[antlr-interest] Interpreting IF statements in ANTLR

[Jim Idle]

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This questions is answered many times in the list :-)

Jim

> -----Original Message-----
> From: antlr-interest-bounces at antlr.org [mailto:antlr-interest-
> bounces at antlr.org] On Behalf Of ZelluX
> Sent: Thursday, July 08, 2010 1:39 AM
> To: antlr-interest at antlr.org
> Subject: [antlr-interest] Interpreting IF statements in ANTLR
> 
> I'm implementing a BASIC-like language, the syntax of if statements is
> almost the same to BASIC:
> 
>     IF a == b THEN
>         PRINT "EQUAL"
>     ELSE
>         PRINT "UNEQUAL"
>     ENDIF
> 
> I have write a grammar file to parse and a tree walker to interpreter
> the
> language:
> 
>     [Expr.g]
>     options {
>         language=Python;
>         output=AST;
>         ASTLabelType=CommonTree;
>     }
> 
>     tokens {
>         BLOCK;
>     }
> 
>     block
>         : stmt* -> ^(BLOCK stmt*)
>         ;
> 
>     if_stmt
>         : 'IF' c=expr 'THEN' t=block ('ELSE' f=block)? 'ENDIF'
>             -> ^('IF' $c $t+ ^('ELSE' $f+))
>         ;
> 
> In the AST walker:
> 
>     [Walker.g]
>     options {
>         language=Python;
>         tokenVocab=Expr;
>         ASTLabelType=CommonTree;
>     }
> 
>     block
>         : ^(BLOCK stmt*)
>         ;
> 
>     stmt
>         : ...
>         | 'IF' expr t=stmt* 'ELSE' f=stmt*
>              {}
> 
> Now I can correctly generate AST for my language, but I don't know how
> to
> handle branch statement. To be more exactly, if the *expr* in if
> statement
> is true, how can I avoid evaluation of the ELSE statement? Thanks
> 
> --
> Best regards,
> Wang Yuanxuan
> Parallel Processing Institute, Fudan University
> 
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