[antlr-interest] Empty AST entry stops parsing

[Jim Idle]

On 09/15/2009 04:41 AM, Kevin Twidle wrote:
> I have been using my PonderTalk (Smalltalk like) language for a while 
> now and I have just noticed that leaving a block empty causes a parser 
> error. This is with ANTLRWorks 1.2.3. I have refined the grammar to 
> show the point. The problem line is ^(statements). With that taken out 
> the parsing works.
>
> grammar Trial;
>
> options {
> output = AST;
>         k = 2;
> }
>
> start : sentences EOF
> ;
> sentences
> : sentence? (DOT sentences)?
> -> sentence? sentences?
> ;
> sentence: WORD | block;
> block : '[' sentences ']'
> -> ^(sentences)
> ;
>
You probably want to use an imaginary token as the root node of your 
block. ->^(BLOCK sentences?). Sentences acan be empty and you cannot 
rewrite it as the root node. Also, you probably want tokens for SENTENCE 
and perahps SENTENCES. If BLOCK is empty then deal with it specially in 
the tree grammar:

block : ^(BLOCK sentences)
           | BLOCK // Empty block
           ;


Jim
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