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Date: Tue, 21 Jul 2009 11:31:03 +0200
From: Georg Bauhaus <rm.dash-bauhaus@futureapps.de>
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Subject: Re: Alternatives to C: ObjectPascal, Eiffel, Ada or Modula-3?
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Date: 2009-07-21T11:31:04+02:00
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Jean-Pierre Rosen schrieb:
> Cesar Rabak a �crit :
>> Ludovic Brenta escreveu:
>>> You focused too much, IMHO, on the range restriction which is not the
>>> important part.  The important part is that, in Ada, this declares a
>>> new *type* (not a "subtype" or "nickname for Float"), which is
>>> *incompatible* with Float or any other type.  
>> More or less. It has all the operations of the derived type, uses the
>> same representation on the machine.  The incompatibility is syntactic no
>> in its essence.
>>
> Here is the crux of the misunderstanding: sure, at /machine/ level they
> are the same, but in the /problem domain/ they are different. The big
> difference between Ada an other languages is that types (even elementary
> ones) reflect the problem domain, not machine types.

It might be worth mentioning that derived elementary types
neet *not* have the same representation.  Gems #27 and #28
talk about opportunities and dangers of using two different
representations for the "same" derived type (as I am certain
you know):

http://www.adacore.com/category/developers-center/gems/

For example, the multiplications in a silly dummy below
use operands of different sizes, as expected.  So they are
really different, even at the machine level.

procedure Rep is

   package P is

      type T0 is range 0 .. 1_000;
      for T0'Size use 16;

      function From_String(Input: String) return T0;


      type T1 is new T0;
      for T1'Size use 64;

   end P;

   package body P is separate;
   use P;

   X0: T0;
   X1: T1;
   Y: T0;

begin

   X0 := P.From_String("3");
   X1 := P.From_String("5");

   X0 := X0 * X0;
   X1 := X1 * X1;

   Y := X0 + T0(X1);

   pragma Inspection_Point(Y);  -- stop ambitious optimizers

end Rep;


--
Georg Bauhaus