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From: Brian Rogoff <bpr@shell5.ba.best.com>
Subject: Re: How to write TYPECASE in Ada 95?
Date: 1999/02/05
Message-ID: <Pine.BSF.4.05.9902051123560.3807-100000@shell5.ba.best.com>#1/1
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Date: 1999-02-05T00:00:00+00:00
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On 5 Feb 1999, Norman Ramsey wrote:
> 
> 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)
> 
> 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
> 
> What's the idiomatic way to express this in Ada?

There is no typecase, but if you want exact tag comparisons you can 
use 

	if p'Tag = Painted_Point then
		... do someting with Painted_Point 
	elsif p'Tag = Tall_Point then
		... yadda yadda yadda 
	...

-- Brian