[antlr-interest] parsing empty function parameters
antlr at mirality.co.nz
Fri Oct 5 16:01:30 PDT 2007
At 09:39 6/10/2007, Andy Tripp wrote:
>It should be that simple, but (at least with my grammar) when arg
>can be empty, ANTLR bombs out.
How is arg/argSpec defined? Does it refer back to argList? Does
it contain loops?
There are three things that tend to result in
OutOfMemoryExceptions (which are actually all variations on one
1. infinite recursion (typically caused by left recursion); ANTLR
is usually pretty good about detecting these, but sometimes it can
miss particularly involuted cases.
2. empty loops. If you've got a (...)* or (...)+ loop that can
result in matching nothing (within the parentheses), this will
create an infinite loop. Be especially careful when putting
optional terms or loops inside other loops. ANTLR will not detect
these cases for you at all -- you have to make sure it's correct
all by yourself.
3. empty lexer rules. (This doesn't sound like your problem, but
I'm including it for completeness.) If a top-level
(non-'fragment') lexer rule can end up matching no characters,
then ANTLR will internally construct an infinite loop generating
that token repeatedly. Again, ANTLR won't help you find these,
you just need to be careful.
Incidentally, to get the behaviour you want, you probably want
this (making sure that 'argSpec' always matches something and
can't be empty, and doesn't have any of the above):
argList : argSpec? (COMMA argSpec?)*;
(Which you'll note can end up matching nothing at all, so make
sure you don't use argList itself inside a list unless there's
something non-optional there too!)
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