Re: Building blocks (Was: Design By Contract)

["John G. Volan" <johnv@ac3i.dseg.ti.com>]

Robert Dewar wrote:
> 
> The specific issue in Ada of whether it should be part of the syntax of
> the language to specify the exceptions that can be raised is of course
> one that was extensively discussed, and the quite deliberate decision was
> made that it is unhelpful to require these to be stated in the syntax.
> 
> The reason is that there are too many exceptions (Storage_Error, or the
> various IO errors if any IO is done, or other application defined system
> wide exceptions) which would be named all over the place and create clutter.
> Note that Program_Error would also have to be named almost everwhere because
> of the Access Before Elaboration (ABE) considerations.

There are a couple of strategies that could be applied to this problem:

(1) Take certain common language-defined exceptions as givens (e.g.,
Storage_Error, Program_Error, Constraint_Error).  That is to say, assume
that they are implicitly part of the interface of every
subprogram/method, and don't require the programmer to explicitly list
them.

(2) Make exceptions first-class objects, and take advantage of class
hierarchies to simplify subprogram/method interfaces.  For instance, IO
exceptions might be organized into a hierarchy rooted at some abstract
IO_Exception class.  A method could declare that it can raise any
IO_Exception, without having to explicitly list all the subclasses.

As I understand it, this is essentially the strategy Java uses.  I must
admit that I like being able to treat exceptions as first-class objects
organized in class hierarchies, and I miss that capability in Ada95.  I
wonder why this notion didn't catch on in the Ada95 design ... is it
because first-class exceptions would have introduced a so-called
"distributed overhead" into the language?

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