David-Sarah Hopwood wrote:
>
> qinglangee wrote:
>> Hi all.
>>
>> I had a toy grammer,
>>
>> grammar cub;
>>
>> prog: (command)+ ;
>> command : (function |var)+;
>> function : (Letter)+',';
>> var : (Letter)+;
>> Letter : 'A'..'Z';
>> WS : ( '\t'|' '|'\r'|'\n'|'\u000C' )+ { $channel = HIDDEN; } ;
>>
>> I expected it can recognise text like:
>>
>> Funone,varone
>> Funtwo,vartwo
>>
>> Here is a tool run:
>> [11:14:04] warning(200): cub.g:4:25: Decision can match input such as
>> "Letter" using multiple alternatives: 1, 2, 3
>> As a result, alternative(s) 3,2 were disabled for that input
>> [11:14:04] error(201): cub.g:4:25: The following alternatives can never
>> be
>> matched: 2
>>
>> I didn't know who can never be matched, and why?
>
> 'command' starts with Letter+ in both its function and var alternatives,
> so they can't be distinguished by an LL parser.
>
> You probably want these to be lexer rules, i.e.
>
> prog : command+ ;
> command : (Function | Var)+ ;
>
> Function : ('A'..'Z')+ ',' ;
> Var : ('A'..'Z')+ ;
> WS : ( '\t'|' '|'\r'|'\n'|'\u000C' )+ { $channel = HIDDEN; } ;
>
> --
> David-Sarah Hopwood ⚥ http://davidsarah.livejournal.com
>
>
>
>
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>
It works. tks .Maybe I shoule understand what is a LL parser first.
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