Re: Exceptions (Was: OO2C problem)

[Ian Rae <ianrae@istar.ca>]

> From: Mark K. Gardner <mkgardne@rtsl3.cs.uiuc.edu>
> *** Now for the controversy ***
>...
> Realizing that Oberon-2 is nearly perfect and therefore being
> reluctance to change it, I attempted to allow the following:
> 
>     ...
>     TRY(handler1);
>       ...
>     CATCH();
>     ...
>     TRY(handler2);
>       ...
>     CATCH();
>   END p;
> 
> which works (in that it doesn't crash) as long as you do not access
> any local state. Not quite as nice, but it doesn't require a change to
> the language or compilers... or so I thought. As was pointed out, the
> language definition prohibits this. So much for no changes to the
> language! (Note that it is also not fool-proof in that it would be
> easy to forget a "catch()" because the compiler is not required by the
> language definition to look for a close to the exception scope.)
> 
> One approach is to use the existing Exception.Mod to add a global
> procedure as a handler at the location of the TRY and to remove it at
> the location of the CATCH. One drawback is that the global handler is
> separated from the code containing the exception block and hence
> requires more effort on the part of the programmer to keep both the
> handler and the code in mind. Some may suggest that having the
> exception handler away from the code in the exception scope is an
> improvement. I could be convinced if it weren't for the next drawback.
> I firmly believe that allowing access to local state is very important
> if exceptions are to be more than a way to print an error message and
> die. It is especially important if recovery is attempted. Even if I am
> only going to print an error message and die, having access to the
> local state allows me to be more specific about why the procedure is
> dying.

I'm not a big fan of exceptions but it's interesting to see how they can be
added to Oberon.  Having access to locate state is not that important if
exceptions are used only for truly exceptional situations where control 
is passed to an outermost level of the program.  The exception object 
itself can include any relevant local info.

--Ian