[antlr-interest] incorrect order for executing actions?

[Marko van Dooren]

Hi,

I use ANTLR to instantiate a metamodel of Java by calling the  
appropriate constructors in actions. Sometimes, I must pass on an  
expression to other rules using dynamic scopes. But in the example  
below, which is based on the Java.g grammar on your website, things go  
wrong. I have removed all subrules that are not relevant for the  
problem.

More specifically, the action of 'selector' , which creates an array  
access expression, is executed (indicating that state.backtracking ==  
0), but it results in a NullPointerException because the target field  
of TargetScope is null. That field, however, is set in the rule of  
'unaryExpressionNotPlusMinus', in the action after 'prim=primary'.  At  
that place, the element that is used to set TargetScope::target is  
checked, and an exception is thrown if it is null. That is also the  
only place where 'selector' is used. This means that the action in  
'unaryExpressionNotPlusMinus' was not executed (backtracking != 0),  
but the action in 'selector' was executed (backtracking == 0). Is that  
supposed to happen? It seems weird to me. I have included the  
stacktrace of the parser, which shows that no synpred methods were  
executed in between, and they seem to be the only methods that modify  
the backtracking field, and they always perform a decrement before the  
exit.

Can somebody help me to solve this problem? I could introduce a  
factory interface with a 'Expression setTarget(...)' method and return  
factory objects instead of directly returning expressions, but that  
seems to defeat the purpose of having dynamic scopes. I would also  
want to avoid inlining because I also use the TargetScope in a number  
of other places where the 'usage dependencies' are much more  
complicated.

If you want to, you can download the complete Java.g file at http://www.cs.kuleuven.be/~marko/Java.g 
. If you want to compile the parser, you'll need to check out the  
following git repositories (and some help from me to sort out the  
classpath ;) :
http://www.cs.kuleuven.be/~marko/gitroot/code/chameleon.git
http://www.cs.kuleuven.be/~marko/gitroot/code/chameleon-support.git
http://www.cs.kuleuven.be/~marko/gitroot/code/jnome.git  (this is the  
Java metamodel which includes Java.g)
http://www.cs.kuleuven.be/~marko/gitroot/code/rejuse.git

Kind regards,

Marko van Dooren






scope TargetScope {
   InvocationTarget target;
}

unaryExpressionNotPlusMinus returns [Expression element]
scope TargetScope;
     :   prim=primary
            {check_null(prim.element);  
$TargetScope::target=prim.element; retval.element=prim.element;}
         (sel=selector
            {$TargetScope::target=sel.element; retval.element =  
sel.element;}
         )*
         (
            '++' {retval.element = new PostfixOperatorInvocation("++",  
retval.element);}
          | '--' {retval.element = new PostfixOperatorInvocation("--",  
retval.element);}
         )?
     ;



// NEEDS_TARGET
selector returns [Expression element]
	:	
        '[' arrex=expression ']'
           {retval.element = new ArrayAccessExpression((Expression) 
$TargetScope::target);
            ((ArrayAccessExpression)retval.element).addIndex(new  
FilledArrayIndex(arrex.element));
           }
	;







46 Parsing /Users/marko/git/jnome/testsource/jutil/src/org/jutil/java/ 
collections/SkipList.java
java.lang.NullPointerException
	at  
jnome 
.core 
.expression.ArrayAccessExpression.setTarget(ArrayAccessExpression.java: 
38)
	at  
jnome 
.core 
.expression.ArrayAccessExpression.<init>(ArrayAccessExpression.java:27)
	at jnome.input.parser.JavaParser.selector(JavaParser.java:16037)
	at  
jnome 
.input.parser.JavaParser.unaryExpressionNotPlusMinus(JavaParser.java: 
15715)
	at jnome.input.parser.JavaParser.castExpression(JavaParser.java:16185)
	at  
jnome 
.input.parser.JavaParser.unaryExpressionNotPlusMinus(JavaParser.java: 
15674)
	at jnome.input.parser.JavaParser.unaryExpression(JavaParser.java:15548)
	at  
jnome.input.parser.JavaParser.multiplicativeExpression(JavaParser.java: 
15217)
	at jnome.input.parser.JavaParser.additiveExpression(JavaParser.java: 
15065)
	at jnome.input.parser.JavaParser.shiftExpression(JavaParser.java:14796)
	at jnome.input.parser.JavaParser.relationalExpression(JavaParser.java: 
14511)
	at jnome.input.parser.JavaParser.instanceOfExpression(JavaParser.java: 
14418)
	at jnome.input.parser.JavaParser.equalityExpression(JavaParser.java: 
14271)
	at jnome.input.parser.JavaParser.andExpression(JavaParser.java:14165)
	at  
jnome.input.parser.JavaParser.exclusiveOrExpression(JavaParser.java: 
14062)
	at  
jnome.input.parser.JavaParser.inclusiveOrExpression(JavaParser.java: 
13959)
	at  
jnome.input.parser.JavaParser.conditionalAndExpression(JavaParser.java: 
13859)
	at  
jnome.input.parser.JavaParser.conditionalOrExpression(JavaParser.java: 
13759)
	at  
jnome.input.parser.JavaParser.conditionalExpression(JavaParser.java: 
13655)
	at jnome.input.parser.JavaParser.expression(JavaParser.java:13270)
	at jnome.input.parser.JavaParser.expression(JavaParser.java:13292)
	at jnome.input.parser.JavaParser.statementExpression(JavaParser.java: 
13150)
	at jnome.input.parser.JavaParser.statement(JavaParser.java:11885)
	at jnome.input.parser.JavaParser.blockStatement(JavaParser.java:10875)
	at jnome.input.parser.JavaParser.constructorBody(JavaParser.java:8334)
	at  
jnome 
.input.parser.JavaParser.constructorDeclaratorRest(JavaParser.java:5874)
	at  
jnome.input.parser.JavaParser.constructorDeclaration(JavaParser.java: 
3952)
	at jnome.input.parser.JavaParser.memberDecl(JavaParser.java:3770)
	at jnome.input.parser.JavaParser.classBodyDeclaration(JavaParser.java: 
3584)
	at jnome.input.parser.JavaParser.classBody(JavaParser.java:3274)
	at  
jnome.input.parser.JavaParser.normalClassDeclaration(JavaParser.java: 
1954)
	at jnome.input.parser.JavaParser.classDeclaration(JavaParser.java:1762)
	at jnome.input.parser.JavaParser.memberDecl(JavaParser.java:3804)
	at jnome.input.parser.JavaParser.classBodyDeclaration(JavaParser.java: 
3584)
	at jnome.input.parser.JavaParser.classBody(JavaParser.java:3274)
	at  
jnome.input.parser.JavaParser.normalClassDeclaration(JavaParser.java: 
1954)
	at jnome.input.parser.JavaParser.classDeclaration(JavaParser.java:1762)
	at  
jnome 
.input.parser.JavaParser.classOrInterfaceDeclaration(JavaParser.java: 
1239)
	at jnome.input.parser.JavaParser.typeDeclaration(JavaParser.java:1135)
	at jnome.input.parser.JavaParser.compilationUnit(JavaParser.java:824)
	at jnome.input.JavaMetaModelFactory.parse(JavaMetaModelFactory.java: 
540)
	at  
jnome.input.JavaMetaModelFactory.lexAndParse(JavaMetaModelFactory.java: 
480)
	at  
jnome 
.input.JavaMetaModelFactory.addFileToGraph(JavaMetaModelFactory.java: 
402)
	at  
jnome 
.input.JavaMetaModelFactory.getMetaModel(JavaMetaModelFactory.java:101)
	at jnome.input.JavaMetaModelFactory.main(JavaMetaModelFactory.java:576)


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