Re: how to resolve if/elseif/else ambiguity?

[SirZooro]

I have performed some additional analysis and found that conflicting rule was
incorrectly created. Therefore closed_else rules should look as following:

[code]
closed_else: ELSE closed_stmt
        | ELSEIF '(' expr ')' closed_stmt closed_else
;
[/code]

Daniel


SirZooro wrote:
> 
> I am trying to create grammar which has if/elseif/else instruction.
> However I am unable to get rid of shift/reduce conflicts. I found
> http://www.parsifalsoft.com/ifelse.html page which helped me a bit, but
> still I have problems. My grammar is as follow:
> [code]
> %token IF ELSE ELSEIF FOR NUMBER STMT '(' ')';
> %%
> prog : stmt;
> 
> expr: NUMBER;
> 
> stmt: open_stmt
>       | closed_stmt;
> 
> open_stmt: IF '(' expr ')' stmt
>       | IF '(' expr ')' closed_stmt open_else
>       | FOR '(' expr ')' open_stmt;
> 
> closed_stmt: STMT
>       | IF '(' expr ')' closed_stmt closed_else
>       | FOR '(' expr ')' closed_stmt;
> 
> open_else: ELSE open_stmt
>       | ELSEIF '(' expr ')' open_stmt
>       | ELSEIF '(' expr ')' closed_stmt open_else;
> 
> closed_else: ELSE closed_stmt
>       | ELSEIF '(' expr ')' closed_stmt
>       | ELSEIF '(' expr ')' closed_stmt closed_else;
> [/code]
> 
> Bison reports following problem:
> [code]
> state 30
> 
>    11 open_else: . ELSE open_stmt
>    12          | . ELSEIF '(' expr ')' open_stmt
>    13          | . ELSEIF '(' expr ')' closed_stmt open_else
>    13          | ELSEIF '(' expr ')' closed_stmt . open_else
>    14 closed_else: . ELSE closed_stmt
>    15            | . ELSEIF '(' expr ')' closed_stmt  [$end, ELSE, ELSEIF]
>    15            | ELSEIF '(' expr ')' closed_stmt .  [$end, ELSE, ELSEIF]
>    16            | . ELSEIF '(' expr ')' closed_stmt closed_else
>    16            | ELSEIF '(' expr ')' closed_stmt . closed_else
> 
>     ELSE    shift, and go to state 20
>     ELSEIF  shift, and go to state 21
> 
>     ELSE      [reduce using rule 15 (closed_else)]
>     ELSEIF    [reduce using rule 15 (closed_else)]
>     $default  reduce using rule 15 (closed_else)
> 
>     open_else    go to state 31
>     closed_else  go to state 32
> [/code]
> 
> 'Rule 15' is the following rule: 
> closed_else: ELSEIF '(' expr ')' closed_stmt
> 
> All suggestions are welcome.
> 
> Daniel
> 

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