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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: How to access an array using two different indexing schemes
Date: Thu, 30 Nov 2017 15:57:10 -0600
Organization: JSA Research & Innovation
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Xref: reader02.eternal-september.org comp.lang.ada:49282
Date: 2017-11-30T15:57:10-06:00
List-Id: <comp.lang.ada>

"Niklas Holsti" <niklas.holsti@tidorum.invalid> wrote in message 
news:f8ak08F916jU1@mid.individual.net...
> On 17-11-30 13:40 , AdaMagica wrote:
>> OK, I dived into the RM:
>>
>> 8.5.1(4) The renamed entity shall be an object.
>> 3.3(12) [An object is a] view conversion of another object.
>> 4.6(5/2) A type_conversion whose operand is the name of an
>> object is called a view conversion if both its target type
>> and operand type are tagged, or if it appears in a call as
>> an actual parameter of mode out or in out; other
>> type_conversions are called value conversions.
>>
>> So my conclusion is:
>>
>> The renamings here are illegal (not tagged).
>> The subprogram calls as shown in my previous post are legal.
>>
>> Didn't try however. Good luck for you!
>
> Perhaps the next Ada revision could extend 4.6(5/2) to include a type 
> conversion in a renaming, as a view conversion?

The first part is already proposed: see AI12-0226-1. The second part would 
be problematic (at least in general), because of the rather wacky semantics 
of view conversions for untagged types (where were designed only for 
parameter passing with copy-in/copy-out semantics). For instance, you'd be 
making the following legal:

           Some_Seconds : Duration;

           Flt_Sec : Float renames Float(Some_Seconds);

           Flt_Sec := <Some_Complex_Float_Expression>;

which would most likely hide some interesting rounding issues (including 
precisely when those would happen). Definitely not just a drop in change.

The array case discussed here would work well, though.

                               Randy.