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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Exception_Occurence and language designers
Date: Mon, 4 Dec 2017 15:31:26 +0100
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Xref: reader02.eternal-september.org comp.lang.ada:49350
Date: 2017-12-04T15:31:26+01:00
List-Id: <comp.lang.ada>

On 04/12/2017 15:00, Mehdi Saada wrote:
> but an "exception" is also an exception that might happen, at least 
> when it's just declared. I still wonder why the same subprograms can 
> take both Exception_Identity and Exception_Occurence, but not objects
> of the mere "exception" type. Mere exceptions are declared, but never
> taken as parameters like normal type instances are, like instances
> of Exception_Identity are.
In Ada 83 exception was not a first-class type. When Ada 95 added 
exception information it introduced Exception_Id to serve as a proper 
type for a class of exceptions and Exception_Occurrence to serve as 
exception instances.

In terms of tagged types it is like this:

    exception type              Foo : exception
    class-wide exception type = Exception_Occurence
    class-wide exception tag  = Exception_Id

Error'Exception_Identity is similar to Object'Tag.

There is no type-specific exception handling because you cannot define 
specific operations on an exception type:

    Foo : exception;
    procedure Bar (Error : Foo); -- This is illegal

You cannot declare type-specific exception instances either:

    Error : Foo; -- This is illegal

You can only declare class-wide instances in the form of exception handler:

    exception
       when Error : Foo | Baz =>

or explicitly:

    Error : Exception_Occurrence;

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