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From: Matthew Heaney <matthew_heaney@acm.org>
Subject: Re: How to write TYPECASE in Ada 95?
Date: 1999/02/06
Message-ID: <m3lnicyvtw.fsf@mheaney.ni.net>#1/1
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Sender: matt@mheaney.ni.net
References: <79fct8$9k3$1@murdoch.acc.Virginia.EDU>
NNTP-Posting-Date: Fri, 05 Feb 1999 20:53:28 PDT
Newsgroups: comp.lang.ada
Date: 1999-02-06T00:00:00+00:00
List-Id: <comp.lang.ada>

nr@labrador.cs.virginia.edu (Norman Ramsey) writes:

> I'm trying to figure out from the reference manual how to identify
> which type extension of a type I have.  For example, if I have
> 
>    type Point is tagged
>          record
>            X, Y : Real := 0.0;
>          end record;
> 
>    type Painted_Point is new Point with
>          record
>            Paint : Color := White;
>          end record;
> 
> 
>    type Tall_Point is new Point with
>          record
>            Height : Real := 0.0;
>          end record;
> 
> I'd like to be able to discriminate at run time:
> 
>    p : Point;
> 
>      ...
> 
>    if p in Painted_Point then
>       ... do something with Painted_Point'(p)
>    else if p in Tall_Point then
>       ... do something with Tall_Point'(p)

You've got it right: use a membership test.  The only problem here is
that you should have said "elsif" instead of "else if".

 
> Even better would be something like Modula-3 TYPECASE:
> 
>   (* modula-3 *)
>   TYPECASE p OF
>   Painted_Point (painted) => ... do something with painted
>   Tall_Point    (tail)    => ... do something with tall
>     ...
>   END

No, you can't use case statement for this sort of thing.
 
> What's the idiomatic way to express this in Ada?

The idiomatic way in Ada --and in any object-oriented language-- is to
not do this at all.  You should be call a primitive operation of the
type that automatically dispatches according to the (type) tag of the
object.

If you insist on doing an explicit test, then use a membership test.