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From: Stefan Lucks <lucks@th.informatik.uni-mannheim.de>
Newsgroups: comp.lang.ada
Subject: Re: Reconsidering assignment
Date: Thu, 7 Jun 2007 09:10:25 +0200
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Date: 2007-06-07T09:10:25+02:00
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On Wed, 6 Jun 2007, Maciej Sobczak wrote:

> 1. Integer and its subtypes (like Positive).
> An Integer object has some value - and only that. Any subtype of
> Integer is defined within the same space, which means that this only
> thing that Integer has is also subject for the constraint check. In
> other words, *every* modification of the Integer (or subtype of)
> object is potentially violating the constraint.

Yes, that is what subtypes are for. And if a subprogram delivers you a 
value in the types range, but outside the subtype's range, a 
Constraint_Error is IMHO the right thing. Note that the Constraint_Error 
is raised at the caller's side, after it got the subprogram's response.
So calling Swap(X,Y) with X: integer := -1; and Y: Natural := 1; gives 
you exactly the same "result" as, say, Y := X*Y. You (the caller) are 
responsible for maintaining the subtypes constraints -- and thus 
for raising the exception ...

> 2. String.
This issue is different from (Of course, this generalises to all types 
with constraints ...). The difference is that the exceptions are raised 
inside the subprogram, so the caller would need the details of the 
subprograms implementation to predict if an exception will be raised. That 
is bad!

I like your suggestion, but i would prefer a simpler syntax. Use some 
attribute Type in this case. Thus, I would prefer

procedure Swap (X: in out String; Y: in out X'Type);

over

> procedure Safe_Swap(X, Y : in out String) with X'Length = Y'Length;

(BTW, your clause ensures that the lengths of the two strings are 
identical, the semantic of X'Type would even ensure X'First = Y'First and 
X'Last = Y'Last ...

Assignment:

Do_Something(X: T, Y: X'Type, <... other params ...>) is
begin
   ...
   -- here you can use X := Y or Y := X without raising Constraint_Error
   -- except, of course, T is limited
end Do_Something;


> 3. Tagged types.

> procedure Safe_Swap(X, Y : in out Object'Class) with X'Tag = Y'Tag;

procedure Safe_Swap(X: in out Object'Class; Y: X'Type) is

> declare
>  T1 : Triangle := ...;
>  T2 : Triangle := ...;
>  C : Circle := ...;
> begin
>  Safe_Swap(T1, T2); -- OK
>  Safe_Swap(T1, C); -- compile-time error
> end;

Comments?

-- 
Stefan Lucks      (moved to Bauhaus-University Weimar, Germany)
------  I  love  the  taste  of  Cryptanalysis  in  the  morning!  ------