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Newsgroups: comp.lang.ada
Date: Fri, 8 Nov 2019 07:55:14 -0800 (PST)
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Subject: Re: How to best make a custom range?
From: AdaMagica <christ-usch.grein@t-online.de>
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Xref: reader01.eternal-september.org comp.lang.ada:57524
Date: 2019-11-08T07:55:14-08:00
List-Id: <comp.lang.ada>

Am Dienstag, 5. November 2019 16:10:46 UTC+1 schrieb Shark8:
>    TYPE Digits is range 0..9;
>    SUBTYPE Odds is Digits with Static_Predicate => Odds in 1|3|5|7|9;
> what should Odds'Pred(1) be? 0? Constraint_Error?
> what about Odds'Pos(1) should it be 0, the first item of the subtype? Or 1, the position in the parent-type?

Attributes work on the base type, e.g.

Natural'Pred (0) = -1

Constraints on the subtype are ignored, Digit'Pred and Odds'Pred are identical. Thus:

Odds'Pred (8) = 7  -- 8-1
Odds'Pos (2) = 2  -- result is universal_integer

Of course, this will raise an exception:

X: Odds := Odds'Pred (7);  -- 6 not in Odds
Y: Odds := Odds'Pos (8);   -- universal_integer 8 converted into Digits not in Odds