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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: Exceptions
Date: Tue, 11 Dec 2007 18:01:43 -0600
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Date: 2007-12-11T18:01:43-06:00
List-Id: <comp.lang.ada>

"Niklas Holsti" <niklas.holsti@tidorum.invalid> wrote in message
news:475e8bde$0$27850$4f793bc4@news.tdc.fi...
...
> And perhaps also
>
>    5. Exceptions as formal generic parameters.

Ada has this (although it isn't obvious): just pass an
Ada.Exceptions.Exception_Id to the generic. That is, given a generic spec
something like:

    generic
         My_Exception : Ada.Exceptions.Exception_Id :=
Constraint_Error'Identity;
    package Gen is ...

you can instantiate it with any exception you like:

     package Inst is new Gen (Program_Error'Identity);

and you can raise the exception inside the generic:

     Ada.Exceptions.Raise_Exception (My_Exception, Message => "Wow!");

and you can even handle the exception with a little cleverness:

    when Occ:others =>
           if Ada.Exceptions.Exception_Identity (Occ) = My_Exception then
                -- Handle My_Exception.
           else
                 raise; -- Not the one we want.
           end if;

(This last is how Janus/Ada has to implement handlers for exceptions
declared inside of generics anyway, so in our case at least, the code cost
is essentially the same.)

All but the last as just as good as a "real" exception formal, and the last
doesn't happen that much in reusable stuff (usually such things are
originating or propagating exceptions, not handling them).

In any case, the fact that it is relatively easy to do this is why there is
no formal exception parameter.

                           Randy.