Re: Parsing an If...End If construct

Scott Nicol <snicol@apk.net>
19 Nov 2004 00:49:50 -0500

          From comp.compilers

Related articles
Parsing an If...End If construct mness1978@hotmail.com (2004-11-17)
Re: Parsing an If...End If construct snicol@apk.net (Scott Nicol) (2004-11-19)
Re: Parsing an If...End If construct vbdis@aol.com (2004-11-19)
Re: Parsing an If...End If construct cfc@shell01.TheWorld.com (Chris F Clark) (2004-11-20)
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From: Scott Nicol <snicol@apk.net>
Newsgroups: comp.compilers
Date: 19 Nov 2004 00:49:50 -0500
Organization: Compilers Central
References: 04-11-047
Keywords: parse, syntax
Posted-Date: 19 Nov 2004 00:49:50 EST

Michael wrote:
> The bison grammar I have so far is listed below. It is has several
> conflicts that I'm having trouble resolving. Can anyone give me some
> pointers on how to deconflict this grammar?
  >[...]
> instruction : END
> | GOSUB destination
> | GOTO destination
> | IF conditions THEN statements opt_else
> | IF conditions statements else_if_list opt_else END IF
> | PRINT print_list
> | RETURN
> ;
>
> else_if_list : else_if_list else_if
> | else_if
> ;
>
> else_if : ELSE IF conditions statements
> | /* empty */
> ;
>
> opt_else : ELSE statements
> | /* empty */
> ;


A few obvious problems. The END IF and ELSE IF are ambiguous beyond 1
token of lookahead. You can fix this by disambiguating in the
scanner, producing ELSEIF and ENDIF tokens. Second problem is that
you are burying the optional else_if_list too deep.


Rewrite your grammar like this:


instruction: END
| GOSUB destination
| GOTO destination
| IF conditions THEN statements opt_else
| IF conditions THEN statements opt_else_if_list opt_else ENDIF
| PRINT print_list
| RETURN
;


opt_else_if_list: /* empty */ | else_if_list;


else_if_list: else_if
| else_if_list else if
;


else_if: ELSEIF conditions statement;


opt_else: /* empty */ | ELSE statements;


You will have 2 shift/reduce conflicts left:
1. the classic if/then/else conflict
2. the BASIC classic colon conflict


both of which you want to shift so it is OK if you leave them alone.


--
Scott Nicol
snicol@apk.net


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