ANTLR 3 RewriteEmptyStreamException on ID/NAME distinction - parsing

Question

Given the Lexer
fragment
FRAGID : ('a'..'z'|'A'..'Z') ('a'..'z'|'A'..'Z'|'0'..'9'|'_')* ;
ID : FRAGID;
NAME: FRAGID ('.' FRAGID)*;
Given the grammar
var_def: type=ID vname=ID ASSIGN expr
-> ^(VARDEF $type $vname expr)
;
with options
options
{
language=CSharp3;
output=AST;
}
and given the code
int i = 0
everything works fine.
However, when I want to allow the use of NAME in assignment (referring to another object)
var_def
: type=(NAME|ID) vname=ID ASSIGN expr
-> ^(VARDEF $type $vname expr)
;
I get at run-time RewriteEmptyStreamException
Antlr.Runtime.Tree.RewriteEmptyStreamException : token type
at Antlr.Runtime.Tree.RewriteRuleElementStream.NextCore() in c:\dev\stringtemplate_main\antlr\antlr3-main\runtime\CSharp3\Sources\Antlr3.Runtime\Tree\RewriteRuleElementStream.cs: line 200
at Antlr.Runtime.Tree.RewriteRuleTokenStream.NextNode() in c:\dev\stringtemplate_main\antlr\antlr3-main\runtime\CSharp3\Sources\Antlr3.Runtime\Tree\RewriteRuleTokenStream.cs: line 62
Doing some more investiagion, with the grammar
var_def
: type=NAME vname=ID ASSIGN expr
-> ^(VARDEF $type $vname expr)
;
I get a
Antlr.Runtime.Tree.RewriteEarlyExitException : Exception of type 'Antlr.Runtime.Tree.RewriteEarlyExitException' was thrown.

Answer 1

The 'i' in:
int i = 0
will always become an ID token. Because both ID and NAME match a single FRAGID, and since ID is defined before NAME, there will never be a NAME token (in case of a single FRAGID). It will always become an ID token.
That is why this won't work:
var_def
: type=NAME vname=ID ASSIGN expr
-> ^(VARDEF $type $vname expr)
;
You must realize that the lexer does not create tokens depending on what token the parser is trying to match at a specific time. The lexer works independently from the parser.
Try avoiding the assignment of a label to a parenthesized group of tokens/rules. Instead of:
var_def
: type=(NAME|ID) vname=ID ASSIGN expr -> ^(VARDEF $type $vname expr)
;
do this:
var_def
: type ID ASSIGN expr -> ^(VARDEF type ID expr)
;
type
: NAME
| ID
;