[antlr-interest] transforming AST nodes

[Andy Tripp]

My advice would be to keep your parsing, tree transformation, and output 
parts of your
program separate.

If this is the only transformation you'll be doing, it's fine to have 
the ANTLR parser do
more than parse - transform the AST. Or to use StringTemplate to do more 
than output
the AST - to output a transformed AST.

But if things get any more complicated, it would be nicer 
architecturally to keep the
AST transformation separate.

And then I'd just write vanilla Java (or whatever language) code to do 
the tree transformation.

Andy
                
       

Tom Smith 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;}
>     ;
>
>