Re: Tagged Types and Generics

["Robert C. Leif, Ph.D." <rleif@RLEIF.COM>]

To: Nick Roberts et al.
From: Bob Leif

I am absolutely in agreement with you and what you posted below!  I was
going to add the last_update date and time to money. The discussion has
never been tagged types versus generics.  It has been about constructing a
tagged type with generic fields and instantiating a generic with a tagged
type which can include generic fields.  Both constructs are very useful and
should NOT be mutually exclusive.  Unfortunately, I do not know how to do
it.  Even if Nick Roberts and I are both dead wrong on money, it is still
necessary to explain how to combine tagged types with generics.

I might add that part of the problem is historical. The text books first
explain generics, which existed in Ada 83 and then the Ada 95 addition,
tagged types.  I have not seen a generic in a tagged type with a class-wise
procedure or operator. Is very nice to start with a money tagged type and
then create many children: US_Dollars, Canadian_Dollars, Yen,
Deutsch_Marks, Italian_Lira, Turkish_Lira, Gold_Oz, etc. This is much
cleaner than a variant record with over 50 values for the discriminant.  A
tagged type also provides the possibility of using wide characters and
different methods for showing negative values.

-------------------------------------------------------------
Date:    Sun, 27 Apr 1997 00:49:14 GMT
From:    Nick Roberts <Nick.Roberts@DIAL.PIPEX.COM>
Subject: Re: Tagged Types and Generics

Robert Dewar <dewar@merv.cs.nyu.edu> wrote in article
<dewar.862004577@merv>...
> Bob Leif says
>
> <<Background: Recently, Tom Moran, Randy Brukard and I wrote a paper.
"Ada
> 95, The Language Speaks for Itself".  This paper will appear in the May
> Object Magazine.  In this paper we modeled money as a decimal type and
did
> inheritance directly via a generic.  I had suggested modeling money as a
> tagged type.  My co-authors very wisely vetoed that approach.  After
the>>
>
> I can't see any reason for wanting to model money as a tagged type -- you
> lose literals, and you lose a lot of other useful semantics of decimal
> types, and you gain nothing  that I can see.


I think I can see a few possibilities: for example, what if you wanted to
be able to use multiple currencies easily? You can get literals reasonably
easily by the use of simple conversion functions. For example:

package Money is
   type General_Monetary_Amount is limited private;
   type US_Dollars is delta 0.01 digits 12;
   type Lire is delta 100 digits 14;
   ...
   function To_General(Amount: in US_Dollars) return
General_Monetary_Amount;
   function To_General(Amount: in Lire) return General_Monetary_Amount;
   ...
private
   ...
end;

and then a 'literal' can be constructed with something like

To_General(US_Dollars'(59.23))

This is slightly clumsy, of course, but not too bad.

If we had defined General_Monetary_Amount as tagged, we could extend it,
perhaps to provide a date (for aiding exchange calculations):

package Money.Dated is
   type Dated_Monetary_Amount is new General_Monetary_Amount;
   ...
private
   type Dated_Monetary_Amount is new General_Monetary_Amount with
      record
         Date: Calendar.Time;
      end record;
end;

And so on. Just thoughts (and it's gone midnight).

Nick.

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