(a) X ::= C C ~ 'cc' B B ~ 'bb' A A ~ 'a' :discard ~ A Forbidden. <A> is required to be a lexeme since it is a discard symbol, and cannot appear on the RHS of an L0 rule.
(b) X ::= C C ~ 'cc' B ~ 'bb' A A ~ 'a' :discard ~ A <B> is *not* unreachable. It is an L0 lexeme, since it is on an L0 LHS but not on any L0 RHS. It might be helpful to imagine that, in L0, the lexemes are what the start symbol is in G1. And, of course, start symbols are reachable by definition. Re (6). When the lexer finds discard and non-discard symbols at the same location, the discard symbols are ignored. Discarded symbols are not passed up to G1 -- they're discarded! -- jeffrey On Wed, Mar 29, 2017 at 10:44 PM, Andreas Kupries <akupr...@shaw.ca> wrote: > > > (4) X ::= B B ~ 'bb' A A ~ 'a' :discard ~ A > > > > "A is both used as discard, and on the RHS of the lexeme B Is that > allowed > > ?" > > No, it is not. > > > > > "What if B is not a lexeme?" I'd want to see an example of what you > have > > in mind before answering this. > > (a) X ::= C > C ~ 'cc' B > B ~ 'bb' A > A ~ 'a' > :discard ~ A > > B is a simple L0 symbol, used in the lexeme C (*) > A is now on the RHS of non-lexeme match B, and in discard. > > The effective structure (language?) we get after inlining > everything is > > X ::= 'ccbba' > discard ~ 'a' > > (*) This is a minimal case. We can nest arbitrarily deep away > from the lexeme. > > I am fine with saying that this is also forbidden. > > (b) X ::= C > C ~ 'cc' > B ~ 'bb' A > A ~ 'a' > :discard ~ A > > In this case B is a simple L0 symbol which is not reachable > from any lexeme, and thus not from the start symbol either. > > This could be ok, given that B is unreachable. I am fine > however with looking at this from the pure syntax level and > saying that it is forbidden. > > > (5) X ::= B B ~ A A ~ 'a' :discard ~ A > > > > As you said, a variant of (4), and same answer. <A> is specified as a > > lexeme and a lexeme can never occur on the RHS of an L0 rule. > > Actually <A> is not a lexeme. <B> is. > Even so, same answer as (4), A not allowed as both discard and RHS of > lexeme B. > > > (6) X ::= B B ~ 'a' A ~ 'a' :discard ~ A > > > > A bit strange, > > Oh yes. Edge case. I am trying to think up all the ways a user may > unintentionally screw up, or intentionally try to break the system. > > > but should be allowed. > > Ok. Would have been a devil to check for if not. > > > My guess is that it accepts any string of 1 or more "a"'s, using the > > first "a" in the parse and throwing away the rest. > > My reading is that > > - The lexer attempts to match B and A in parallel. > At the end both match and the lexeme is passed up to G1. > This for every 'a'. > > - Only exception is when B is not acceptable to G1. > Then LATM excludes it and only A matches and is discarded. > > > ! hope these guesses were all correct and/or that this helps! > > It does. > > -- > See you, > Andreas Kupries <akupr...@shaw.ca> > <http://core.tcl.tk/akupries/> > Developer @ SUSE > > Tcl'2017, Oct 16-20, Houston, TX, USA. http://www.tcl.tk/community/ > tcl2017/ > EuroTcl 2017, Jul 8-9, Berlin/DE, http://www.eurotcl.tcl3d.org/ > ------------------------------------------------------------ > ------------------- > > > > > -- > You received this message because you are subscribed to the Google Groups > "marpa parser" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to marpa-parser+unsubscr...@googlegroups.com. > For more options, visit https://groups.google.com/d/optout. > -- You received this message because you are subscribed to the Google Groups "marpa parser" group. To unsubscribe from this group and stop receiving emails from it, send an email to marpa-parser+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
