On 06/02/2010 12:45 PM, Ken Williams wrote:
> Hi,
>
> I have the following toy grammar:
>
> -------------------------
> date : DIGIT+ SLASH DIGIT+ SLASH DIGIT+ ;
>
> SLASH : '/' ;
> DIGITS : DIGIT+ ;
> fragment DIGIT : '0'..'9' ;
> -------------------------
>
> (No options{} declarations etc.)
>
> I know I shouldn't be using fragments in this way, but it was an accident -
> 'DIGIT' used to be a lexer rule, but then I changed it into a fragment,
> added DIGITS as a new lexer rule, and forgot to remove DIGIT+ from the
> parser rule.
>
> The problem I encountered was that even though the grammar check succeeded
> and the lexer/parser classes were successfully generated, the 'date' rule
> doesn't match the intended input.
How can it? When the lexer runs (before the parser, and without any
knowledge of the parse state) when it finds DIGIT+, it creates a DIGITS
token. This is your problem, not the fragment DIGIT. What happens when
you change your date rule to:
date : DIGITS SLASH DIGITS SLASH DIGITS
;
?
Does it work then?
> It seems like the right solution would be for Antlr to generate an error
> when a fragment is used in a parser rule.
>
> Or is there a reason I'm not thinking of to allow them?
>
> Thanks.
--
Kevin J. Cummings
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