[antlr-interest] unexpected nondeterminism...

[Terence Parr]

Hi.  I think this is a limitation of the linear approximate lookahead 
not your understanding.  There is usually one place in a grammar where 
the approximation bites you.  ANTLR will do the right thing by 
consuming the OP greedily, but it confuses itself and then gives you a 
warning.  My suggestion is to just leave it and add "options { 
greedy=true; } :" to the subrule (OP)? so that you can inform ANTLR you 
intended to do this.  It will obediently shut up and do the right 
thing. :)

Sorry for the hassle.

Ter

On Mar 26, 2004, at 4:21 PM, Mark wrote:

> Call me dense, but I'm not getting why this results in a 
> nondeterminism:
>
> class TestParser extends Parser;
> 	options { k = 2; }
>
> term:		ID ;
> expression:	term ( OP term )* ;
> statement:	expression ( assignment )? ;
> assignment:	( OP )? AOP expression ;
>
> The warning given is:
>
> ANTLR Parser Generator   Version 2.7.3 (20040322-1)   1989-2004 
> jGuru.com
> foo.g:7: warning:nondeterminism upon
> foo.g:7:     k==1:OP
> foo.g:7:     k==2:ID
> foo.g:7:     between alt 1 and exit branch of block
>
> Surely, with k=2 look ahead, the sequence [ OP, ID ] can only be 
> alternative one of the
> block.  There doesn't seem to be a way in which OP can be followed by 
> anything other than
> ID (in which case it is a continuation of the ()* block in 
> expression), or AOP, in which case
> it is an assignment.  Doesn't the k=2 ensure that in the code for 
> expression, it can look
> ahead and say if k==1:OP, k==2:ID then continue the loop, otherwise 
> exit?
>
> Any help to clear my thick-headed understanding would be appreciated!
>
> - Mark
>
>
>
>
> Yahoo! Groups Links
>
>
>
>
>
>
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