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From: Jeffrey Carter <jeffrey.carter@boeing.com>
Subject: Re: Direct_IO problem -- generic package problem
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Date: Thu, 12 Jul 2001 16:58:10 GMT
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Date: 2001-07-12T16:58:10+00:00
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In Ada 83, you could say

generic -- Example
   type T is limited private;
package Example is
   ...
end Example;

and then, before supplying a body for Example, write something like

package P is new Example (T => String);

Then you could supply a body for example that creates a variable of type
T:

package body Example is
   V : T;
   ...
end Example;

This is the same as writing

   V : String;

which is clearly illegal. This would be caught eventually, but was a
"gotcha" that was removed in Ada 95. Now the generic formal type as
given means that it must be possible to declare a variable of the type
(the type must be "definite"). The new generic formal syntax

type T (<>) is [limited] private;

allows the generic to be instantiated with an indefinite type, but means
the generic cannot declare any uninitialized objects of the type.

Ada.Direct_IO declares the generic formal type

type Element_Type is private;

so the type used to instantiate Direct_IO must be definite, while your
type Matrix is indefinite. You have a few possibilities

Use a subtype in place of Small_Nat with a small enough range that you
can provide defaults for all discriminants without encountering
Storage_Error if you declare an unconstrained variable.

Instantiate Direct_IO with constrained subtypes of Matrix for the types
of interest.

Good luck.

-- 
Jeffrey Carter