ANTLR AST rules fail with RewriteEmptyStreamException
- by Barry Brown
I have a simple grammar:
grammar sample;
options { output = AST; }
assignment
: IDENT ':=' expr ';'
;
expr
: factor ('*' factor)*
;
factor
: primary ('+' primary)*
;
primary
: NUM
| '(' expr ')'
;
IDENT : ('a'..'z')+ ;
NUM : ('0'..'9')+ ;
WS : (' '|'\n'|'\t'|'\r')+ {$channel=HIDDEN;} ;
Now I want to add some rewrite rules to generate an AST. From what I've read online and in the Language Patterns book, I should be able to modify the grammar like this:
assignment
: IDENT ':=' expr ';' -> ^(':=' IDENT expr)
;
expr
: factor ('*' factor)* -> ^('*' factor+)
;
factor
: primary ('+' primary)* -> ^('+' primary+)
;
primary
: NUM
| '(' expr ')' -> ^(expr)
;
But it does not work. Although it compiles fine, when I run the parser I get a RewriteEmptyStreamException error. Here's where things get weird.
If I define the pseudo tokens ADD and MULT and use them instead of the tree node literals, it works without error.
tokens { ADD; MULT; }
expr
: factor ('*' factor)* -> ^(MULT factor+)
;
factor
: primary ('+' primary)* -> ^(ADD primary+)
;
Alternatively, if I use the node suffix notation, it also appears to work fine:
expr
: factor ('*'^ factor)*
;
factor
: primary ('+'^ primary)*
;
Is this discrepancy in behavior a bug?
