Re: [Grammatica-users] Catch All production

[Anant Mistry]

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
> address@hidden
> http://lists.nongnu.org/mailman/listinfo/grammatica-users