Private enum members + Descent

[Denis Koroskin]

On Sat, 24 Oct 2009 19:00:10 +0400, Justin Johansson <no at spam.com> wrote:

> Yigal Chripun Wrote:
>
>> On 24/10/2009 01:16, Justin Johansson wrote:
>> > Sorry; subject line mod'ed  just so Ary doesn't miss it.
>> >
>> >> enum Color { private UNINITIALIZED = -1, RED, GREEN, BLUE }
>> >
>> >> (btw. Interestingly, typing this code into Eclipse with the Descent  
>> plug-in
>> >> causes a Java out of heap space malfunction.)
>> >
>>
>> enum Color { RED, GREEN, BLUE }
>>
>> void foo(Color* c) {
>>    if (c !is null) handleColor(*c);
>> }
>> what's the need for that UNINITIIALIZED member?
>
> Okay; that's one work around for a corner case of my cited use-case, but
> you don't always want to, or perhaps it is not convenient/elegant to,  
> use a pointer
> to data that conveniently fits into a machine register.
>
> Perhaps I am wrong but I thought uninit was a good metaphor to  
> demonstrate
> the various useful purposes that private enum members might have.
>
> Here is another example that might make the concept jell.  Again I may  
> well be wrong.
>
> Consider this hypothetical enum definition together with plausible  
> comments:
>
> enum Color {
>   RED, GREEN, BLUE,  // these 3 members are available for public  
> consumption
>   private RED_WITH_BLUE_POKER_DOTS,  // this value is used internally  
> and is not for public consumption and that's why it is marked private
>   private RED_OR_GREEN,  // ditto; internal routine to cater for  
> red-green color-blindness
> }
>
> The above demonstrates a set of entities that are meaningful to some  
> possible internal
> function but otherwise not externally meaningful.
>
> Another use-case lies in the API programmer's want for "private" ..
> so, for example, consider that "private" may well be a synonym for
> "pleasedontusethismemberbecauseitisalikelycandidateforfuturedeprecation"  
> **
>
> ** Using Walter Bright insignificant whitespace/separator notation :-)
>
> Thanks Yigal for commenting and perhaps your further comment?
>
> Justin
>

Another useful use-case I see is enforcing explicit initialization:

enum Color
{
     private Uninitialized = 0,
     Red,
     Green,
     Blue,
}

Color color; // error: Color.Uninitialized is not accessible

It would work similar to private struct ctors.