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From: Mats Weber <matsw@mail.com>
Subject: Re: Problem with function return
Date: 2000/04/03
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Date: 2000-04-03T16:06:03+00:00
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Robert Dewar wrote:

> >    type color_kind is (nil, red, green, blue);

> This seems a bad idea to me, it will result in C-like errors
> of the caller forgetting to check for an error, yes it may
> be caught, but it is much to easy for it to get swallowed
> up, e.g. by the else clause of an if.
> 
> Much better to raise an exception. if the caller wants to handle
> it they can put an exception handler at the call site. If they
> don't want to handle it, or more likely they forget, then the
> exception gets raised and unhandled, which is often better than
> blindly carrying on!

Of course an exception is best when feasible, but I have used the Nil
technique quite often when I need the enumerated type as a discriminant
and a default value is required, as in:

type R (Color : Color_Kind := None) is
   record
      case Color is
         when None =>
            null;
         when Red =>
            ....
      end case;
   end record;