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From: Jacob Sparre Andersen <sparre@nbi.dk>
Newsgroups: comp.lang.ada
Subject: Choice of variant record discriminant (design question)
Date: Wed, 26 Oct 2005 14:21:53 +0200
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Date: 2005-10-26T14:21:53+02:00
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I have declared a record type like this:

   type Preprocessed_Parameters (Measurement_Count : Natural := 0) is
      record
         case Measurement_Count is
            when 0 =>
               null;
            when others =>
               Mean_Value      : Scalar;
               Scaled_Variance : Scalar;
         end case;
      end record;

I would like to return an object of that type from a function using
this piece of code:

   if Count = 0 then
      return (Measurement_Count => 0);
   else
      [...]
      return (Measurement_Count => Count,
              Mean_Value        => Mean,
              Scaled_Variance   => Sum);
   end if;

I can't do this, since the discriminant has to be static.  The obvious
solution to the problem is to change the record declaration into:

   type Preprocessed_Parameters (Empty : Boolean := True) is
      record
         case Empty is
            when True =>
               null;
            when False =>
               Measurement_Count : Positive;
               Mean_Value        : Scalar;
               Scaled_Variance   : Scalar;
         end case;
      end record;

but I have a feeling there ought to be a more elegant solution.  Does
somebody have a suggestion or two?

Jacob
-- 
ACRONYM: A Contrived Reduction Of Nomenclature Yielding Mnemonics