[antlr-interest] Multiple alternatives warning

[Pavel Grinfeld]

I'm sorry, you did tell me how to fix it!

On 6/22/2010 11:59 AM, John B. Brodie wrote:
> Greetings!
>
> On Tue, 2010-06-22 at 11:08 -0400, Pavel Grinfeld wrote:
>    
>> Hi,
>>
>> I'm begging to get my bearings on this.... But the following gives two
>> "multiple alternatives" warnings.
>>
>> How come?
>>
>> Many thanks in advance,
>>
>> Pavel
>>
>> grammar PGTeX;
>>
>> doc
>> :
>> (a=text{System.out.print(">"+$a.value+"<");}|b=command{System.out.print($b.value);})+
>> EOF;
>>
>> command returns [ String value ]
>> :'\\' a=word '\{' b=word '\}' {$value=$a.value+"+"+$b.value;};
>>
>> text    returns[String value]
>> :{$value="";}(a=word{$value += $a.value;} | WS{$value += $WS.text;} )+;
>>
>> word    returns[String value]
>> :WORD  {$value = $WORD.text;}  ;
>>
>> WS  :   ( ' '
>>           | '\t'
>>           | '\r'
>>           | '\n'
>>           )+;
>>
>>
>> WORD:    ('a'..'z')+;
>>      
> your grammar is ambiguous because there is no way to know how to divide
> up multiple text instances and supply them to the loop in the doc rule.
>
> my explanation is probably really confusing but lets look at an example.
>
> consider this input: a b
>
> so the input is just 3 tokens: a WS b
>
> now there are at least 4 derivations (e.g. parse trees) for this input
> under your grammar:
>
> doc
>     text
>        word=='a'
>        WS==' '
>        word=='b'
>
> or
>
> doc
>     text
>        word=='a'
>     text
>        WS==' '
>     text
>        word=='b'
>
> or
>
> doc
>     text
>        word=='a'
>        WS==' '
>     text
>        word=='b'
>
> doc
>     text
>        word=='a'
>     text
>        WS==' '
>        word=='b'
>
> all of these are perfectly valid under your grammar and there is no way
> for the Tool to decide which derivation you really want (i'm guessing
> you want the first...)
>
> I think if you ignore the warning and try your grammar you will get the
> first derivation, not sure.
>
> of course the better solution is to re-work your grammar in order to
> remove the ambiguity.
>
> is this Knuth's TeX typesetting language? I have not used TeX directly,
> but have used Lamport's LaTeX quite a bit. And in LaTeX commands and
> text are interleaved. is that true for TeX also? so maybe:
>
> doc : command+ ( text command+ )+ EOF;
>
> Hope this helps...
>     -jbb
>
>
>