#clojure – 2008-02-14

2008-02-14

12:03	Chouser	When I try to use assert, I get: clojure.lang.Compiler$CompilerException: REPL:1265: No such var: clojure/prstr
12:03	rhickey	should be pr-str, fixed in svn already
12:14	Chouser	ok, thanks
15:42	Chouser	I think I wish more functions that currently return true or false instead returned an arg's value or false.
15:42	rhickey	which ones?
15:42	Chouser	like "zero?", "or", etc.
15:42	Chouser	I'm not sure I've thought through the implications.
15:43	Chouser	Was it a concious decision to not do that?
15:43	rhickey	user=> (or nil 2 3)
15:43	rhickey	2
15:43	Chouser	Even things like < and > could do it
15:43	Chouser	oh. :-)
15:44	rhickey	The others are boolean though
15:44	Chouser	zero? wouldn't work
15:44	Chouser	but pos? might
15:45	Chouser	if < returned the larger when true, you could chain them: (< (< 5 i) 10)
15:46	rhickey	(< 5 i 10)
15:47	Chouser	hmph.
15:47	rhickey	doesn't negate your idea
15:47	Chouser	you keep shooting down all my good ideas with .. better ones that ... already exist.
15:47	Chouser	:-)
15:48	Chouser	ok, so the only one I've stumbled on in my actual code is pos?
15:49	rhickey	there are many, just not or
15:49	Chouser	(some (fn [i] (and (pos? i) i)) seq)
15:49	bgeron	Chouser: what would (< a b) return?
15:49	rhickey	pos? emulates CL plusp
15:50	Chouser	bgeron: either false or b
15:50	bgeron	why b?
15:50	Chouser	because it's the larger
15:50	bgeron	that way (< a (< b c)) doesn't work as expected
15:51	bgeron	and < is commonly called "lesser-than" or the like
15:52	Chouser	bgeron: that's a pretty good point.
15:53	Chouser	I suppose that since < already supports multiple args, the (< (< a b) c) and (< a (< b c)) use cases aren't good ones to look at.
15:53	bgeron	it would be cool if we could use math notation to write a < b <= c
15:53	rhickey	Chouser: your problem might be with some returning the predicate return rather than the found value
15:53	bgeron	that is a good point also
15:53	rhickey	another CL-ism
15:54	Chouser	rhickey: maybe, but I think it makes sense to use the predicate return, since that at least gives you the power to return the found valoue or something else.
15:54	Chouser	I'll go with power and simplicity over brevity.
15:55	Chouser	I guess if it gave you the found value you could wrap it in another (map ...). Hm.
15:55	rhickey	CL has separate find-if
15:56	Chouser	yeah, I'm also in favor of fewer builtins
15:57	Chouser	oh, zero? would work. So is there a reason for zero?, pos?, and neg? not to return their arg when true?
15:58	rhickey	possibly not - running now, will think about it
16:00	Chouser	bgeron: any objections? you had some good ones against <
16:03	bgeron	maybe that it's not consistent with other predicates in returning values other than false/true, but it's really not a problem
16:05	bgeron	I think returning only false/true is more pure, but it's more practical to return the value itself in zero?,pos?,neg?
17:42	scramflot	hi
17:43	scramflot	in the definition of the filter function, on page: http://clojure.sourceforge.net/reference/sequences.html , what does "pred" mean?
17:43	scramflot	(filter pred coll)
17:43	scramflot	Returns a lazy seq of the items in coll for which (pred item) returns true.
17:45	scramflot	so, like, (filter (= x 1) [4 1 3 1]) -> [1 1] ??
17:46	scramflot	or it's a function that takes a parameter and tests it, returning true or false
17:57	Chouser	pred is short for predicate, which means a function.
17:57	Chouser	In this case at aleast, a function that returns true or false
17:58	Chouser	(= x 1) is a function call, not a function.
17:58	Chouser	(filter (fn [x] (= x 1)) [4 1 3 1])
23:38	jh06	Hi guys. I had a question which stems mainly from my misunderstanding of how Lisp works. How come I can redefine if, but the redefined version acts exactly the same as the old one?