Re: Half Constrained Array Types and Slices

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On 7 Mar 2006 11:02:49 -0800, Jeff Carter wrote:

> Dmitry A. Kazakov wrote:
>>
>> BTW, in the second case there is an interesting possibility, I didn't think
>> much about. If the language had supertyping, then the compiler could create
>> an anonymous supertype of the index type with one or two more values. These
>> imaginary values could then be returned as 'First and 'Last. The idea is
>> same as in IEEE's positive and negative infinities. Of course these values
>> could have different representation than natural index. Yet the following
>> will be illegal:
>>
>> I : Index := A'First; -- This may cause C_E while type conversion
> 
> I suppose this could be handled much as we handle integer types, with a
> base type that is different from the 1st-named subtype. So a
> declaration such as
> 
> type Single is (One);
> 
> would be implemented as
> 
> type Single'Base is (First, One);
> 
> subtype Single is Single'Base range One .. One;
>
> Then, given
> 
> type Degenerate is array (Single range <>) of Whatever;
> 
> V : Degenerate (null range);
> 
> we can say that V'Last is of type Single'Base. V'First = One, V'Last =
> First.
>
> Probably we'd do this for all enumeration types; that would allow ""
> for all string types. Since we define Single'Pos (One) to be zero,
> Single'Base'Pos (First) would have to be -1. Would that work? Or would
> it be better for position numbers to start at 1?

Maybe it will work. At least it looks consistent.

An interesting related question is:

type Single is (One);
for Single'Size use 0; -- Can I have it really 0?

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de