Re: Use of %prec breaks associativity

[Akim Demaille]

Le 14 nov. 08 à 11:04, Jeroen Ketema a écrit :

> %nonassoc '+'
> %nonassoc PLUS_PREC
>
> proc '+' proc %prec PLUS_PREC

This grammar states that when there are s/r conflicts between the rule  
"proc + proc" and the token "+" then Bison should use their own  
precedences to resolve the conflict.  The precedence of the rule is  
the one of PLUS_PREC, just as you stated via %prec, and that of "+" is  
that of "+".

Therefore, since PLUS_PREC has higher precedence, the rule wins, which  
in the end makes "+" behave left-associatively.

Now, in the case of "%nonassoc '+' PLUS_PREC" both the rule and the  
token have the precedence, so we use their associativity to decide.   
Here, it is non-assoc, so this token is not allowed to "follow this  
rule", so indeed + is non assoc.

%nonassoc, %left etc. express nothing about associativity, they are  
understood as hints to favor either the rule, the token or neither in  
s/r conflicts.

If you want to express associativities, stick to the book, and write

%nonassoc '+';
foo: foo '+' foo;

It suffices.  By default, the precedence/associativity of a rule is  
that of its last token.