[antlr-interest] Advice on effective expr parsing

[Greg Lindholm]

Ask yourself this; If the parser sees "LPAREN expr RPAREN" should 
it match a "primary" or a "row_expr" ? Since the COMMA separated 
list is optional and a row_list_element can be an expr, this is 
ambiguous.

The even more trivial example is that a row_expr can be an expr, so
how would the parser know which to match.   

My suggestion is to combine expr and row_expr together (eliminate 
row_exp) adding the COMMA separated list to expr.
Then later during semantic analysis you could check for comma 
separated expr lists and report errors if invalid in the context.

Hope that helps.

Greg


--- Ruslan Zasukhin <sunshine at public.kherson.ua> wrote:
> Hi Guys,
> 
> Assume I have the following rules in grammar.
> It is easy to note that rule row_expr has ambiguity between 1 and 2
> branches
> on LPAREN expr RPAREN
> 
> -----------------------------------------------------------------
> expr
>     :    primary( (PLUS | MINUS) primary)*
>     ;
> 
> primary
>     :    LPAREN expr RPAREN
>     |    unsigned_value_specification
>     |    column_reference
>     |    set_function_specification
>     |    subquery
>     |    case_expression
>     |    cast_specification
>     ;
> 
> row_expr
>     :    LPAREN row_list_element ( COMMA row_list_element )* RPAREN
>     |    row_list_element
>     ;
> 
> row_list_element
>     :    expr
>     |    "null"
>     |    "default"
>     ;
>
----------------------------------------------------------------------
> 
> 
> Yes, using syntax predicate this can be handled as
> 
> row_expr
>     :    (LPAREN row_list_element COMMA) =>
>             LPAREN row_list_element ( COMMA row_list_element )*
> RPAREN
>     |    row_list_element
>     ;
> 
> 
> But I afraid this is NOT effective way.
> Because expr in general case can be VERY LONG and COMPLEX.
> 
> So I wonder does exists any "secret" "cool" trick to resolve this
> problem in
> row_expr ?
> 
> What is the best way ?
> 
> -- 
> Best regards,
> Ruslan Zasukhin      [ I feel the need...the need for speed ]
> -------------------------------------------------------------
> e-mail: ruslan at paradigmasoft.com
> web: http://www.paradigmasoft.com
> 
> To subscribe to the Valentina mail list
> send a letter to valentina-on at lists.macserve.net
> -------------------------------------------------------------
> 
> 
>  
> 
> Your use of Yahoo! Groups is subject to
> http://docs.yahoo.com/info/terms/ 
> 
> 


__________________________________________________
Do You Yahoo!?
Yahoo! Finance - Get real-time stock quotes
http://finance.yahoo.com

 

Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/