this also gets rid of initValue issue. We don't really know how to
return a "neutral" value when the stack is empty.
Ter
On May 2, 2008, at 2:19 PM, Terence Parr wrote:
> JimPoints out also that we should not be guarding our scope stacks.
> Let's begin with the guards we added for referencing rule properties.
> In version 3.0.1, if the else clause is not executed, s is null and
> you get a null pointer at $s.st.
>
> ('else' s=stat)? ... {$s.st}
>
> For 3.1, we have guarded all of those property references with s!
> =null?
> s.st:null. That is good.
>
> Now Imagine how we guard stack stuff. consider the following rule:
>
> block
> scope { int x; }
> : '{' stat+ '}'
> ;
>
> Ref $block::x currently checks that the stack is not empty before
> calling peek() to avoid a stack underflow exception. Jim argues that
> we should take that guard back out because it is a programming error
> to refer to $block::x when the stack is not properly set up. I think
> I'm agreeing with him. Is like accessing the local variable in an
> enclosing function but when there is no invoking function.
>
> We would need to change these templates:
>
> scopeAttributeRef(scope,attr,index,negIndex) ::= <<
> <if(negIndex)>
> ((<scope>_stack.size()-<negIndex>-1)>=0?
> ((<scope>_scope)<scope>_stack.elementAt(<scope>_stack.size()-
> <negIndex>-1)).<attr.name>:<initValue(attr.type)>)
> <else>
> <if(index)>
> ((<index>\<<scope>_stack.size())?
> ((<
> scope
>>
> _scope
> )<scope>_stack.elementAt(<index>)).<attr.name>:<initValue(attr.type)>)
> <else>
> ((<scope>_stack.size()>0)?
> ((<
> scope>_scope)<scope>_stack.peek()).<attr.name>:<initValue(attr.type)>)
> <endif>
> <endif>
>>>
>
> scopeSetAttributeRef(scope,attr,expr,index,negIndex) ::= <<
> if ( <scope>_stack.size()>0 ) {
> <if(negIndex)>
> ((<scope>_scope)<scope>_stack.elementAt(<scope>_stack.size()-
> <negIndex>-1)).<attr.name> =<expr>;
> <else>
> <if(index)>
> ((<scope>_scope)<scope>_stack.elementAt(<index>)).<attr.name> =<expr>;
> <else>
> ((<scope>_scope)<scope>_stack.peek()).<attr.name> =<expr>;
> <endif>
> <endif>
> }
>>>
>
> to avoid (<scope>_stack.size()>0) tests.
>
> Is it okay for me to remove the stack guards for dynamic scoping
> stuff? quick change to these two templates.
>
> Ter
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