Greetings!
On Tuesday 28 April 2009 08:10:43 am Gabriel Petrovay wrote:
> Hi all,
>
> I have the parser grammar posted at the bottom of this e-mail. (there
> is a corresponding lexer grammar that provides all the necessary
> tokens).
>
> If I try to build the parser for this grammar, I get the following warning:
> -------
> warning(200): exit_test.g:13:3: Decision can match input such as
> "IF" using multiple alternatives: 1, 3
> As a result, alternative(s) 3 were disabled for that input
> -------
>
>........ remainder of original posting snipped...
This is very similar, to me, to the classical if-then-else ambiguity.
Consider this, apparently valid, sentence in your language:
REPLACE EXIT WITH EXIT WITH EXIT
now there are 2 ways to parse this sentence.
is it REPLACE EXIT WITH (EXIT WITH EXIT)
or REPLACE (EXIT WITH EXIT) WITH EXIT
where the parens group together the p_ExitExpr within
each version of the parse. the other instance of the word
EXIT in each parse is a p_NCName.
and here are the parse trees for each hopefully making
the ambiguity clearer:
1) REPLACE p_ExprSingle WITH p_ExprSingle
| |
p_StepExpr p_ExitExpr
| |
p_NCName EXIT WITH p_ExprSingle
|
|
EXIT
p_StepExpr
|
p_NCName
|
EXIT
2) REPLACE p_ExprSingle WITH p_ExprSingle
| |
p_ExitExpr p_StepExpr
| |
EXIT WITH p_ExprSingle p_NCName
| |
p_StepExpr EXIT
|
p_NCName
|
EXIT
and so your grammar is ambiguous, the warning is correct, predicates have
nothing to do with your problem.
Hope this helps...
--jbb
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