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From: "Jeff Carter" <jcgoogle@earthlink.net>
Newsgroups: comp.lang.ada
Subject: Re: Half Constrained Array Types and Slices
Date: 7 Mar 2006 11:02:49 -0800
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Date: 2006-03-07T11:02:49-08:00
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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?

--
Jeff Carter

jrcarter commercial-at acm [period | full stop] org