[antlr-interest] Why does ANTLR generate code that will never call an OR'd alternative?

[Gerald Rosenberg]

  Most likely, the parser generation analysis determined that isToken 
can never be reached.  Check your error log for an alt skipped message.



------ Original Message (Saturday, August 21, 2010 1:01:20 
AM) From: Avid Trober ------
Subject: [antlr-interest] Why does ANTLR generate code that will never call
an OR'd alternative?
> For this rule,
>
>
>
> identifier
>
>                  :       isToken | IDENTIFIER;
>
>
>
> ANTLR generates code that would never calls the isToken rule
> (target=CSharp2):
>
>
>
>      public MYParser.identifier_return identifier()    // throws
> RecognitionException [1]
>
>      {
>
> .
>
>              // .  : ( isToken | IDENTIFIER )
>
>              int alt30 = 2;
>
>              int LA30_0 = input.LA(1);
>
>
>
>              if ( (LA30_0 == IDENTIFIER) )   //<== token must be IDENTIFIER
> to call isToken???
>
>              {
>
>                  int LA30_1 = input.LA(2);
>
>
>
>                  if ( ((isToken(input.LT(1)))) )  //<== why must LA30_0 ==
> IDENTIFIER to call isToken?
>
>                  {
>
>                      alt30 = 1;
>
>                  }
>
>                  else if ( (true) )
>
>                  {
>
>                      alt30 = 2;
>
>                  }
>
> .
>
>              else                         //<== since not IDENTIFIER, why
> not call isToken here???
>
>              {
>
>                  NoViableAltException nvae_d30s0 =
>
>                      new NoViableAltException("", 30, 0, input);
>
>
>
>                  throw nvae_d30s0;
>
>              }
>
>
>
> I would think it's something to do with DFA optimization?   Perhaps that's
> why IDENTIFIER is checked first.
>
> But, if IDENTIFIER is false, why not call isToken???    Afterall, the rule
> is IDENTIFIER  ****OR***** isToken.
>
>
>
> Thanks,
>
> Trober
>
>
>
>
>
>
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>


-- 

Gerald B. Rosenberg, Esq.
NewTechLaw
260 Sheridan Ave., Suite 208
Palo Alto, CA 94306-2009
650.325.2100 (office) / 650.703.1724 (cell)
650.325.2107 (facsimile)

www.newtechlaw.com

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