On 08/12/2009 05:45 PM, Nikolay Ognyanov wrote:
> Hi everybody,
>
> Please have a look at a toy grammar for language consisting of 2
> statements :
>
> grammar Ambiguous;
>
> expr
> : expr1
> | expr2
> ;
> expr1
> : PREFIX_1 expr2 SUFFIX
> ;
> expr2
> : PREFIX_2
> | PREFIX_2 SUFFIX
> ;
>
> PREFIX_1 : 'prefix_1';
> PREFIX_2 : 'prefix_2';
> SUFFIX : 'suffix';
> WS : (' ' | '\r' | '\n' | '\t')+ {$channel=HIDDEN;};
>
>
> Please do not advise how to fix it :) I know that but the question is why
> ANTLR considers rule for expr2 ambiguous? Here is a tool run:
Because at the time it is in expr2, it is ambiguous as to whether it
should match PREFIX_2 and exit, or match PREFIX_2 SUFFIX and then exit.
You only have 1 token of lookahead (by default). You might be able to
correct with syntactic predicates or a higher value of k, but your
grammar is not well defined because of the ambiguity.
--
Kevin J. Cummings
[email protected]
[email protected]
[email protected]
Registered Linux User #1232 (http://counter.li.org)
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