# IRC log for #marpa, 2014-09-02

All times shown according to UTC.

Time Nick Message
00:42 jeffreykegler ronsavage1: My apologies re http://irclog.perlgeek.de/​marpa/2014-08-30#i_9273196
00:43 jeffreykegler I missed this when backlogging -- that's why I did not respond.
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01:36 shadowpaste "jeffreykegler" at 162.232.214.245 pasted "An ambiguous marked middle recursion that runs in linear time" (55 lines) at http://scsys.co.uk:8002/420820
01:37 jeffreykegler I ran a check on Marpa's handling of marked middle recursions, which I've just pasted.
01:38 jeffreykegler It's this grammar S ::= a S a   S ::= a a x a
01:38 jeffreykegler It obviously a middle recursion, but it's marked -- only a middle can contain a 'x'
01:39 jeffreykegler It's also ambiguous.  Until you see an 'x', after initial couple of 'a'
01:39 jeffreykegler ... couple of "a"'s,
01:44 jeffreykegler every 'a' can be either a first 'a' in the rule S ::= a S a, or one of the first two in S ::= a a x a
01:45 jeffreykegler And it's linear, as you can see by varying the length of the input string.  The Earley sets keep track of all three possibilities, until they see an 'x'.  No Earley table contains more than 5 entries, so the cost of tracking this is low.
01:45 jeffreykegler It's all little abstract I admit, but if you're trying to figure our what's efficient in Marpa, and how it works, it is a very instructive, very short example.
01:46 * jeffreykegler is having Internet troubles again.  Apparently this stuff bores the gremlins who control his Internet connection. :-)
01:52 Aria Good explanation!
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04:24 jeffreykegler New blog post, announcing Ron's web site: http://jeffreykegler.github.io/Ocean-of-Awa​reness-blog/individual/2014/09/website.html
04:25 jeffreykegler You may notice it has Yet Another Rewrite of the "about the Marpa algorithm" language.
04:26 jeffreykegler Ron: you might want to wait until it settles, has typos removed, etc., before updating your page.
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07:58 ronsavage1 Waiting for the gremlin dust to settle.....................................
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15:34 MintberryCruNCH hey guys, very new to marpa, had an issue id like to discuss, anyone around
15:36 lucs MintberryCruNCH: Just post away, the experts (not me! :-) are usually not very far away, and they'll see it on the log (see /topic) anyway.
15:41 MintberryCruNCH well i had a small issue, im trying to put G1 rules for parsing hex and binary numbers like so Number ::= '0b' BinDigits action => bin_number  | '0x' HexDigits action => hex_number, and then declare the L0
15:41 MintberryCruNCH as HexDigits ~ [\dA-Fa-f]+ \nBinDigits ~ [01]+. Now when i enter say 0b1010, it doesnt get detected in the rule, but gets treated as HexDigits. Is there any way (perhaps a better character class or prioritizing rule over L0 to ensure it hits the rule?) forgive my noob level explanation, but you probably get the idea of it
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15:59 rns MintberryCruNCH: please check this https://gist.github.com/rns/df918056abc656eb0d01
16:01 rns Basically you need to make (Hex|Bin)Digits G1 rules because G1 is much better at handling ambiguity.
16:10 rns topic
17:22 MintberryCruNCH Thanks a lot rns!, sorry i was away for dinner, that worked
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20:06 jdurand Re https://gist.github.com/jdd​urand/59d2f01a62820619d335 - my attempts using L0 is at https://gist.github.com/jdd​urand/59d2f01a62820619d335
20:08 jdurand rns answer and mine show well the difference between values when things are coded at G1 or L0 level. At G1 level (rns), all characters appears one by one in the AST. IN the L0 level (rns), BinDigits and HexDigits appear as a single block.
20:09 jdurand I have posted the output as a comment to both gists, just to make things clear.
20:12 jdurand But G1 level also has its very big disadvantage as soon as you have a :discard rule. For example, rns gist will happily parse 0b 0101
20:13 jdurand Re http://irclog.perlgeek.de/​marpa/2014-09-02#i_9286672 : should have been read "Re http://irclog.perlgeek.de/​marpa/2014-09-02#i_9285615 etc..." ahem
20:16 jdurand Errata: In the L0 level (jdd) - sorry
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20:46 rns MintberryCruNCH: jdd's suggestion is more correct and efficient. You have to use it, I think.
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