd	jcrites: I believe rhickey's been adding some explicit native arithmetic. But normally, I think most stuff is boxed right now.
15:10	meredydd	jcrites: That's to do with being a Lisp[2], rather than a Lisp[1] which is what Clojure is.
15:10	jcrites	how does it perform well then? :/ I would expect boxing and unboxing between every operation would be fairly expensive
15:11	jcrites	meredydd: what does that mean? [1] and [2] ?
15:11	meredydd	jcrites: For numerical applications, sure. However, most people don't write numberical apps
15:11	fyuryu	and you can put type hints for the compiler
15:12	jcrites	but any application works with integers quite a lot.... iterators in a loop for instance
15:12	meredydd	If you're using iterators in Clojure, it's almost always implicitly, as a (for) or a (seq) - and the iterators themselves are written in Java using native arithmetic
15:13	jcrites	ahh
15:13	meredydd	As for the Lisp-1/Lisp-2 distinction, check out http://www.nhplace.com/kent/Papers/Technical-Issues.html
15:14	meredydd	Basically, Lisp-1s (like Clojure, and I believe Scheme) have functions in the same namespace, whereas Lisp-2s (like Common Lisp) have separate namespaces for functions and everything else
15:14	yrb	I think that would be a micro optomisation also, any who knows what the jvm is doing...
15:14	yrb	anyway*
15:15	yrb	there is some really awesome tech in the jit
15:15	jcrites	true
15:15	jcrites	I suppose it would detect if the boxed numbers are not being used as objects
15:16	yrb	I think it will even generate fall through code for "hot" code paths
15:17	jcrites	fall through code?
15:17	jcrites	(I don't know a ton about optimization)
15:18	meredydd	Oh, yuck. Chouser, any idea how to do backquote-style quoting explicitly (ie like (quote))
15:20	yrb	like say you call a function with 4 params and it ends up at run time 9 times out of 10 3 params are the same it can gen native code specifcally for that case
15:21	yrb	but it isn't iimited to functions it is entire code paths i think
15:26	yrb	Does anyone know of any lispy gui toolkits that have a good design, I am tring to write a Swing wrapper (called Cling ;) ) just looking for a good design to get inspired
15:27	jcrites	well, you might take lessons from the declarative interface kits
15:27	jcrites	in Yegge's recent post he talks about ...
15:27	jcrites	in Rhinos and Tigers: http://steve-yegge.blogspot.com/2008/06/rhinos-and-tigers.html
15:28	yrb	That is the idea, I have been having a look at jelly swing
15:28	jcrites	ah, ok :)
15:35	Chouser	I believe (almost?) all clojure functions take boxed args and return boxed, so no boxing/unboxing.
15:36	jcrites	well, I mean if you have (+ n 1) there has to be some boxing
15:36	Chouser	...except when calling Java functions, or the new native number stuff, in which case it tries to use native... somehow...
15:40	Chouser	jcrites: numeric literals are boxed, probably at compile time
15:44	Chouser	meredydd: I don't quite understand. you want ' is to quote as ` is to _x_ ?
15:45	meredydd	Yep.
15:45	Chouser	hm, I don't think there is such a beast.
15:45	meredydd	Looking at what macroexpand-1 says, it's all done with a very hairy hack inside the reader
15:45	Chouser	yeah, sound about right
15:46	meredydd	so I just rewrote the macro to require the user to backquote the args that go in.
15:46	meredydd	Less pretty, but livable.
15:46	Chouser	http://n01se.net/paste/Yiq -- Clojure Compiler.java RecrExpr.emit
15:47	Chouser	...that appears to emit a bunch of local variable manipulation followed by a goto.
15:53	jcrites	hmmm
15:53	jcrites	(first nil) => nil ?
15:54	yrb	meredydd: Just out of interest why did you need it `ed
15:54	meredydd	yrb: I'm writing a unit test framework.
15:55	jcrites	bah
15:55	meredydd	There's a utility macro for asserting that a macro expands as it should
15:55	jcrites	I found the bug
15:55	jcrites	can you spot what's wrong with this function:
15:55	meredydd	(defmacro assert-macro [macro-expr expanded-expr]
15:55	meredydd	`(assert (= (macroexpand-1 ~macro-expr) ~expanded-expr)))
15:55	jcrites	http://pastebin.com/d45395948
15:56	jcrites	tricky bug I wrote there :-)
15:56	meredydd	yrb: Ideally, I'd like to do something like ~(quote expanded-expr) and ~(quote macro-expr)
15:56	jcrites	causes infinite recursion
15:57	jcrites	what's the problem? the problem is that if both sequences are nil, (first nil) returns nil, meaning my function recurses infinitely :(
16:00	jcrites	now I'm getting wrong number of arguments passed to cons... sheesh... I suck >_<
16:00	jcrites	is 2 not enough?!! IS IT NOT ENOUGH?!
16:09	jcrites	hmmmm
16:09	jcrites	what have I done wrong with this function?
16:09	jcrites	(defn const-p [cq q] (if (and cq (first-eq cq q)) (cons (first cq) (const-p (rest cq) (rest q)) nil)))
16:09	jcrites	I keep getting the error: wrong number of args passed to cons
16:10	Chouser	(cons (first...) (const-p...) nil)
16:11	jcrites	oh I thought I put the nil on the if
16:11	jcrites	did I really put it in the cons >_<
16:11	jcrites	oh shit you're right. thanks!
16:11	Chouser	this is why you need an editor to help
16:11	jcrites	I agree :(
16:11	Chouser	what are you using?
16:13	jcrites	nothing
16:13	jcrites	working on the REPL
16:15	yrb	emacs works really well with lisps
16:35	yrb	meredydd: does this not work? (defmacro macro-equal [me ee] `(assert (= (macroexpand-1 (quote ~me)) (quote ~ee))))
16:36	meredydd	yrb: Nope. The macroexpand will add namespace qualifications to symboles, whereas (quote) won't.
16:36	meredydd	*symbols
16:38	yrb	ah I was assuming the user must also add namespace quals
16:41	meredydd	...which is what I've fallen back to. But it's a PITA.
16:41	yrb	how about doing this? (defmacro macro-equal [me ee] `(assert (= (macroexpand (quote ~me)) (macroexpand (quote ~ee)))))
16:42	meredydd	Ooh, goodness. That's going to look ugly as sin on the debugging, but should cure the other stuff...
16:42	meredydd	Good idea.
16:43	meredydd	(The wheels on the bus go "hack hack hack"...)
16:44	yrb	that so isn't a hack... rofl... well not when you look back at java/c++ land
16:46	yrb	but your right will look ugly as sin debugging
17:14	meredydd	*blinks
17:25	jcrites	hmmmm
17:25	jcrites	I love how easy it is to represent algorithms in Lisp
17:26	jcrites	none of the... what he called "ceremony" :)
17:26	jcrites	but it's hard to figure out the right way to do it some of the time
17:26	jcrites	what I'm trying to do (recursively, which is perhaps why it's hard) is check if one sequence is a sub-sequence of another sequence. if so, return it, otherwise return nil
17:27	jcrites	without writing too many conditionals...
17:28	jcrites	I feel like there should be some elegant way to do this without the "if this - if that "
17:28	meredydd	yrb: If you're interested, it turns out that even that wasn't quite right.
17:28	jcrites	my algorithm now is basically..... if (both sequences are nil) true (else .....
17:29	jcrites	else if the first element of each sequence is equal, then call self recursively, otherwise return nil
17:29	yrb	meredydd: Interesting, where does it fail?
17:29	meredydd	Because the args still had non-namespaced stuff, which (macroexpand) wouldn't touch
17:30	meredydd	and apparently, nested `s don't work, so you can't invoke that quote on it inside the macro
17:32	meredydd	It could probably be done with a macroexpand, followed by a code-walk to force the namespace qualifiers onto any unqualified symbols
17:32	meredydd	(that is, "a full macroexpand, like you were suggesting, followed by a code-walk...")
17:33	meredydd	But I think it's just passed my "can-be-arsed" threshold, and I'm just mandating backquotes on args passed in :P
17:34	yrb	I agree :)
18:03	pjb3	When I am in the Clojure REPL
18:03	pjb3	If I define a function that will run a function in a thread
18:03	pjb3	like this:
18:03	pjb3	(defn in-thread [f] (.start (new Thread f)))
18:04	pjb3	When I call it like this:
18:04	pjb3	(in-thread #(prn "hi"))
18:04	pjb3	it doens't print
18:04	pjb3	but as soon as I evaluate something else
18:04	pjb3	like:
18:04	pjb3	()
18:04	pjb3	Then I see the output
18:05	pjb3	Any idea as to what causes that?
18:07	JamesIry	pjb3: just a guess, but could it be that the repl is buffering stdio so that output in other threads doesn't appear randomly as you are typing.
18:08	pjb3	yeah, I suppose that makes sense
18:08	JamesIry	Should be easy to test...make in-thread do something with file IO instead of stdio
18:09	JamesIry	Then go look at the file in a separate window
18:13	Chouser	always use (flush) after any debugging (prn)s
19:54	jcrites	hey, anyone know how I can convert a sequence to a vector?
19:54	rhickey	(vec your-seq)
19:55	jcrites	(vector my-seq) ?
19:55	jcrites	I tried:
19:55	rhickey	no, vec
19:55	jcrites	my version doesn't have that. is that new?
19:55	rhickey	In the latest release 20080612
19:56	jcrites	let me get that one. thanks :)
19:56	rhickey	if you are on an old release, use (apply vector your-seq)
19:56	jcrites	hmmm
19:56	jcrites	how can (f x) be different from (apply f x) ?
19:57	jcrites	or is it just with values of f which are special forms?
19:57	rhickey	the last arg to apply must be a seq, which gets 'unrolled' into args
19:57	rhickey	(apply + [1 2 3])
19:57	jcrites	ahhhh, that makes sense then
19:58	jcrites	thank you
19:58	rhickey	vs (+ [1 2 3])
19:58	rhickey	np
20:06	jcrites	what would be the right way to append an element to the end of a sequence? (returning a new sequence)
20:07	jcrites	I guess I am too dumb to do it recursively with recur :)
20:08	jcrites	hmmm maybe I can do it!
20:08	jcrites	oh, nope, I can't...
20:09	jcrites	i'm not sure it's possible to do tail recursively because the cons can't be called until after the recursion returns
20:09	jcrites	maybe you could do it somehow by reversing the sequence?
20:09	slava	jcrites: (reverse (cons elt (reverse seq)))
20:10	jcrites	would you consider that better or worse than converting it to a vector, vector-appending, and converting to seq? (the last step which may not be much of a "conversion")
20:10	slava	well, the underlying issue here is that you're peforming an operation which the data structure is unsuited for
20:10	slava	if you need to do this once or twice, no big deal, if you're doing it in a loop consider just using a vector
20:11	jcrites	I guess you're right. I'm thinking in the non-functional mindset
20:11	jcrites	I'm trying to implement Sparsec/Parsec combinators in Clojure as an exercise actually :-) the DS I'm discussing will actually be my "parse string"
20:11	jcrites	I was hoping to get around Sparsec's ParseResult and have parsers return nil on no-match
20:12	jcrites	then that means I need an end-of-string symbol
20:12	jcrites	so this question came up from how can I append :eof to (seq "some string to parse")
20:12	jcrites	and it looks like I should just do something like (defn append [v e] (assoc v (count v) e))
20:12	meredydd	rhickey: The sourceforge redirects are broken from what I can see.
20:13	jcrites	they are totally broken :/
20:13	jcrites	AFAICT a "/" got lost somewhere
20:13	slava	i don't think parsec needs to be implemented with an 'end of string' symbol
20:14	jcrites	no, it doesn't need to be, I just think it would make it more elegant
20:14	slava	the way its usually done is with a list monad
20:14	jcrites	I mean in Scala for example, oh, I guess you aren't necessarily familiar with that, I forgot
20:14	slava	i am, somewhat
20:14	jcrites	Scala has an implementation called Sparsec, with a structure called ParseResult which is a boolean and some stuff
20:14	jcrites	I was hoping for my Lisp (Clojure) version to return nil/non-nil
20:15	slava	in haskell's parsec, parsing yields a lazy list of results
20:15	jcrites	the problem with that is, what if your parser matches the whole sequence?
20:15	slava	where a parse result is some object (otuput by action parsers) together with the rest of the sequence
20:15	jcrites	the "rest of the sequence" is nil, then, which is the same as the error case
20:15	jcrites	since they need to be distinguishable, I was considering adding a magic character on the end, designated :eof
20:15	jcrites	in that way, all parsers will always terminate with "next" pointing to the eof character
20:16	slava	the error case outputs an empty list of parse results
20:16	slava	not a list containing nil
20:16	jcrites	so parsers cannot match nothing?
20:16	jcrites	how do you match * ?
20:16	slava	they can, in which case you get a list contianing nil as the parse result
20:16	jcrites	ah, interesting
20:17	jcrites	I hadn't thought of returning a list of parse results at all :) although I was pondering how to implement a "fork" combinator
20:17	slava	that's how parser combinators work
20:17	slava	each parser returns a list of parse results
20:17	jcrites	it makes sense why the list monad then
20:17	slava	parser concatenation is 'append' lifted to hte list monad
20:17	jcrites	aren't most of the parsers deterministic though? why do they need to return a list of results?
20:18	jcrites	the only reason I had considered a list of results was for a nondeterministic "fork" operator
20:18	jcrites	oh, is the list of results items matched in series?
20:18	slava	well, consider a parser like (a)(a)
20:18	jcrites	I was thinking of them as alternatives up until now
20:18	slava	if you parse aaaa, there are 4 possible results
20:18	rhickey	meredydd: clojure.sourceforge.net goes to clojure.org, no?
20:18	slava	a\|aaa aa\|aa aaa\|a
20:18	jcrites	rhickey: it's broken somehow, try some google searches
20:18	slava	oh, and \|aaaa aaaa\|
20:19	jcrites	so all of those are returned.... oic ...
20:19	slava	( scratchpad ) "aaaa" "a" token <*> parse llength .
20:19	slava	5
20:19	slava	there are 5 results there, "", "a", "aa", etc
20:19	slava	but if we insist that it must patch the whole string
20:19	slava	( scratchpad ) "aaaa" "a" token <*> just parse llength .
20:19	slava	1
20:19	slava	'just' filters out all parse results where the 'rest' is not nil
20:20	slava	parser combinators are all operations on lazy lists
20:20	jcrites	interesting.... the Scala versions are a deterministic version of that, I guess
20:20	slava	then they're not parser combinators
20:20	jcrites	I am not very familiar with the original Parsec
20:20	slava	they're parsing expression grammars
20:20	slava	pegs don't backtrack after a successful match
20:21	slava	so you either get one parse result or nothing at all
20:21	jcrites	yes, that's how they are
20:21	slava	so for example, with pegs, (a*)a will never match aa
20:21	slava	because (a*) 'eats up' the aa
20:21	slava	and it doesn't backtrack after that, ever
20:21	jcrites	hmmm
20:21	slava	with parsec, it fails to match the second 'a', and backtracks, then suceeds
20:21	meredydd	rhickey: Yes, but the sub-URL forwarding is borked.
20:21	meredydd	rhickey: http://clojure.sourceforge.net/reference/namespaces.html (just to pick one out of the air)
20:21	jcrites	thanks for the insight; I think you have saved me from some hours of bad design decisions ;-)
20:21	slava	jcrites: parsec parsers are generally less efficient
20:22	slava	but they're more expressive
20:22	rhickey	some of them will be as they use subdirectories which my new wiki doesn't support
20:22	slava	however i'm pretty sure anything you can parse with parsec can be parsed with peg, except sometimes the peg grammar can get convoluted
20:22	jcrites	I was actually thinking of trying to generate a greedy \| non-greedy fixed implementation from the combinators or something
20:22	slava	regular expressions are very hard to embed in peg for example
20:22	jcrites	you build up a combinator expression then serialize it, compile it, or something
20:22	slava	yes, in peg, everything is 'greedy' unless you add a lot of negations
20:23	slava	in factor, we have parsec parsers, which are built with lazy lists and are inefficient, and peg parsers which compile to machine code but are a bit harder to use
20:23	jcrites	is that generally a problem if you were going to, oh, say totally hypothetically ;-) use it to parse a simple language?
20:23	meredydd	rhickey: I don't think they even get that far - I'm getting "Unknown host 'clojure.orgreference'"
20:23	meredydd	rhickey: Looks like you're missing a '/;
20:23	meredydd	*'/'
20:23	jcrites	meredydd: it's missing a /
20:23	slava	jcrites: well, pegs can be used to parse very complex languages
20:23	slava	like javascript
20:23	rhickey	jcrites: ah, just a sec
20:24	jcrites	oic, a peg is what I want then :)
20:24	slava	and they're more efficient than parsec
20:24	jcrites	however, I was trying to add something quite odd
20:24	jcrites	I was trying to add a "fork" peginator
20:24	jcrites	(peginator, combinator)
20:24	slava	i'm not familiar with that type of thing
20:24	slava	i didn't implement any of this stuff, i just use it a fair bit
20:24	jcrites	which would parse (depending on how I decide to do it) either both arguments, or one
20:24	jcrites	(or fail and nothing)
20:25	jcrites	there are some options for the specifics, but the general idea is to parse both alternatives, sort of like a real parser combinator
20:25	jcrites	except both are valid syntactically
20:25	rhickey	jcrites: ok, you'll get a 404 now
20:25	jcrites	the two valid syntax trees would be the result of the parsing operation
20:25	jcrites	the goal of this being to allow simple ambiguous grammar in a programming language
20:26	jcrites	I know it's crazy; I just want to try it ;-)
20:26	jcrites	one could imagine a Lisp where either (a + b) or (+ a b) are accepted
20:26	jcrites	you could, if you wanted, represent both expressions by the same AST
20:26	slava	what if a evaluates to a function?
20:26	jcrites	or they could be different
20:26	jcrites	if there is semantic ambiguity it would be an error
20:26	slava	at run time?
20:27	slava	sorry, i don't like ambiguity in syntax :)
20:27	jcrites	I don't know as much about dynamic languages :) I was thinking of this from a static perspective
20:27	jcrites	oh I know, it might be a horrible idea
20:27	jcrites	imagine reading (f g h)
20:27	jcrites	who knows what that means
20:27	jcrites	g(f,h)?
20:27	jcrites	f(g,h)?
20:27	jcrites	or maybe it even means h(f,g)! :)
20:28	jcrites	I wasn't expecting this particular example to be a good use of ambiguous syntax
20:28	jcrites	I was thinking maybe about using [ ] to specify a variety of similar data structures or whatnot
20:28	jcrites	the two alternative ASTs might be very different and easily distinguishable by the programmer
20:28	jcrites	that would be a better usage of ambiguous syntax
20:29	jcrites	my goal was just to not not have that option because of technical limitations in the parser ;-) I think I can write semantic analysis algorithms to handle it intelligently if I can support nondeterministic parse trees
20:29	jcrites	but only experimenting with the language feature will tell me for sure if it's a horrible idea, or one worth investigating ;-)
20:30	jcrites	where the program expression mirrors a very numerical form, I might think of it as OK.... (a+b)*(c+d) or something
20:30	jcrites	(in an otherwise Lisp program)
20:30	jcrites	I'm also going to try ambiguous syntax by omission
20:31	jcrites	you could imagine a Lisp program where many parentheses are simply omitted, inferred by other means such as indentation or newlines
20:31	jcrites	in that case, one way I could implement it is via ambiguous syntax... I'm not sure exactly how to do that without an explosion of AST cases, but the general idea makes sense I think
20:40	rhickey	meredydd: links better now?
20:40	meredydd	Much, thanks :)
20:40	rhickey	thanks for the heads up
20:50	drewr	Is trunk@906 broken?
20:51	drewr	Exception in thread "main" java.lang.NoSuchMethodError: clojure.lang.RT.cons(Ljava/lang/Object;Lclojure/lang/ISeq;)Lclojure/lang/ISeq;
20:55	slava	do you guys have continuous integration?
20:56	drewr	We need to get rhickey on a dvcs.
20:57	drewr	These "interim checkins" are unnecessary.
20:57	meredydd	drewr: I dunno...I like being able to experience something he's playing with
20:57	meredydd	The real problem is that svn sucks at branch-and-merge
20:57	jcrites	a dvcs is not necessary for development commits to be made on a development branch ... eh ?
20:57	drewr	You'd still get the history. He could just push it when it's functional.
20:57	jcrites	what sucks about it?
20:58	slava	we have a bunch of machines which compile and run tests, if everything passes they tag the revision they just built
20:58	dudleyf	jcrites: Merging is way harder than it needs to be
20:58	slava	so people can check in broken stuff and users can just pull the 'clean' tag
20:58	slava	when you support 11 architectures its unrealistic to expect every developer to test on every platform before checking in changes
20:58	jcrites	what's harder about it? forgive me, I haven't used svn very often in any kind of production environment
21:00	meredydd	jcrites: You need to specify revisions manually. You can't do sensible things like "merge all changes made to meredydds-devel since it diverged from trunk"
21:00	dudleyf	svn makes you track down the branch point manually, then apply a diff, then fix any conflicts, then commit
21:00	jcrites	oh, so effectively you can only merge revisions, and not branches?
21:00	jcrites	that does sound painful.
21:00	meredydd	jcrites: What you do, effectively, is get a diff between two revisions on one branch, and apply it to another.
21:01	meredydd	So, I'd take the diffs between 503 (when I branched) and now, on my branch, and apply it to trunk. That sort of thing.
21:01	jcrites	I wonder what their rationale is for that kind of merge behavior .... ?
21:03	meredydd	"It's one hell of a lot better than CVS" :D
21:03	meredydd	I don't think it's that bad, actually - if there were sensible automated ways of figuring out which revisions you wanted to take, I think I'd be quite happy with it
21:04	jcrites	what happens if you specify an earlier revision? does the merge get all messed up, or just fail?
21:04	jcrites	(a revision already merged, say)
21:04	meredydd	Uhh...it tries as best it can.
21:05	meredydd	It's exactly like applying a patch from the wrong revision
21:09	drewr	Ah, 906 isn't broken. I had to ant clean myself of the cruft of many revisions ago.
21:18	slava	ant still doesn't support automatic recompilation of dependencies? :)
21:58	pjb3	I'm working on adapting the server socket repl example from the wiki into a toy web server
21:58	pjb3	http://pastie.org/215618
21:59	pjb3	Weirdest thing though, if I redefine the handle-request function and just change the value of msg
21:59	pjb3	the next request I make from a web browser uses the old version of msg
21:59	pjb3	but the 2nd request uses the updated version