Re: [SimpleParse] Graceful failure ?

[H. T. H. <hi...@co...>]

On Sun, Mar 21, 2004 at 03:58:16PM -0500, Mike C. Fletcher wrote:
> H. T. Hind wrote:
> ...
> 
> >>>>column_specs := '<s>',ws,('<c>'?,ws)+,'\n'  
> >>>>
> ...
> 
> >The data had '<S>' , i.e a capital S instead of a lower case s
> >and that resulted in the app going into a loop. 
> > 
> >
> But not from the section described if spelled like this:
> 
> 	column_specs := '<s>',ws,('<c>',ws)*,'\n'
> 
> that is, you can't get a loop *from this part of the grammar* because 
> there's no null-matching construct.  When writing a grammar, you write 
> the grammar to include everything which is valid and reject that which 
> is not valid.  An '<S>' presented anywhere to the above production will 
> simply cause a fail, not a loop.

Thank you for the pointer. I looked at the grammar and indeed
there was a null-matching construct. 
 
> >My issue is , that it may not be possible to define all the valid
> >input that we'll get. In the case where the input is invalid, I'd
> >want the  application to fail instead of going into an infinite
> >loop. 
> > 
> >
> Understood, what I'm saying is, you don't need to define all possible 
> inputs, you just need to avoid defining constructs that can successfully 
> match a NULL string.  You might also want to look at the "cut" or 
> "errorOnFail" directive, which can raise SyntaxErrors if part of a 
> construct fails (doesn't help with null-matching constructs, however).

Avoiding the NULL string was it. Now it correctly fails . 

Thanks,
HTH