[antlr-interest] Empty AST entry stops parsing

[Gavin Lambert]

At 01:20 16/09/2009, Kevin Twidle wrote:
>line 1:6 no viable alternative at character ' '
>line 1:8 no viable alternative at character ' '
>line 1:11 no viable alternative at character ' '

Those are because you haven't defined any lexer rules that can 
match whitespace.

>tokens{
>         BLOCK = '_block';
>}

That is *not* an imaginary token.  Remove the '= ...' part or 
it'll actually try to match that in the input.

>Exception in thread "main" 
>org.antlr.runtime.tree.RewriteEmptyStreamException: rule sentences
>at 
>org.antlr.runtime.tree.RewriteRuleElementStream._next(RewriteRuleElementStream.java:158)

This is because your 'sentences' rule may match nothing; in that 
case you need to use 'sentences?' in the rewrite in the block 
rule.

>start:sentences EOF
>;
>sentences
>:sentence? (DOT sentences)?
>-> sentence? sentences?
>;
>sentence:WORD | block;
>block:'[' sentences ']'
>-> ^(BLOCK sentences)
>;

Is it valid to have dots without anything else?  As your grammar 
stands at the moment, it'll accept '.....' in sentences.

If not, then you should express the optionality externally:

start : sentences? EOF ;
sentences : sentence (DOT sentence)* -> sentence+ ;
sentence : WORD | block ;
block : '[' sentences? ']' -> ^(BLOCK sentences?) ;

(I've also changed the sentences rule to use iteration rather than 
recursion, which is more efficient.)