Re: Tagged Types and Generics

["Nick Roberts" <Nick.Roberts@dial.pipex.com>]

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.