Re: Pre-condition vs. Post-condition

[jls@rutabaga.Rational.COM (Jim Showalter)]

>When teaching Ada, immediately after giving that definition I point out to the
>students that it does not require all exceptional conditions to be handled
>by exceptions.

Exactly. Some are better handled by status parameters, or status fields
in abstract data types, or whatever.

>     What's happening here is that we're in an area where a lot of design
>decisions have to be made.  I'm not sure that there is a rule that can be
>applied to all cases and I am sure that if there is one we haven't found
>it yet.

Exactly. Although I do think there are heuristics, such as the infrequency
criterion. For example, I have no problem with blowing up on underflow
instead of pre-checking it. But I wouldn't use an exception to implement
a cyclic type, since the whole point of a cyclic type is that it will
be cycled through, so using the exception on 'succ is poor form.

In short, I use exceptions whenever their being raised would signal to
someone in a debugger that something undesirable has occurred. Underflowing
a stack is undesirable. Cycling a type is normal, so trapping that in
a debugger would confuse the hell out of the maintenance programmer (it
brings in the oxymoronic notion of a "normal exception"!).
--
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