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From: Jeffrey Carter <jeffrey.carter@boeing.com>
Subject: Re: Container reqs
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Date: 2001-10-18T22:29:28+00:00
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Ehud Lamm wrote:
> 
> I have no problem with a Queue package paraemtrized by an Element type
> (specified as private). Even novice Ada programmer must know how to
> instantiate such packages if he uses Text_IO.

How the Element type is specified in the generic formal part should
reflect how the generic uses the type. A generic formal part is a
specification of what the generic must be given to perform its service.
A generic formal private type specifies that the generic will use ":="
and "=" for the type. If the generic doesn't use both, then the generic
is incorrectly specified.

If the generic only uses assignment, then the only way to specify this
in Ada is

   type Element is limited private;
   with procedure Assign (To : in out Element; From : in Element);

This is a failing in Ada, IMO. It might have been better to have
something like

type T is private; -- No operations defined, like Ada's limited private
type T is private with ":="; -- Only assignment defined; no analog in
Ada

and allow the definition (in package specifications) or import (in
generic formal parts) of "=".

> What I dodn't like is the need for several, possibly nested, instantiations
> simply to get a simple queue or hash table.

I wholeheartedly agree. Wright's posting here a few days ago of the
sequence of instantiations needed to obtain a simple structure with the
Booch components clearly indicates that they fail this requirement.

-- 
Jeffrey Carter