From:	"Göran Halfvarson" <m-12716 AT mailbox DOT swipnet DOT se>
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: Bison optionality
Date:	11 Feb 1997 03:12:35 GMT
Organization:	IGL
Lines:	41
Message-ID:	<01bc17c8$79608d00$7e58f482@nic.swip.net>
References:	<5dgqht$nu3 AT news00 DOT btx DOT dtag DOT de>
NNTP-Posting-Host:	dialup112-4-15.swipnet.se
NNTP-Posting-User:	s-12716
To:	djgpp AT delorie DOT com
DJ-Gateway:	from newsgroup comp.os.msdos.djgpp

I am not very experienced in Bison or Yacc programming 
but this works fine for me: 

...
| circle expression expression expression opt_radius {circle($2, $3, $4,
$5);}

opt_radius: 	/* empty is OK */	{ return NULL; }
		| expression		{ return $1; }
		;
....

I think this is a standard technique. 
If anyone has a better approach I would be happy to learn. 

Hope this helps!
/Göran 
goran AT igl DOT se


Gunnar Beushausen <GBeushausen AT t-online DOT de> wrote in article
<5dgqht$nu3 AT news00 DOT btx DOT dtag DOT de>...
> Hi there!
> 
> Does anyone know how to implement optionality into a bison script?
> I'm creating a command that's called "circle" with the arguments
> x_position, y_position, radius and color. But i want color to be an
> optional argumend. My current code looks like this
> 
> | circle expression expression expression expression {circle();}
> 
> But when i execute the code i can't leave out the last expression,
> otherwise there would be a parse error. 
> 
> Can somebody help me please?
> 
> ---
> Gunnar Beushausen
> GBeushausen AT t-online DOT de
> http://www.hof.de/~gbasic
> 

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