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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: stopping a loop iteration without exiting it
Date: Sun, 7 Jan 2018 13:05:38 +0100
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Xref: reader02.eternal-september.org comp.lang.ada:49802
Date: 2018-01-07T13:05:38+01:00
List-Id: <comp.lang.ada>

On 2018-01-07 12:36, Niklas Holsti wrote:
> On 18-01-06 19:20 , Dmitry A. Kazakov wrote:

>> T is a single type. T'Class is a range of types similar to
>> 5..Integer'Last. Certainly definite ranges of types must be supported as
>> well:
>>
>>    X in T..S
>>
>> which same as
>>
>>    X in T'Class and T in S'Class and (X in S or else X not in S'Class)
> 
> That is a bit hard to understand.

If you want to have mutually exclusive alternatives you need type 
ranges. If S is a descendant of T then in order to build a case you have 
to spell choices like:

    when T..S'Parent =>
    when S'Class =>

X in T..S'Parent = X in T'Class and X not in S'Class and T in S'Class. 
The equivalent cannot be written as an alternative. And you cannot 
declare a corresponding set of types either, because there is no support 
for set-theoretic operations on types and classes: e.g.

    X'Class - S'Class = X..S'Parent

> However, I do think there is a clear (and perhaps valuable) difference 
> between the forms
> 
>     when Same_X : T =>
> 
> where the object Same_X would not be class-wide, and
> 
>     when Same_X : U'Class =>
> 
> where the object Same_X would be class-wide. One could admit both kinds 
> of "when" in the same "case", of course, as long and they are 
> non-overlapping.

It is the same form only the tests and resulting types are different.

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