Re: [Grammatica-users] Catch All production

[Marielle Fois Pelayo]

Hej Anant,

The error you are getting is telling you your grammar is ambiguous and 
in which production lies the ambiguity. It seems that when it tries to 
apply the 'reference' production, it cannot tell whether to apply 
'fedPrefix' or 'default' because both match. You should rewrite your 
grammar to fix this ambiguity.

Hope this helps!

Marielle


Anant Mistry wrote:

> I suppose it's hard to describe, so I've cut out an example out the
> grammar I'm struggling with.
>
> %tokens%
>
> dash    = '-'
> period  = '.'
> slash   = '/'
> colon   = ':'
> lparen  = '('
> rparen  = ')'
> letter  = <<[A-Z]>>
> digit   = <<[0-9]>>
> WHITESPACE = <<[ \t\n\r]+>> %ignore%
>
> %productions%
>
> reference       =   fedPrefix | default ;
>
> alpha           = ( letter )+ ;
> number          = ( digit )+ ;
>
> delimiter       = dash | period | slash | colon | lparen | rparen ;
>
> fedPrefix       = fedHead dash letter dash ;
>
> fedHead         = alpha ;  // Production code will ensure that this
> alpha can only contain up to 3 of the same character from A-Z
>
> default         = ( alpha [number] ) | ( number [alpha] ) ;
>
> Gives me the following when I run it with inpfile of "DDD-C-"
>
> bandikoot$ java -jar lib/grammatica-1.4.jar test.g --parse inpfile
> Error: in test.g: line 36:
>     inherent ambiguity in production 'reference' starting with token
>     <letter>
>
> reference       =   fedPrefix | default ;
>
>
> The fedHead is a production that you had provided an answer for in a
> previous email. I matches the generic definition of an alpha, but the
> parse production code will ensure that there can be up to 3 of the same
> character, e.g. AAA or FF or V
>
> The default production is the "catch all" because a doc number could
> start with a numeric or an alpha .... but it causes a problem in the
> parsing.
>
> Any thoughts?
>
> Anant
>
> On Tue, 2005-03-22 at 23:24 +0100, Per Cederberg wrote:
>
>
> > Well, there is not really any support for a default "exception"
> > production. Depending on what you want though, you can create
> > it yourself by modifying your grammar in like this:
> >
> > 1. Add a default token (last) to skip any unmatched characters:
> >   SKIP_ANY = <<.|\n>> %ignore%
> >
> > 2. Add dummy productions on places where you wish to skip stuff:
> >   Prod = A Skippy* B ;
> >   Skippy = Token1 | Token2 | ... | TokenN ;
> >
> > Doing this well might take some practice so that you can accept
> > all the "common" errors without enlarging the grammar too much.
> > If you really want accept any erroneous input, maybe Grammatica
> > isn't the perfect tool for the job.
> >
> > The Grammatica error handling does something similar though,
> > attempting to recover from broken input by skipping tokens until
> > it can continue parsing. Maybe one could modify that to just
> > skip unrecognized input instead of stopping the parse tree
> > construction and logging parse errors.
> >
> > /Per
> >
> > On tue, 2005-03-22 at 13:40 -0700, Anant Mistry wrote:
> >
> > > Does grammatica have a way of defining a "catch all" production i.e.
> > > if your input does not meet the parsing rules of the grammar, then
> > > jump to a generic or default  production?
> > >
> > > Thanks
> > >
> > > Anant
> >
> >
> >
> >
> > _______________________________________________
> > Grammatica-users mailing list
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> > http://lists.nongnu.org/mailman/listinfo/grammatica-users
>
>
>
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