Re: Exceptions

[Robert A Duff <bobduff@shell01.TheWorld.com>]

"Peter C. Chapin" <pchapin@sover.net> writes:

> 2. X is an abstract type representing an XML document.
> Has_Acceptable_Value returns true if X is valid according to its
> declared schema. P1 does some XML processing but it assumes the document
> given to it is well formed and raises exceptions if that is not the
> case. Since valid documents are also well formed, those exceptions won't
> occur once Has_Acceptable_Value has signed off on X. Indeed, this is one
>  of the main reasons why validating documents before processing them is
> desirable: it simplifies later error handling.

Right.  A similar example is a compiler.  E.g. in GNAT, the semantic
analysis phase assumes that the tree produced by the parser is
syntactically correct.  And the code generator assumes that the
program is legal.

> Suppose the above is inside a loop that runs over a collection of
> documents. Do we really want to include a handler for
> Not_Well_Formed_Exception in this procedure just because the contract on
> Process_Document says it might raise such an exception?

Well, we at least want a comment saying why the exception that "might"
be raised will not be raised in this case.  And "when Blah => raise
Bug; -- ..." seems like a good way to say that.

>...Do we really
> want to claim that we propagate that exception when we clearly don't?

No.  That would spread noise around the whole program.

> You might say that in a careful program (for example in a high integrity
> program) unexpected exceptions should not be treated in such a cavalier
> manner. I would agree with that. The question is should the language
> attempt to force that degree of care on all programs? It's a balance
> between usability and safety. As with all things related to security,
> overly aggressive policies can backfire when people feel the need to do
> silly things to work around them. I suppose at the end of the day it's
> really just a matter of taste... which is what makes debates like this
> possible. :-)

How can it be a matter of taste?  Checked exceptions either improve the
code or they don't.  Maybe it depends on the application area.  But I
don't see how it can depend on taste.

- Bob