Re: Misunderstanding %prec?

[Luca]

Using your grammar :

%token NEG FUNCALL SEMICOLON ID MINUS LPAREN RPAREN

%left NEG
%left FUNCALL

%%

input:
| input exp SEMICOLON
;

exp:
 ID
| MINUS exp %prec NEG
| exp LPAREN exp RPAREN %prec FUNCALL
;

%%

bison (2.4.1) complains

exp:
   4    | MINUS exp
...

state 6

   4 exp: MINUS exp .
   5    | exp . LPAREN exp RPAREN

   LPAREN  shift, and go to state 8

   LPAREN    [reduce using rule 4 (exp)]
   $default  reduce using rule 4 (exp)

state 8

   5 exp: exp LPAREN . exp RPAREN

   ID     shift, and go to state 3
   MINUS  shift, and go to state 4

   exp  go to state 9


because when, in state 6, it sees a MINUS, followed by an expression 
and  the lookahed symbol is LPAREN, the parser can shift the symbol 
LPAREN (expecting a function) or reduce MINUS exp to an exp (expecting a 
exp). The latter is the default, so -f(); will be never recognized as an 
function call.

You can rewrite the grammar in the following way, this is dirty but right:

%token NEG FUNCALL SEMICOLON ID MINUS LPAREN RPAREN

%left NEG
%left ID

%%

input:
| input exp SEMICOLON
;

exp:
 ID
| MINUS exp %prec NEG
| ID LPAREN exp RPAREN
;

%%

If you don't like it, you have to introduce a precedence using the rules 
rather than Context-Dependent Precedence (% prec % left % right ans so 
on) breaking the "expression rule" in many rules.
A useful example is the C grammar:
http://www.lysator.liu.se/c/ANSI-C-grammar-y.html
where postifix expression has an higher precedence than "numeric 
expression" but no Context-Dependent Precedence is used. The grammar has 
only one conflict, due to the "dangling else", but the default action of 
bison fixes it.

%token NEG FUNCALL SEMICOLON ID MINUS LPAREN RPAREN
%left NEG

%%

input:
| input postfix_expression SEMICOLON
;

postfix_expression:
 ID LPAREN exp RPAREN
 ;

exp:
 ID
| MINUS exp %prec NEG

;

%%

A hint: you can use directly the token '(' rather than LPAREN in .y 
file; of course lex has to return the char and not the token.

Luca

Tom Lieber ha scritto:
> I either don't understand shift/reduce yet, or I don't understand
> %prec. I believe the conflict in my grammar is that it can't decide
> whether -a(b) is -(a(b)) or (-a)(b), which I thought using %prec would
> solve. The essence of it is below, along with the corresponding
> section of the output file:
>
> %left NEG
> %left FUNCALL
>
> %%
>
> input:
> /* empty */
> | input exp SEMICOLON
> ;
>
> exp:
>   ID
> | MINUS exp %prec NEG
> | exp LPAREN exp RPAREN %prec FUNCALL
> ;
>
> 7: shift/reduce conflict (shift 9, reduce 4) on LPAREN
> state 7
>         exp : MINUS exp .  (4)
>         exp : exp . LPAREN exp RPAREN  (5)
>
>         LPAREN  shift 9
>         SEMICOLON  reduce 4
>         RPAREN  reduce 4
>
>