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From: eachus@spectre.mitre.org (Robert I. Eachus)
Subject: Re: private type discriminants ignored?
Date: 1998/05/06
Message-ID: <EACHUS.98May6164528@spectre.mitre.org>#1/1
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References: <rz84szje1ol.fsf@cs.williams.edu> <35468F93.C371940@hso.link.com>
 <Es7569.4n2@world.std.com> <199804301352.PAA20159@basement.replay.com>
 <6ib6t4$63f$1@goanna.cs.rmit.edu.au>
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Organization: The Mitre Corp., Bedford, MA.
Newsgroups: comp.lang.ada
Date: 1998-05-06T00:00:00+00:00
List-Id: <comp.lang.ada>


In article <6ib6t4$63f$1@goanna.cs.rmit.edu.au> Dale Stanbrough <dale@goanna.cs.rmit.edu.au> writes:

  > It seems from the preceeding set of rules about discriminants that 
  > they too are a complicated morrass. Are discriminants really that 
  > useful to be worth the effort? I look at all the rules (in the LRM)
  > for them, and the code in Gnat, and it seems there is a lot of work
  > trying to get them right. Perhaps I don't write the sort of code
  > that they were intended to make simple, but I've rarely found them
  > that useful.

   There are several different cases that individually very useful.
For example, record types containing arrays whose bounds depend on a
discriminant with defaults.  (Whew!)  We really need a name for those.
They allow you to create types which correspond to char(N) varying in
PL/I:

    subtype Index is Integer range 0..100;

    type VString(Size: Index := 0) is record
      Value: String(1..Size);
    end record;

    Objects of type VString can contain strings of different sizes:

    V: VString;
    ...
    V := (6,"String");
    V := (27, "This is a test of VStrings.")

    Of course, instead of counting and using aggregates, you provide
operations for the type that do all that for you:

    V := + "This is a much longer string to assign to V.";

    By the way, in Ada 95, there is a package for varying strings.
But 1) the above allows the package body to be written in Ada. and
    2) you occaisionally need this feature for types other than
strings.

--

					Robert I. Eachus

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