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

[Rudolf Sykora <rudolf.sykora@gmail.com>]

I must say, many thanks!
Ruda

2009/10/21 Russ Cox <rsc@swtch.com>:
>> Ok, thanks, this seems to have solved it.
>> So the %nonassoc says to the parser that
>> (REP block) ATOM
>> is the right decision as opposed to
>> REP (block ATOM)
>> right?
>
> %token declares its arguments as tokens but
> does not give them any precedence level.
>
> %left, %right, and %nonassoc also declare their
> arguments as tokens.  in addition, each such line
> introduces a new precedence level stronger than
> the ones introduced by previous lines.
>
> if a shift/reduce conflict involves different precedences,
> the stronger precedence always wins.
>
> if a shift/reduce conflict is a tie between identical precedences,
> the resolution depends on which of the three lines
> (%left, %right, or %nonassoc) introduced the precedence.
>
> precedences introduced by %left resolve the tie
> by reducing; this creates left-to-right associativity (x-y-z).
>
> precedences introduced by %right resolve the tie
> by shifting; this creates right-to-left associativity (x^y^z in hoc).
>
> precedences introduced by %nonassoc do not resolve
> the tie.  they leave it as a conflict that gets reported.
>
> if you're defining a precedence that is not intended
> to be associative, much of the time it doesn't matter
> which you pick, because your grammar is likely to
> be such that ties never happen.  but %nonassoc is
> still the safe choice.
>
> in the running example, %nonassoc by itself
> doesn't say which of those two is right.  it just
> defines a precedence for ATOM, which is used
> as the precedence for shifting ATOM.
> because the REP block and block block rules
> have precedences too, the ambiguity can be
> resolved by comparing the precedences.
> which way things get resolved depends on whether
> the %nonassoc line comes before or after the
> other precedences, not on the meaning of %nonassoc.
>
> i said
>
>> %left '+'
>> %left REP
>> %nonassoc ATOM
>
> and that will give you REP block ATOM == REP (block ATOM)
> which is probably not what you want.  to tweak it,
> just move the %nonassoc line above the two %left lines.
>
> russ
>
>