[antlr-interest] transforming AST nodes

[Tom Smith]

In the absence of any advice to date, I've implemented Option 1, which
appears to work.  However any advice on improvements are still
welcome!

Thanks,
Tom.

On Dec 6, 2007 10:37 AM, Tom Smith <yotommy at gmail.com> wrote:
> Hello,
>
> I am seeking advice on good ways to transform the structure of some
> AST nodes.  My example involves translating a language that supports
> multiple assignment into one that does not.  The grammar (shown below)
> parses input of the form:
>
>     a, b = c, d
>
> into
>
>     ^(ASSIGN ^(LIST a b) ^(LIST c d))
>
> However I would like a structure more like:
>
>     ^(LIST ^(ASSIGN a c) ^(ASSIGN b d))
>
> I've considered two ways of accomplishing this.  I'd appreciate
> feedback on these, or suggestions for a better alternative.
>
> Option 1.  Change the rewrite expression for multAssign such that it
> uses custom Java code to create a new tree node with the desired
> structure.
>
> Option 2.  Create a tree grammar with output=AST that performs the
> desired transformation.  However if I understand correctly, this is
> only available in antlr 3.1.  Is that correct, and is it available
> somewhere?  And will I end up doing something like Option 1 in this
> grammar anyway?
>
> Thanks,
> Tom.
>
> grammar MultAssign;
>
> options {
>         output = AST;
> }
>
> tokens {
>         ASSIGN = '=';
>         LIST;
> }
>
> multAssign
>         :       assignList ASSIGN assignList
>         ->      ^(ASSIGN assignList assignList)
>         ;
>
> assignList
>         :       IDENT ( ',' IDENT )*
>         ->      ^(LIST IDENT+)
>         ;
>
> IDENT
>    :  ('a'..'z'|'A'..'Z'|'_'|'$') ('a'..'z'|'A'..'Z'|'_'|'0'..'9'|'$')*
>    ;
>
> WS  :  (' '|'\r'|'\t'|'\u000C'|'\n') {$channel=HIDDEN;}
>     ;
>