#clojure log - Mar 04 2008

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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 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 a symbol is an object

16:04 it might name a var(iable)

16:04 it might name an immutable local

16:04 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 of course it could be misused

16:08 but once you have a globally accessible name, you have a var anyway

16:09 so why piggy-back on the metadata?

16:09 arbscht: mainly to overload it

16:10 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 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 otherwise they should be used mutably only after per-thread binding

16:14 which is safe mutation

16:14 or put refs in vars

16:14 or agents

16:14 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 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 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 union, difference, intersection, index, project, select, join

18:32 docs coming

18:32 relations are sets of maps

19:05 sets now (really) up, with docs

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