Re: Building blocks (Was: Design By Contract)

[dewar@merv.cs.nyu.edu (Robert Dewar)]

<<In the original posting I was trying to be brief.  I have since
explained about design by contract and the Eiffel exception mechanism.>>

It is not at all the case that the abstract notion of design by contract
dictates the design of the detailed syntax of the exception mechanism.
Of course (as anyone familiar with the Ada design process are very aware),
the Ada design of specifications is entirely focussed on the issue of what
elements of the interface contract belong in the syntax of the language and
what are best left to comments. The same statement can of course be made
in Eiffel, so the theoretical bases, or more accurately, the fundamental
design principles, are in fact very similar.

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.

SO there is no question of a presence or lack of theory here, rather it is
a pragmatic issue of what should and should not go into the syntactical
interface. Eiffel generally decides to put more into the syntax, but there
is no real evidence to suggest that this is a good thing, it's mostly a
subjective issue.