[antlr-interest] antlr should throw NoViableAltException

[David Holroyd]

On Sun, Apr 15, 2007 at 08:11:39PM +0800, femto gary wrote:
> also, generating parser will also produce the following warning:
> [20:08:36] warning(200): Rubyv3.g:101:32: Decision can match input
> such as "SEMI" using multiple alternatives: 1, 2
> As a result, alternative(s) 2 were disabled for that input
> 
> but for the grammar:
> statement
> 	:	expression (modifier_line)* SEMI? -> ^(STATEMENT expression
> (modifier_line)*)
> 	|       SEMI!
> 	;
> input SEMI shouldn't cause an ambiguity, because expression can't be empty,
> so either match the alt1 or alt2, why does it will report that warning.
> Anybody has any ideas? Thanks.

I think that since the grammar allows,

  statement*

the ambiguity is between the alternatives of,

 1) matching 'SEMI?' right now, in this invocation of 'statement', and,

 2) not matching 'SEMI?', exiting this invocation of the 'statement' rule
    and then matching the 'SEMI!' alternative the next time around the
    'statement*' loop

Is "1;" to be parsed as

  (STATEMENT 1) (STATEMENT ;)
or
  (STATEMENT 1;)



i.e. the decision being referred to in the message is probably the one
at the '?', not the one at the '|', if that helps at all :)


ta,
dave

-- 
http://david.holroyd.me.uk/