Re: Tell a rule to shift instead of reduce?

[Akim Demaille]

Le 22 déc. 2013 à 21:17, Adam Smalin <address@hidden> a écrit :

> > Please, post a self contained and minimal example.
> 
> Ok. I didn't test it by parsing actual code. I only looked at the conflict

> %left ','
> %right '='
> %left '.'
> 
> %%
> 
> program:
>     newline_many global_loop newline_many
> global_loop:
>       global_expr
>     | global_loop global_expr
> global_expr:
>     VAR '(' ')' code_block
>     
> code_block: '{' newline_many body newline_one_or_more '}'
> 
> newline_many: | newline_many '\n'
> newline_one_or_more: '\n' | newline_one_or_more '\n'
> body:
>       body_expr
>     | body newline_one_or_more body_expr
>     | body_expr '=' body_expr ',' body_expr %prec '.'
>     | body_expr ',' body_expr '=' body_expr
>     | body_expr ',' body_expr '=' body_expr ',' body_expr
> body_expr:
>       body_expr '=' body_expr
>     | body_expr '.' VAR
>     | VAR
> 
> %%

Honestly, I find your grammar hard to read.  Plenty of
opportunities for ambiguities here, not just conflicts.  It
shows a lot of redundancy, which calls for refactoring.  Have
another look at my proposal with lvals and rvals.

> state 27
> 
>    12 body: body_expr '=' body_expr . ',' body_expr
>    15 body_expr: body_expr . '=' body_expr
>    15          | body_expr '=' body_expr .  [',', '\n']
>    16          | body_expr . '.' VAR
> 
>     '='  shift, and go to state 29
>     '.'  shift, and go to state 22
> 
>     $default  reduce using rule 15 (body_expr)
> 
>     Conflict between rule 15 and token ',' resolved as reduce (',' < '=').
>     Conflict between rule 15 and token '=' resolved as shift (%right '=').
>     Conflict between rule 15 and token '.' resolved as shift ('=' < '.').
> 
> As you can see it says "Conflict between rule 15 and token ',' resolved as 
> reduce (',' < '=').". As you can see my rule is written as "| body_expr '=' 
> body_expr ',' body_expr %prec '.'"

You are referring to rule 12, bison is referring to rule 15.
Rule 15 has no explicit precedence token, so the last one is
issued, '=', whose precedence is greater than that of ','.

The output file includes:

    0 $accept: program $end

    1 program: newline_many global_loop newline_many

    2 global_loop: global_expr
    3            | global_loop global_expr

    4 global_expr: "VAR" '(' ')' code_block

    5 code_block: '{' newline_many body newline_one_or_more '}'

    6 newline_many: %empty
    7             | newline_many '\n'

    8 newline_one_or_more: '\n'
    9                    | newline_one_or_more '\n'

   10 body: body_expr
   11     | body newline_one_or_more body_expr
   12     | body_expr '=' body_expr ',' body_expr
   13     | body_expr ',' body_expr '=' body_expr
   14     | body_expr ',' body_expr '=' body_expr ',' body_expr

   15 body_expr: body_expr '=' body_expr
   16          | body_expr '.' "VAR"
   17          | "VAR"