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From: "Jeffrey R. Carter" <spam.jrcarter.not@spam.not.acm.org>
Newsgroups: comp.lang.ada
Subject: Re: How to write "Hello" to a text file without a line terminator
Date: Sun, 14 Aug 2016 10:05:06 -0700
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Xref: news.eternal-september.org comp.lang.ada:31440
Date: 2016-08-14T10:05:06-07:00
List-Id: <comp.lang.ada>

On 08/14/2016 06:28 AM, AdaMagica wrote:
> 
> No problem for a type like this (without the defaulted discriminant, you'll have indeed a problem, as for strings):
> 
>   type Disc is (A, B, C, D);
>   type Rec (X: Disc := A) is record
>     case X is
>       when A => I: Integer;
>       when B => F: Float;
>       when C => B: Boolean;
>       when D => null;
>     end case;
>   end record;

This type is definite, unlike String, which is indefinite. Both Sequential_IO
and Direct_IO can have unconstrained elements, but Sequential_IO allows an
indefinite subtype for the elements, while Direct_IO requires a definite
subtype. The definition of (in)definite subtypes is given in ARM 3.3(23/3): "A
subtype is an indefinite subtype if it is an unconstrained array subtype, or if
it has unknown discriminants or unconstrained discriminants without defaults
(see 3.7); otherwise, the subtype is a definite subtype (all elementary subtypes
are definite subtypes)."

In practice, a subtype S is definite if you can declare an object of the subtype
without adding any additional information:

V : S:

and indefinite otherwise.

-- 
Jeff Carter
"Frankie Wolf, wanted by Federal authorities for
dancing with a mailman."
Take the Money and Run
143