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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: About String
Newsgroups: comp.lang.ada
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Date: Sun, 15 Jun 2008 22:52:45 +0200
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Date: 2008-06-15T22:52:47+02:00
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On Sun, 15 Jun 2008 15:38:08 -0400, Robert A Duff wrote:

> "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:
> 
>> On Sun, 08 Jun 2008 17:19:11 -0400, Robert A Duff wrote:
>>
>>> I agree.  I suppose it comes from Pascal, where declarations and
>>> executable code are completely separated.  It makes no sense in
>>> Ada, where declarations are just as executable as statements.
>>
>> I disagree. I think it does make sense because it clearly defines the scope
>> of the declared variable. Expression (result) /= named object. And
>> reversely, if you don't need the variable, you are free to put expressions
>> anywhere outside the declaration part.
> 
> If Ada allowed:
> 
>     for I in A'Range loop
>         X : constant Character := F(A(I)); -- Not Ada!
>         Do_Something(X);
>     end loop;
>
> the scope of X is clear -- it's the entire loop body.

No, then it should better be something like:

   for I in A'Range;
        X : constant Character := F(A(I)); -- Not Ada!
   loop
      Do_Something(X);
   end loop;

The declarative part of a loop is between "for" and "loop."

However I find nested declare/begin/end more readable because it clearly
disambiguates between the scope of the loop and the scope of one iteration
of.

> You could still say:
> 
>     for I in A'Range loop
>         begin
>             X : constant Character := F(A(I)); -- Not Ada!
>             Do_Something(X);
>         end;
>         Do_Something_Else;
>     end loop;

But what would happen if I said:

   begin
      raise Foo_Error;
      X : constant Character := F(A(I)); -- Not Ada!
      Do_Something (X);
   exception
      when Foo_Error =>
         Do_Something_Interesting (X);
   end;

---------------
What I miss is things like:

procedure Foo (X : T'Class) is
begin
   case X is
      when S_X : S'Class =>
          Do_S_Method (S_X);
      when others =>
          Do_Something_Else;
   end case;
end Foo;

procedure Bar (Ptr : access T'Class) is
begin
   case Ptr is
      when Ref : not null access T =>
          Ref.Foo; -- No more checks
      when others =>
          null;
   end case;
end Bar;

case I is
   when Checked_I : A'Range =>
      ... A (Checked_I) ... -- No range checks
   ...
end case;

case L - 123 is
   when Sum : Positive =>
       -- Sum = L - 123 and it is positive
       ...
   when Sum : Integer =>
       -- Sum = L - 123 and it is not positive
      ...
   when others =>
       -- It would overflow, but no exception was propagated
      ...
end case;

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de