Re: [PHP-DEV] Re: Bug #838 Updated: Parser error in nested if From: Antonio Garcia Mari (agarcia <email protected>)
Date: 10/09/98

> Nope, it's not clear at all, and it's impossible to implement using an LALR
> parser. Why is it not 'clear'? Because compilers aren't human beings and
> can't apply intelligence. The parser has a working set of rules, with
> rules of what to do in each case, using one lookahead token at the most.

I know nothing about LALR, i only remember some theorics i learned in my
old school times, so perhaps all what i'm going to say is stupid or perhaps
i'm right and you missunderstood what i was saying.

Theese are the rules i found in language-parser.y:

        | IF '(' expr ')' { cs_start_if (&$3 _INLINE_TLS); } statement elseif_list else_single {
cs_end_if ( _INLINE_TLS_VOID); }
        | IF '(' expr ')' ':' { cs_start_if (&$3 _INLINE_TLS); } statement_list new_elseif_list
new_else_single { cs_end_if ( _INLINE_TLS_VOID); } ENDIF ';'

So, rules for colon if's and non colon if's are completely different. except for the
cs_start_if, cs_end_if. I deduce (correct me if i'm wrong) that theese functions
register the nesting if's.

What i'm saying is that a colon if is a different instruction (a different language construct ) that a non
colon if. If i'm right, the register functions must be different.

Considere the language construct AF like the colon IF:

AF '(' expr ')' ':' { cs_start_af (&$3 _INLINE_TLS); } statement_list new_elseaf_list new_elsea_single {
cs_end_af ( _INLINE_TLS_VOID); } ENDAF ';'

Then if i've understood the semmantics, this construction is correct:
AF ($a==1):
        if ($b==2) echo "hello world";
ELSEAF ($a==2):
        echo "hello world";
ENDAF;

Can you do that without changing IF by AF or am i completely obfuscated?

> We've designed the language to use the standard solution for the dangling
> else problem, which means that an else (or an elseif) applies to the least
> recently nested if. When the parser spots an else statement, it expects it
> to match the least recently nested if. It doesn't scan the whole block and
> tries to make sense out of it. Of course, you could do that, but it
> wouldn't have been an LALR parser and it would have been slower by orders
> of magnitude (all popular languages can be described by LALR grammars,
> including C, Perl, Java and just about anything else you can think of). It
> expects you to follow the language grammar, and an else (or elseif) at that
> spot is supposed to match that last if. Because of that, it expects that
> else to have the same syntax as the last if, i.e., if the last if wasn't
> using colon mode, the else shouldn't either. Since that's not the case, it
> results in a parse error.
>
> Zeev
> --
> Zeev Suraski <zeev <email protected>>
> For a PGP public key, finger bourbon <email protected>
>
>

Antonio Garcia Mari
AT4.net Internet y Comunicación
http://www.at4.net/ 

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