Exception transparency

[Alex Buckley]

I believe the short answer to this is "yes".

Alex

On 6/10/2010 2:41 PM, Peter Levart wrote:
> Hello Maurizio,
> 
> What if compiler would track internally any type variable with Throwable bound in such a way 
> that when used in any context except "throw" statement it would mean lub(X,Y,Z,...) as defined 
> today but when used in throw statement it would mean X|Y|Z|...
> 
> class Foo<X> { void set(X x) {} }
> 
> class A extends Exception {}
> 
> class B extends Exception {}
> 
> class Test {
> 
>   static <E extends Exception> Foo<E> test(E e1, E e2) throws E {
>     switch (new Random().nextInt(3)) {
>       case 0: throw e1;
>       case 1: throw e2; 
>       default: return new Foo<E>();
>     }
>   }
> 
>   public static void main(String args) {
>     try {
>       test(new A(), new B()).set(new Exception());
>     }
>     catch (A a) {}
>     catch (B b) {}
>   }
> 
> }
> 
> 
> Would that work? Would that be back compatible?
> 
> Regards, Peter
> 
> 
> On Tuesday, June 08, 2010 02:25:40 pm Maurizio Cimadamore wrote:
>> On 08/06/10 13:13, Maurizio Cimadamore wrote:
>>> On 08/06/10 12:54, Peter Levart wrote:
>>>> On 06/08/10, Maurizio Cimadamore wrote:
>>>>>>> needs to guess any intentions where it will affect realistic code.
>>>>>> Neal,
>>>>>> I have trouble to figure out an example of such rare places.
>>>>>>
>>>>>> Do you have an example of such code ?
>>>>> Could be this?
>>>>>
>>>>> class A<X>   extends Exception {}
>>>>> class B extends A<String>   {}
>>>>> class C extends A<Integer>   {}
>>>>>
>>>>> <throws E>   E choose(E e1, E e2) { ... }
>>>>>
>>>>> choose(new B(), new C()); //what is the inferred type for E???
>>>>>
>>>>> In JDK 5/6 E is inferred to be A<? extends Object&   Comparable<?
>>>>> extends Object&   Comparable<?>>>. In JDK 7 with the proposed new
>>>>> semantics for 'exception-bound' type-vars the inferred type would be
>>>>> something like A<String>   | A<Integer>, which, I guess, would need to
>>>>> be rejected as an ill-formed type on the grounds of type-disjointness
>>>>> (A<String>   and A<Integer>   are not provably distinct).
>>>> Are you suggesting to relax the rule that generic classes can not extend
>>>> Throwable?
>>> Good catch - my example is illegal; no incompatibility here.
>> I tried harder ;-) :
>>
>> class Foo<X> { void set(X x) {} }
>>
>> class A extends Exception {}
>> class B extends Exception {}
>>
>> class Test {
>>     static <E extends Exception> Foo<E> test(E e1, E e2){ return new
>> Foo<E>(); }
>>
>>     public static void main(String args) {
>>        test(new A(), new B()).set(new Exception());
>>     }
>> }
>>
>> This compiles in JDK 5/6 (E inferred to be Exception); it fails to
>> compile under JDK 7 if we assume that type-variables with Exception
>> bound have a 'varadic' semantics; in fact in this case the type A|B
>> would be inferred. But this means that you cannot pass an Exception
>> where an A|B is expected.
>>
>> Maurizio
>>
>>> Maurizio
>>>
>>>> Peter
>>>>
>>>>> Maurizio
>