Re: Exception_Occurence and language designers

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On 2017-12-04 21:27, G. B. wrote:
> Dmitry A. Kazakov <mailbox@dmitry-kazakov.de> wrote:
> 
>> In Ada 83 exception was not a first-class type.
> 
> Still isn’t.

But its "class-wide" type Exception_Occurrence is.

>> In terms of tagged types it is like this:
>   
> Exceptions may be understood to be part
> of control structure. Doing so lets you see
> them as neither objects nor subprograms.
> So, in exceptional situations, the program
> isn’t moving along *if*s and *loop*s and calls;
> rather, a different mechanism of jumping is
> triggered. You can handle the situation by
> referring to its name, the exception’s,
> in a handler.

That was Ada 83. Then it became clear that the exception should carry 
user-defined and other information. One could of course invent new 
syntax in order to query and handle that information (e.g. to save and 
restore exception). Ada 95 decided to use normal types and values 
instead. So Exception_Id, Exception_Occurrence, Raise_Exception, 
Save_Occurrence appeared.

> Other answers explain what to do if the
> name is not in scope and how to print
> interesting information about what named
> exception has caused the situation.

Exception_Id is like Ada.Tag, it never actually dies. That is the 
problem with making exception full types like in C++ (along with loosing 
nice Ada's flat exception catch syntax in favor of ugly C++'s 
walkthrough catcher).

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