Re: [SimpleParse] Graceful failure ?

[Mike C. F. <mcf...@ro...>]

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.

>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).

HTH,
Mike

_______________________________________
  Mike C. Fletcher
  Designer, VR Plumber, Coder
  http://members.rogers.com/mcfletch/