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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: formal array types and default values
Date: Tue, 2 Jan 2018 18:52:11 -0600
Organization: JSA Research & Innovation
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Date: 2018-01-02T18:52:11-06:00
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"Niklas Holsti" <niklas.holsti@tidorum.invalid> wrote in message 
news:faqhpnFljnfU1@mid.individual.net...
> On 17-12-30 23:42 , Mehdi Saada wrote:
>> I have several questions more or less related.
>
>> First: why aspect DEFAULT_COMPONENT_VALUE can apply only
>> to components of scalar type ?
>
> Don't know. Perhaps the thought was that if the component is a record 
> type, then there is already a way to give default values to the components 
> of the record type.

Correct; we didn't want multiple ways of doing the same thing.

> However, the AARM explains that this is a representation attribute, and 
> why that is so, and (therefore?) requires the default component value to 
> be defined by a /static/ expression, which means that it can only be a 
> scalar or a string. In principle, a string might define the default 
> component value of an array with elements of a constrained String subtype, 
> but that is probably not a very common case.

Correct again. We did worry a bit about there not being a good way to set a 
default for an array of arrays, but we decided those aren't very common and 
the default can always be set for the ultimate (non-array) element type.

>> It would be nice to be able  to set a default value to an array type
>> in a generic package, equal to a generic object parameter.
>
> Agreed. Possibly this case was forgotten, or prevented by such a generic 
> object parameter not being a static value.

Not really. All of these defaults for for a *type*, not a subtype, so there 
is only one per type. (There are thorny semantic problems with per-subtype 
defaults, we decided they were not worth the trouble.) Thus, it wouldn't 
make sense to specify it for generic formal parameters, as presumably the 
actual type already has such a default (and if it doesn't, you can't add one 
in the generic unit).

                                         Randy.