[antlr-interest] help me to understand nondeterminism warnings please

[Greg Lindholm]

I would suggest having your lexer throw away all the white space (S
rule).  In most languages the white space is not significant except
to separate tokens.
Use the { $setType(Token.SKIP); } command to have the lexer discard
all the whitespace characters.

S : (' ' | '\t' | '\n' | '\r')+ { $setType(Token.SKIP); };

Then remove all the 'S's from your parser.
This would greatly simplify your grammer as you wouldn't have (S)? 
between every token. And... the nondeterminism will likely go 
away or be clearer.

(If you need to perserve the whitespace for generation later you
can use the Hidden token stream mechanism.)

I know this doesn't really answer your question but it's good
advice and if you search this group for "nondeterminism" you will
find hundreds of similar questions and occasionally a helpful
answer :)

Greg


--- "davidjpenton2002 <nwestreb at arrowsash.com>"
<nwestreb at arrowsash.com> wrote:
> I would very much appreciate tips on understanding nondeterminism 
> warnings from antlr.  I suppose it is probably not appropriate to 
> just dump a grammar into a post and ask y'all to debug it, so, I'll 
> only include the relevant snippets (which may be insufficient to 
> identify the problem, I guess).
> 
> As I would like to gain a fairly complete grasp of antlr, I expect 
> that replies to this post be to point me in the right direction in 
> the FAQ, the reference manual, or other sources. Such advice would be
> 
> much appreciated.
> 
> Anyway, here is what I get:
> 
> *** antlr output: ***
> 
> ANTLR Parser Generator   Version 2.7.1   1989-2000 jGuru.com
> grammar.g:137: warning: nondeterminism upon
> grammar.g:137: 	k==1:S
> grammar.g:137: 	k==2:S
> grammar.g:137: 	between alt 1 and exit branch of block
> grammar.g:92: warning: nondeterminism upon
> grammar.g:92: 	k==1:S
> grammar.g:92: 	k==2:S
> grammar.g:92: 	between alt 1 and exit branch of block
> 
>   ***
> 
> Here is the rule at line 137:
> 
> notationType
>     :  "NOTATION" S LEFTPAREN (S)? name 
>        ((S)? VERTICALBAR (S)? name)*
>        (S)? RIGHTPAREN 
>     ;
> 
> And here is the production at line 92:
> 
> enumeration
>     : LEFTPAREN (S)? nmtoken
>       ((S)? VERTICALBAR (S)? nmtoken)* (S)? RIGHTPAREN
>     ;
> 
> and, if it helps, the lexer rule for S is:
> 
> S : (' ' | '\t' | '\n' | '\r')+;
> 
> I don't really understand what the ambiguity is, which is probably 
> just another way of saying I do not yet understand antlr and LL(k) 
> parsing yet. More specifically, I don't know how to read the warning 
> message.  What are 'alt 1' and the 'exit branch'?
> 
> The nondeterminism seems to exist regardless of k.  I guess I don't 
> really know if I should expect to spot the problem by looking only at
> 
> the rules at the line numbers reported by antlr, or if I must think 
> more globally, i.e. look at the rules that include or are included by
> 
> the offending rules as reported by antlr?
> 
> My attempt to sort this out entailed stripping my grammar down to the
> 
> offending rules (the ones above), the rules that refer to them, and 
> the related lexer rules.  This produced the odd (to me) effect of 
> causing the problem to go away, i.e. no more warnings.
> 
> How should I approach getting an understanding of this?
> 
> 
> 
>  
> 
> Your use of Yahoo! Groups is subject to
> http://docs.yahoo.com/info/terms/ 
> 
> 


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