Re: [9fans] bison problem, not plan9 related

[Skip Tavakkolian <9nut@9netics.com>]

i think this is what you want. untested:

pair: REP ATOM
	| REP '[' block ']'

block: pair
	| block pair

> Hello,
> sorry for an off-topic thing. But I guess somebody here could help me...
> I have a problem with bison grammer
>
> Having
>
> %token	ATOM
> %left	'+'
> %left	REP
>
> and a grammar:
>
> block:	  ATOM
> 	| REP block
> 	| block '+' block
> ;
>
> is ok. Having another grammer:
>
> block:	  ATOM
> 	| REP block
> 	| block block %prec '+'
> ;
>
> has 2 shift/reduce conflicts, similar to
> state 7
>
>     5 block: REP block .
>     6      | block . block
>
>     ATOM  shift, and go to state 3
>
>     ATOM      [reduce using rule 5 (block)]
>     $default  reduce using rule 5 (block)
>
>     block  go to state 9
>
> or
> state 9
>
>     6 block: block . block
>     6      | block block .
>
>     ATOM  shift, and go to state 3
>     REP   shift, and go to state 4
>
>     ATOM      [reduce using rule 6 (block)]
>     $default  reduce using rule 6 (block)
>
>     block  go to state 9
>
> What I want is to have a parser that can read e.g. (the spaces are
> left out by lex, they are not in what bison sees; I only write them
> here for better readability)
> 12 Au 13 Cu 2 Ag
> the former grammer (REP is for repetition) is able to read
> 12 Au + 13 Cu + 2 Ag
> but I don't like those pluses, which are redundant.
>
> Also important: I have those 'block' non-terminals there, since I want
> to add another rule
> block: '[' block ']'
> so that I can use brackets and can parse things like
> 12 [ 2 Cu 3 Co]
>
> Could anyone explain to me what goes wrong?
> I can't figure it out...
>
> Thanks a lot
> Ruda
>
> PS.: the grammer is actually identical to a grammer that can evaluate
> expressions with +, *, and brackets, with usual operator precedence.