I have narrowed down my problem. The validating semantic predicate
properly throws a FailedPredicateException in a regular tree grammar,
but does not when filter=true.
I added the following TreeMatcher.g to the VecMath example from the
new book. (I invoke this TreeMatcher before the Printer.) Note that
the INT alternative has a validating semantic predicate that is
guaranteed to fail. However when I run the program, no sign of the
FailedPredicateException.
Am I wrong to expect this?
Thanks,
Tom.
tree grammar TreeMatcher;
options {
tokenVocab=VecMath; // use token vocabulary from VecMath.g
output = AST;
ASTLabelType = CommonTree;
filter = true;
}
topdown: expr;
// START: expr
expr: ^('+' expr expr)
| ^('*' expr expr)
| ^('.' expr expr)
| ^(VEC expr (expr)*)
| INT {false}?
| ID
;
// END: expr
On Tue, Aug 4, 2009 at 2:02 PM, Tom Smith<[email protected]> wrote:
> I have a rule (in a tree pattern matcher) that uses a validating
> semantic predicate. When the predicate returns false, I expected to
> catch a FailedPredicateException and take some custom action.
> However, the FailedPredicateException is not getting thrown. Looking
> at the generated code, the problem appears to be that the exception
> only gets thrown conditionally (if state.backtracking==0). For some
> reason that is beyond my meager understanding, state.backtracking = 1
> when the semantic predicate fails.
>
> Any clues for me?
>
> Thanks,
> Tom Smith.
>
> tree grammar Trans;
>
> options {
> tokenVocab=SsipAST;
> output=AST;
> ASTLabelType=CommonTree;
> filter = true;
> }
> ...
> transition
> scope {
> ...
> }
> @init {
> ...
> }
> : ^(TRANSITION s1=stateName s2=stateName arcBody[$s1.text])
> { checkTransition($s1.text, $s2.text) }?
> { System.err.println("Transition from " + $s1.text + " to " +
> $s2.text + " is OK"); }
> ;
> catch [FailedPredicateException fpe] {
> String msg = String.format("In transition from \%s to \%s,
> liveness
> is not consistent", $s1.text, $s2.text);
> throw new SemanticException(msg);
> }
>
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