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

Henning Thielemann lemming at henning-thielemann.de
Mon Mar 30 18:03:32 EDT 2009

On Sun, 29 Mar 2009, John Lato wrote:

> On Sat, Mar 28, 2009 at 9:49 PM, Henning Thielemann
> <lemming at henning-thielemann.de> wrote:
>> On Sat, 28 Mar 2009, John Lato wrote:
>>> Honestly, me neither, until recently.  I'm only barely starting to
>>> understand it, and I do think there's a great deal of overlap.  Even
>>> if an error is a bug that can be fixed by the programmer, certain
>>> exceptional situations can also be fixed by the programmer by handling
>>> the exception, even if they can't be detected in advance.
>> For example?
> A file not being written because of a permissions error.  This can't
> be detected in advance due to effects from other processes, but it's a
> predictable enough exception that the programmer should handle it for
> IO.

Indeed. However I still do not see the overlap of errors and exceptions. 
  Handling of exceptions may mean that you print a message and abort the 
affected operation, that is you often do not need a specific exception 
handling. E.g. if the user wants to load a file into an editor, and the 
file is read-protected, then the editor reports, that the file cannot be 
loaded and does not load it. The editor should keep running, in contrast 
to when it detects a programming error. The same should happen when the 
editor encounters a memory overflow. So I cannot follow the argument of 
"memory exhaustion shouldn't happen anymore today, so there is no need to 
handle it". I remember this was claimed in this thread, too. However, 
recovering from a memory exhaustion can be tricky. Can the garbage 
collector still free memory, when it cannot allocate memory temporarily?

>> Btw. not handling an exception is an error.
> Agreed generally.  But some exceptions are likely in given contexts,
> others are not.  I don't think it's necessary to handle every possible
> exception, just the ones that are likely and predictable for a given
> activity.

Since handling of exceptions often consist of reporting the exception and 
aborting an operation, it should always be managable to handle all 
possible exceptions.

>  Excluding generic "The impossible happened, file a bug report" 
> handlers.

This is the one special case, where program errors are catched in order to 
allow debugging.

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