[J3] [EXTERNAL] Private enumerators

[Reuben D. Budiardja]

Hello,

On 10/19/2019 01:37 PM, Van Snyder via J3 wrote:
> Gary objected to private enumerators.
> 
> Access specs control access to identifiers, not objects.
> 
> He didn't like the possibility that one might be able to get the value
> of an enumerator for which the module did not publish the name, using,
> for example, INT(foo)+1 or foo%next().

I share the concern regarding access to private enumerators. Being able 
to get the ordinal value of a private enumerator with NEXT() and turn it 
back using the constructor seems to defeat the point of the enumerator 
being private, and therefore a vulnerability.

For example, suppose enumerators are being used to identify some state, 
then being able to iterate over the private enumerators with foo%next(), 
one can deduce how many (internal) states there are, and perhaps use 
this state as argument, etc, introducing vulnerability.

I think it also defies "regular user's" expectation. I understand that 
we say ENUM is not derived type and enumerators are not component, but 
some similarities hold. With PRIVATE component, there is no access to 
the name or value of the component outside the module unless explicitly 
given, e.g. via getter and setter subroutines in the module. With ENUM 
as proposed, this is effectively provided and there is no way to disable it.

I was thinking perhaps a solution is to require that all private 
enumerators to be defined after the public one, so that the ordinal 
values for private enumerators are always higher than the public one. 
Then, the requirements could be that NEXT() inside the module would 
traverse all the enumerators, while outside the module would only 
traverse the public enumerators. The same thing with LAST().

Some examples:

Example 1:

enum, public :: color
   enumerator, public  :: red, green
   enumerator, private :: purple   ! <-- compile error,
                                   ! require all private after public
   enumerator, public  :: blue
end enum


Example 2:

module enum_def

   enum, public :: color
     enumerator :: red, green, blue
     enumerator, private :: purple, black
   end enum color

...

   type(color) :: my_car = color%blue
   my_car%last !-- would give "black"
   my_car = my_car%next() !-- would give purple

end module enum_de


module my_object

   use enum_def
...
   type(color) :: my_house = color%green
   my_house%last !-- would give "blue" since this is
                 !   the last of the public enumerator

   my_house = my_house%next()  !-- would give blue
   my_house = my_house%next()  !-- ERROR terminated

end module my_object


End examples.


I would be interested in feedback.

Best,
Reuben

-- 
Reuben D. Budiardja, Ph.D.
reubendb at ornl.gov | (865) 576-9519
National Center for Computational Sciences
Oak Ridge National Laboratory