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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Half Constrained Array Types and Slices
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Date: Mon, 6 Mar 2006 19:31:16 +0100
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On 6 Mar 2006 09:50:16 -0800, Jeff Carter wrote:

> We seem to be fine as long as we don't do V'Last. I can't figure out
> how such a language should handle 'First and 'Last for such a
> value/object.

Compile error, if the index type isn't ordered (= does not have 'First,
'Last, "<", "=").

Otherwise, Constraint_Error.

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 know no good way to name the type. Even Index'Class would be wrong.

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