[antlr-interest] Empty alternative causes error

[Jamie Penney]

You are correct. I am working on an unfinished grammar with Johannes 
Luber. The extra symbols in the rewrite rule are needed by Johannes for 
his project. Any chance you (or anyone else) could have a quick look at 
it and give us some pointers? This is the Java target version, as I was 
having difficulties debugging the C# target version.

Thanks,
Jamie Penney

Jim Idle wrote:
> My educated guess is that you are trying to write a C# parser and 
> falling in to the trap of typing in the language spec too literally. The 
> spec is not a spec for writing a grammar and so it creates ambiguities 
> all over the place. Basically, you just cannot write it by following the 
> language spec like that, you must left factor everything so it is a 
> sensible grammar.
>
> I suspect that what is happening is that when you remove the empty alt, 
> a rule higher up creates the ambiguity. There is a difference between 
> generic declarations and generic invocations and you also need to be 
> able to distinguish between generics and the < operator in some 
> circumstances. 
>
> Finally, you don't need the CLOSE_GENERICS element of the tree you are 
> building.
>
> There are no bugs in this area that I am aware of, I am afraid that you 
> probably need to rethink your grammar.
>
> Jim
>
>
>
>
>   
>> -----Original Message-----
>> From: Jamie Penney [mailto:jpen054 at ec.auckland.ac.nz]
>> Sent: Monday, February 18, 2008 2:05 PM
>> To: antlr-interest at antlr.org
>> Subject: [antlr-interest] Empty alternative causes error
>>
>> Hi All,
>> I have the following rules:
>> type_argument_list_handler
>>     :    (type_argument_list)=> type_argument_list -> ^(OPTIONAL
>> type_argument_list)
>>     |    /*empty*/ -> ^(OPTIONAL)
>>     ;
>>
>> type_argument_list
>>     :    LT type_parameters GT -> ^(OPEN_GENERICS[$LT] type_parameters
>> CLOSE_GENERICS[$GT])
>>     ;
>>
>> If I try to remove the empty alternative, I get warnings about
>> type_argument_list_handler being able to handle LT using multiple
>> alternatives, yet there are no alternatives. If I leave it there, I 
>>     
> get
>   
>> a test for the UP token generated from that rule, yet UP does not
>> exist.
>>
>> I have two possible answers to this problem.
>> 1) The grammar is wrong. Is there another way of specifying empty
>> rules?
>> 2) ANTLR is wrong. I know that Token.UP is really what it is searching
>> for, yet it uses UP instead. Is this possibly a bug?
>>
>> Thanks,
>> Jamie
>>     
>
>
>   

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