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From: dewar@merv.cs.nyu.edu (Robert Dewar)
Subject: Re: Tagged Types and Generics
Date: 1997/04/27
Message-ID: <dewar.862144456@merv>#1/1
X-Deja-AN: 237706646
References: <3.0.32.19970426224603.00702f6c@mail.4dcomm.com>
Organization: New York University
Newsgroups: comp.lang.ada
Date: 1997-04-27T00:00:00+00:00
List-Id: <comp.lang.ada>


Robert Leif said

<<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.>>

I disagree, this is using a shotgun to kill an ant. Simple derivation is
perfectly fine for dealing with different kinds of currency. Sure you
can tag with the type, but about all that buys you is a display routine
that puts FF or UKL in as appropriate, and that's not worth the heaviness
of using tagged types in this connection.

I prefer to keep thinks as light and simple as possible (I must say that
in general it surprises me how many Ada programmers have a great love of
complex abstraction mechanisms, and seem to delight in trying to use
all kinds of complex features all over the place -- I am not talking
of Robert's suggestoin specifically here, but just reacting to some of
the code we see submitted to us -- I guess people must find the GNAT
code very simple minded -- one of my students said it was "painfully
easy to read" :-) :-)