Re: About String

[Robert A Duff <bobduff@shell01.TheWorld.com>]

"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:

> 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."

I don't see why a loop needs a declarative part.
My idea is that every statement list is a scope.

> 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.

I don't get it.  You declare something inside a loop body, or the 'then'
part of an 'if' -- it's local to that.

>> 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;

As I said earlier, exception-handler regions deserve their own syntax,
not connected with declaration blocks.

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

Not sure how these are related to local decls...

> 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;

- Bob