[Haskell-cafe] Re: Exception handling in numeric computations

[John Lato]

On Fri, Mar 27, 2009 at 10:01 AM, Gregory Petrosyan
<gregory.petrosyan+haskell at gmail.com> wrote:
> Thanks a lot for the answer!
>
> On Thu, Mar 26, 2009 at 4:36 PM, John Lato <jwlato at gmail.com> wrote:
>> Languages with checked exceptions usually use them for two purposes:
>>
>> 1.  Exceptional conditions - disk full, CPU on fire, etc.
>> 2.  Error handling - invalid arguments to a function, attempt to
>> invert non-invertible matrix, etc.
>
> Is there any good rule someone can use to decide whether it is error
> or exception?
> For me, this is the most important thing, because IMHO you (as library
> writer) often can't
> say what is it, it's up to client of your code to decide.

An exception is caused by some sort of interaction with the run-time
system (frequently a hardware issue).  The programmer typically can't
check for these in advance, but can only attempt to recover after
they've happened.

An error is some sort of bug that should be fixed by the programmer.

There is some overlap for certain cases, notably divide by 0.
Dividing by 0 is an error, because it's something that the program
should never do, and it can be detected and dealt with by the
programmer in advance.  However most systems allow the divide function
to be called with a 0 denominator.  The function has a precondition,
meaning the onus is on the programmer to not do it, however this is
not enforced by the language.  If a program does this in error, the
result is an exception because the result is not a valid number (this
is codified with NaN for IEEE floats).  In this case, a programming
error results in an exception.  The proper solution is to fix the
source of the problem, the error, instead of trying to clean up the
results.

>
>> Henning T., FYI your constant advocacy has gotten at least one person
>> around to this view.
>
> Can you please provide me some links about error/exception separation?

http://haskell.org/haskellwiki/Error
http://haskell.org/haskellwiki/Exception