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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: Q: Trouble creating array and discriminated type
Date: Mon, 23 Jan 2017 08:19:21 +0000
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Date: 2017-01-23T08:19:21+00:00
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"G.B." <bauhaus@notmyhomepage.invalid> writes:

> Defaults for discriminants of tagged types would not be permitted,
> only if objects of the type can not have their discriminants changed
> after creation (tagged limited). The reason is stated in AARM 3.7(9.*)
> and it seems to say "It's complicated".
>
> The paragraph suggest that for some use cases, one can declare a
> subtype that sets the discriminants to default values. IIUC, these
> subtypes can then be used in derivations also.

Not sure about use in derivations, but I have

   generic
      Maximum_Size : Positive;
   package BC.Containers.Collections.Bounded is

      type Unconstrained_Collection
        (Maximum_Size : Positive) is new Abstract_Collection with private;

      subtype Collection
         is Unconstrained_Collection (Maximum_Size => Maximum_Size);

I've never been happy about Maximum_Size there, but that's the sort of
thing that happens when you already have a published interface that
you're reluctant to change!