[antlr-interest] ANTLR3 AST construction with options and alternatives

[Foolish Ewe]

Hello All:

I think the previous approach had an error in it, which was detected at 
run-time.
The problem lies in that a "zero repetition" scenario may leave an attribute 
uninitilized,
in which case it should not be inserted into the AST.

This can be fixed by refactoring rules to use recursion and not variable 
repetition operators.
Note that the proposed changes may impact treewalking. So a rule of the 
form:

a: (b=B)? c:C->^(A $b $c);

may run into trouble if the optional B is not encountered, since $b will be 
uninitialized
We can fix this by doing:

a: b=optionalB c:C->^(A $b $c);

optionalB: b=B->^(OPTIONALB $b)
             |      ->^(OPTIONALB) // empty case
             ;

And if we have a case like say

a: (B | C)+  D;

we can then create an AST using something like
tokens{
    BORC; // B or C
}

a: (bc+=bOrC)+ d=D    ->^(A $bc+ $d)

bOrC:  b=B                 ->^(BORC $b)
       | c=C                  ->^(BORC $c)
       ;

At this point however, I am less certain about how to handle a zero or more 
repetition case
such that we don't try to insert an uninitialized attribute into the AST, 
e.g.

What should we do with:

a: (B|C)* D;

Regards:

Bill M.

>From: "Foolish Ewe" <foolishewe at hotmail.com>
>To: antlr-interest at antlr.org
>CC: FoolishEwe at hotmail.com
>Subject: ANTLR3 AST construction with options and alternatives
>Date: Mon, 13 Nov 2006 02:27:05 +0000
>
>Hello All:
>
>I've not heard back on a previous question about how to generate AST's.
>
>I have a construct in a grammar that looks something like:
>
>// allows optional override of default version id
>versionInfo: (id+=versionId)? (f1+=fieldType1 | f2+=fieldType2 | 
>f3+=fieldType3)*
>   ->^(VERSIONINFO $id+ $f1+ $f2+ $f3+);
>
>but it doesn't work the way I would like, meaning that I run the debugger 
>for a test case that
>has well defined ASTs are constructed for some of the fields, but some some 
>are not populated.
>This debugger halts and won't proceed to construct the VERSIONINFO node, 
>although
>before I decorated the grammar with action annotations for AST 
>construction, the grammar
>created a well formed parse tree.   I looked at the wiki  for AST related 
>stuff, and
>I am still not sure how to fix this problem or if it is a bug in ANTLR3.
>
>Regards:
>
>Bill M.
>
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