[bitc-dev] Error in specification of DO

[Swaroop Sridhar]

Kevin Reid wrote:
> On Jul 10, 2008, at 14:57, Jonathan S. Shapiro wrote:
> 
>> Unfortunately, it seems to me that the labeled BLOCK escape, which is
>> the generalization of these constructs, can't be handled by any
>> stateless implementation if it is possible for the label to be  
>> captured.
>> The problem is that we need to disarm the label when its associated
>> block exits. What we need here is dynamic scope rather than dynamic
>> extent, which seems to require something like:
>>
>>  (define (f x)
>>    (block myBlock expr))  =>
>>
>>  (define (f x)
>>    (let ((myBlock-armed #t)))
>>      (defexception myBlock '#a)
>>      (try
>>          ...body...
>>       (catch id
>>          ((myBlock exitValue)
>>             (if myBlock-armed
>>                 exitValue
>>                 (throw DynamicLabelExtentViolation))))))
>>      (set! myBlock-armed #f)))
> 
> It seems to me that one of these is the case:
> 
> 1. myBlock-armed will never be seen to be false, in that if the throw  
> occurs outside after myBlock-armed is set to #f, it is outside the  
> catch as well.
> 
> 2. You meant (defexception myBlock '#a) as a "static" (as in the C  
> storage class) definition rather than a lexical one; in which case  
> your implementation is incorrect anyway, as this example demonstrates:
> 
> (define (call-with-return f)
>    (block myBlock
>      (f (lambda (x) (return-from myBlock x)))))
> 
> (call-with-return (lambda (b1)
>    (call-with-return (lambda (b2)
>      (b1 "ok")))
>    "broken"))

I think that the interpretation of the local defexception is lexical.
However, the check for armed-ness is done at the wrong place.
It must be checked within the `expr' before throwing/invoking the
return-via (similar to array bounds check) and not at the enclosing
catch block.

Swaroop.