[antlr-interest] transforming AST nodes

[Tom Smith]

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;}
    ;