Re: resolving shift/reduce in ActionScript3

[Hans Aberg]

On 26 Nov 2008, at 14:43, Matthias Kramm wrote:

> I'm writing a parser for ActionScript3, and I'm stuck
> with the following problem.
>
> In ActionScript, you can omit the semicolon (;). Furthermore, function
> pointers are allowed.
>
> I have the following (simplified) grammar:
>
> PROGRAM : EXPRESSION
>         | PROGRAM ';' EXPRESSION
>         | PROGRAM EXPRESSION //omit ;
>
> EXPRESSION : T_IDENTIFIER '(' EXPRESSION ')' // functioncall
>            | T_IDENTIFIER                    // variable
>            | '(' EXPRESSION ')'
>            | EXPRESSION "++"
>            | EXPRESSION "--"
>            | EXPRESSION '+' EXPRESSION
>            | EXPRESSION '-' EXPRESSION
>            | EXPRESSION '/' EXPRESSION
>              (etc.)
>
> Now, when parsing the code
>
>     function ( var++ )
>
> it's naturally unclear to the parser whether to treat this as
> "function(var++);" or "function;var++;"
>
> Interestingly enough, bison seems to choose the latter (from the
> -r report):
>
> state 1
>     4 EXPRESSION: T_IDENTIFIER . '(' EXPRESSION ')'
>     5           | T_IDENTIFIER .  [$end, T_IDENTIFIER, ';', '(',  
> ')', "++", "--", '+', '-', '/']
>     '('  shift, and go to state 5
>     '('       [reduce using rule 5 (EXPRESSION)]
>     $default   reduce using rule 5 (EXPRESSION)
>
> Is there some way to tell bison to prefer shifting to reducing in this
> case?

If one should use Bison token precedence, then T_IDENTIFIER needs to  
be expanded in the grammar, so that there is a token immediately  
before the parsing "." in the shift-reduce conflict above.

But if your identifiers have been defined and pu on a look-up table  
which the lexer can read, them change the rule above to
  EXPRESSION:
    FUNCTION '(' EXPRESSION ')'
where the lexer returns the token FUNCTION.

> (I'd rather not go down the GLR road just yet. I know I could add a
>  second '(' token and make the lexer generate that instead of the
>  "normal" '(' whenever at the "start" of an expression- but I was  
> hoping
>  there might be an easier way I'm missing)

Much can be done without GLR with such tweaking, even Yaccify C++:
  http://www.parashift.com/c++-faq-lite/compiler- 
dependencies.html#faq-38.11

  Hans