Re: F9X twister & ADA (was: n-dim'l vectors)

[Geoff Bull <geoff@research.canon.com.au>]

James Giles wrote:
>  How do you *not* provide a
> handler when you don't even know what it is that you're trying
> not to handle?

Documentation, tools and testing.

> 
> In any case, for most of my own code I much prefer halting
> immediately. 
You might prefer it, but I don't. Forced halting would suck
from my point of view.

> Most "graceful" termination involves doing
> things that cover up evidence that might be needed to discover
> the cause of the fault (closing files, deleting temporaries, leaving
> the program counter and/or the registers in different states than
> the context in which the error arose, etc.).  It's only for code
> written for naive public consumption that "graceful" termination
> is appropriate.

Actually, in my case the point of flushing files is to *not* throw away 
the evidence of where the program was up to when it crashed.
Also graceful termination might be disabling of an impending missile
launch, or doing a retry using a backup system,
or sending a failure notice to a remote system.

> >>  Mystically skipping over several
> >> levels of the call tree is messy.
> >
> >I don't understand what you mean here.
> 
> I can't see any opportunity for confusion here.  

Ada exceptions are neither mystical nor messy, so I thought
you must be talking about something else.


>  Yes, this means that when you add code which may
> raise exceptions to a procedure, you have to recompile (and maybe
> even rewrite) code which uses that procedure.  This is no different
> than any other case in which you changed a procedure in a way
> in which its public (interface) properties have changed.

Going back to your earlier example, if you decided to raise 
an exception in Z and you really did want to handle it in A 
(because of requirement of graceful operation for naive users),
you would have to go back and modify procedures B .. Y to
either handle or pass on the exception.


>  Or, are
> you claiming that an exception is not a public property of the
> procedure which raises it?
No.
I agree with your sentiments.
But I can't think of a workable implemetation.
Certainly the way Java does it can be a headache.
Other difficulties with Java style exceptions
in Ada have been previously discussed in comp.lang.ada.

Cheers
Geoff