[antlr-interest] Allow empty file/input

[Jim Idle]

Looks like your tree grammar does not allow an empty input. I find it useful
to always have a single base node:

eat: b+=bananas+ EOF
 
	->^(TRANSLATION_UNIT $b*)
;

Then your tree rule is:

eat: ^(TRANSLATION_UNIT bananas+)
   | TRANSLATION_UNIT
       // Empty input
;


Jim

> -----Original Message-----
> From: antlr-interest-bounces at antlr.org [mailto:antlr-interest-
> bounces at antlr.org] On Behalf Of Rui Vilão
> Sent: Monday, November 22, 2010 11:03 AM
> To: antlr-interest at antlr.org
> Subject: [antlr-interest] Allow empty file/input
> 
> Hi there,
> 
> I implemented a simple grammar and I want to add support to allow empty
> input. Empty input can be for example comments too. So I defined it
> this way
> 
> eat: (bananas)+
>     |
>     ;
> 
> bananas: <lots of stuff here>
> 
> LINE_COMMENT   : '//' ~('\r' | '\n')*   {skip(); };
> 
> ETC...
> 
> The thing is if I put anything that does not match the grammar, i.e.
> empty string, comment, whatever I get the following error when I try to
> get the object that will represent my DSL:
> 
> java.lang.NullPointerException: null
>         at
> org.antlr.runtime.tree.BaseTreeAdaptor.isNil(BaseTreeAdaptor.java:70)
> ~[antlr-runtime-3.2.jar:na]
>         at
> org.antlr.runtime.tree.CommonTreeNodeStream.nextElement(CommonTreeNodeS
> tream.java:93)
> ~[antlr-runtime-3.2.jar:na]
>         at
> org.antlr.runtime.misc.LookaheadStream.fill(LookaheadStream.java:94)
> ~[antlr-runtime-3.2.jar:na]
>         at
> org.antlr.runtime.misc.LookaheadStream.sync(LookaheadStream.java:88)
> ~[antlr-runtime-3.2.jar:na]
>         at
> org.antlr.runtime.misc.LookaheadStream.LT(LookaheadStream.java:119)
> ~[antlr-runtime-3.2.jar:na]
>         at
> org.antlr.runtime.tree.CommonTreeNodeStream.LA(CommonTreeNodeStream.jav
> a:116)
> ~[antlr-runtime-3.2.jar:na]
>         at
> com.feedzai.pulse.service.kpi.parser.KpiTree.script(KpiTree.java:85)
> ~[classes/:na]
> 
> Does anyone know how can I achieve this?
> 
> Thank you in advance,
> 
> Rui Vilao
> 
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