[mercury-users] Exceptions and unique modes

[Ralph Becket]

>From Peter Ross on 21/02/2001 12:21:36
> > 
> An interesting point was raised by Paul Massey here at MC, and that is a
> predicate should return either a status indicator or throw an exception
but
> never do both.  So you could argue that the IO predicates should report
> EOF as an exception as well or return everything in the status
> indicator.

But, I claim, EOF is not an exceptional condition and hence should be a
valid result!  An error is an exceptional condition.

Your example problem, as I understand it, can be paraphrased as

	- carry out some operation, allocating resources along the way
	- if something goes wrong, deallocate any resources that did
	  get allocated and clean up the structure of interest

You say that carrying an io__state around to do this is unacceptable, but 
don't say why.  Admittedly it might be inconvenient, but it *is* the place
where you need to put persistent state, such as the information required
by the recovery code.

--
Ralph Becket      |      MSR Cambridge      |      rbeck at microsoft.com 

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