[antlr-interest] default ANTLR behavior

[Terence Parr]

Ok, so after trying to plug in the Java from language spec and also  
to convert Grimm's Rats!-based java grammar to ANTLR v3, I think the  
default mode for antlr should be more forgiving.  While I'm ok with  
making the grammar conform to v3, the deluge of errors emitted by  
ANTLR can be really discouraging.  I am proposing that we start ANTLR  
out with "k=1 + auto-backtracking" or PEG mode.  In other words,  
accept any grammar w/o error.  If there is an ambiguity, ANTLR will  
automatically backtrack over the alts in that decision and take the  
first one that matches.  The LL(1) will make it more efficient than a  
normal PEG implementation.  note that I do memoization so parsing is  
still linear.

Then, if you want to optimize your grammar for speed, you can turn on  
an "expert" mode or something that will engage full LL(*) and manual  
backtracking, which does some amazing optimizations.  The only  
negative with LL(*) at the moment is that sometimes it hits a  
"landmine" and cannot finish analyzing the decision.  I suspect this  
is because of my blunt indicator (raw DFA conversion time, size)  
about when ANTLR has gone off into the weeds, but to ensure  
acceptance of the tool I want to provide a much less intimidating  
interface for the uninitiated.

The default mode would put a syn pred on every alt but of course  
would only use them when LL(1) failed.  You could set k=2 for the  
grammar and it would backtrack when k=2 failed.  So, this is like  
current antlr accept you won't get any warnings, it will just  
backtrack when fixed k fails.  perhaps when I get more experience  
with large grammars and LL(*) we can make this the default.

Thoughts and comments are appreciated.

Ter