#clojure – 2008-03-04

2008-03-04

16:00	albino	Why in this screencast can you not use the word variable instead of symbol?
16:01	arbscht	which one?
16:01	albino	clojure data strcutures part 1
16:01	arbscht	right. which point in the screencast are you referring to?
16:03	albino	well I wasn't refering to anything specifically, someone (I think rhickey) is saying symbols
16:03	albino	I would like to understand what differentiates a symbol from a variable
16:03	rhickey	not all uses of symbols are variables, so I didn't want to equate them
16:03	rhickey	a symbol is an object
16:04	rhickey	it might name a var(iable)
16:04	rhickey	it might name an immutable local
16:04	rhickey	but symbols themselves don't have storage slots, unlike CL
16:06	arbscht	can you fake that with metadata?
16:07	rhickey	why?
16:07	arbscht	just wondering. it might not be good style
16:08	rhickey	symbols do support metadata, mostly to allow hinting to the compiler
16:08	rhickey	of course it could be misused
16:08	rhickey	but once you have a globally accessible name, you have a var anyway
16:09	rhickey	so why piggy-back on the metadata?
16:09	arbscht	mainly to overload it
16:10	arbscht	like you might with SYMBOL-PLIST in CL
16:10	rhickey	but you'll have to pass them around - two symbols with the same name can be different objects, with different metadata they definitely are
16:10	arbscht	right
16:11	rhickey	Clojure global vars are much closer to CL symbols than Clojure symbols
16:11	rhickey	CL interned symbols at least
16:12	albino	so variable implies mutability and we want to get away from that notion?
16:13	rhickey	pretty much, global vars are mutable mostly to allow function redefinition/fixes
16:13	rhickey	otherwise they should be used mutably only after per-thread binding
16:14	rhickey	which is safe mutation
16:14	rhickey	or put refs in vars
16:14	rhickey	or agents
16:14	rhickey	the idea is to used the controlled mutation strategies built in to Clojure
16:15	albino	well I hear the CL folks talk about symbols all the time
16:15	albino	kind of weird coming from the C++ background, variables man variables!
16:15	rhickey	C++ variables disappear after compilation, CL/Clojure symbols are objects
16:30	Chouser	Maybe I'm misusing keywords, but I find I do (. (str k) (substring 1)) a lot
16:31	Chouser	I guess I wouldn't want "str" to string the : off, even though "keyword" tacks it on.
16:31	rhickey	(name :foo) -> "foo"
16:31	Chouser	woohoo! Thanks!
18:31	rhickey	set ops and relational algebra are up:
18:31	rhickey	union, difference, intersection, index, project, select, join
18:32	rhickey	docs coming
18:32	rhickey	relations are sets of maps
19:05	rhickey	sets now (really) up, with docs