[antlr-interest] Managing more than one node as root exceptions

[Andreas Stefik]

Hi folks,

I have a relatively simple grammar, which works well under most
conditions, but in one case it has issues. Here is a snippet of the
grammar:

expression :	or 	;
or 	:	and (OR ^ and)* 	;
and 	:	equality (AND ^ equality)* 	;	
equality:	comparison ((EQUALITY ^ | NOTEQUALS ^) comparison)* 	;
comparison:	add ((GREATER ^| GREATER_EQUAL ^| LESS ^| LESS_EQUAL^) add)* 	;
add	:	multiply ((PLUS ^| MINUS^) multiply)* 	;
multiply:	combo_expression ((MULTIPLY ^| DIVIDE ^|MODULO^) combo_expression)* 	;
combo_expression
	:	atom
	|	NOT atom
	|	CAST LEFT_PAREN assignment_declaration COMMA expression RIGHT_PAREN
	;
atom 	: qualified_name
	(
		LEFT_SQR_BRACE expression RIGHT_SQR_BRACE //this condition breaks
	|	(COLON ID)? LEFT_PAREN (expression (COMMA expression)*)?
RIGHT_PAREN //so does this one
	)?
	| (MINUS)? INT
	| BOOLEAN
	| (MINUS)? DECIMAL
	| STRING
	| NULL
	| LIBRARY_CALL LEFT_PAREN expression COMMA expression COMMA
expression RIGHT_PAREN
	| INPUT LEFT_PAREN expression RIGHT_PAREN
	| LEFT_PAREN expression RIGHT_PAREN -> ^(expression) ;

So far as I can tell from testing, this works fine, except in the case
where you have an expression with surrounding parentheses and the item
inside of it is a qualified_name, with either square braces or the
part of the rule after it. In this particular case, the following
exception is thrown:

java.lang.RuntimeException: more than one node as root (TODO: make
exception hierarchy)
	at org.antlr.runtime.tree.BaseTreeAdaptor.becomeRoot(BaseTreeAdaptor.java:150)
        at org.sonify.vm.hop.parser.HopParser.atom(HopParser.java:4954)
//notice it is in the parser, not the tree grammar
	at org.sonify.vm.hop.parser.HopParser.combo_expression(HopParser.java:4403)

To give an example of what would break in the language (not all rules
shown), this would work fine:

boolean array a
a[0] = 10
a[1] = a[0]

and this would throw the above exception

boolean array a
a[0] = 10
a[1] = (a[0])

I've tried everything obvious (to me) that I can think of (e.g.,
pulling out the rules one by one), but am perplexed as to why that
rule throws a double root exception.
I'm pretty sure the problem is relative to this portion of the
grammar, but just in case, here is the full grammar:

https://sourceforge.net/apps/trac/sodbeans/browser/trunk/sodbeans/Compiler/src/org/sonify/vm/hop/parser/Hop.g
and the full tree grammar:
https://sourceforge.net/apps/trac/sodbeans/browser/trunk/sodbeans/Compiler/src/org/sonify/vm/hop/parser/HopSymbolTableWalker.g

When I run the code in the ANTLRWorks interpreter, it generates the
trees just fine. One final clue is that my tree grammar is showing UP
and DOWN tokens, which I know means there is a mismatch somewhere ---
where, however, and how to fix, I'm not quite sure about, and this
error seems to be coming from the parser anyway, so I don't "suspect"
that that is the cause for this immediate problem.

Any help would be greatly appreciated, as the error is driving me a bit crazy,

Stefik