exit antlr 3 parser early without raising exception - antlr

Question

I am using antlr 3.1.3 and generating a python target. My lexer and parser accept very large files. Based on command-line or dynamic run-time controlled parameters, I would like to capture a portion of the recognized input and stop parsing early. For example, if my language consists of a header and a body, and the body might have gigabytes of tokens, and I am only interested in the header, I would like to have a rule that stops the lexer and parser without raising an exception. For performance reasons, I don't want to read the entire body.
grammar Example;
options {
language=Python;
k=2;
}
language:
header
body
EOF
;
header:
HEAD
(STRING)*
;
body:
BODY { if stopearly: help() }
(STRING)*
;
// string literals
STRING: '"'
(
'"' '"'
| NEWLINE
| ~('"'|'\n'|'\r')
)*
'"'
;
// Whitespace -- ignored
WS:
( ' '
| '\t'
| '\f'
| NEWLINE
)+ { $channel=HIDDEN }
;
HEAD: 'head';
BODY: 'body';
fragment NEWLINE: '\r' '\n' | '\r' | '\n';

Answer 1

What about:
body:
BODY {!stopearly}? => (STRING)*
;
?
That's using a syntantic predicate to enable certain language parts. I use that often to toggle language parts depending on a version number. I'm not 100% certain. It might be you have to move the predicate and the code following it into an own rule.

Answer 2

This is a python-specific answer. I Added this to my parser:
#parser::header
{
class QuitEarlyException(Exception):
def __init__(self, value):
self.value = value
def __str__(self):
return repr(self.value)
}
and changed this:
body:
BODY { if stopearly: raise QuitEarlyException('ok') }
(STRING)*
;
Now I have a "try" block around my parser:
try:
parser.language()
except QuitEarlyException as e:
print "stopped early"