Re: Technical question about yacc and LALR parsers

In article <9104222013.AA09131@florida.HQ.Ileaf.COM>, jar@florida.HQ.Ileaf.COM (Jim Roskind x5570) writes:
|> [...] A second
|> example would would be (again verifiable by the parser generator) when a
|> "grammar preprocessor" back-substituted some rules and in doing so,
|> replicated the code actions.
|> [...]
|>
|> Jim Roskind- Author of a YACCable C++ grammar
|> jar@hq.ileaf.com or ...!uunet!leafusa!jar


Unfortunately, grammar preprocessors (such as one I've developed), often
have subtly different actions:


For example:


stmtlist : stmtlist ';' ? ';;' stmtlist
{ $$ = sequence($stmtlist'1,$stmtlist'2); }


gets expanded into:


stmtlist : stmtlist ';' DOUBLESEMI stmtlist
{ $$ = sequence($1,$4); }
                  | stmtlist DOUBLESEMI stmtlist
{ $$ = sequence($1,$3); }


(Duplicating rather than having a special optional-semicolon rule more often results in an LALR(1) grammar).


Notice how the actions differ in where arguments are found.


John Boyland
boyland@sequoia.Berkeley.EDU
--

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