[Haskell-cafe] New Hackage category: Error Handling

Michael Snoyman michael at snoyman.com
Mon Dec 7 02:00:21 EST 2009

On Mon, Dec 7, 2009 at 8:40 AM, Alexander Dunlap <alexander.dunlap at gmail.com
> wrote:

> On Sat, Dec 5, 2009 at 3:00 PM, Michael Snoyman <michael at snoyman.com>
> wrote:
> >
> >
> > On Sun, Dec 6, 2009 at 12:55 AM, Henning Thielemann
> > <lemming at henning-thielemann.de> wrote:
> >>
> >> On Sun, 6 Dec 2009, Michael Snoyman wrote:
> >>
> >>> I think there are plenty of examples like web servers. A text editor
> with
> >>> plugins? I
> >>> don't want to lose three hours worth of work just because some plugin
> >>> wasn't written
> >>> correctly. For many classes of programs, the distinction between error
> >>> and exception is
> >>> not only blurred, it's fully irrelevant. Harping on people every time
> >>> they use error in
> >>> the "wrong" sense seems unhelpful.
> >>>
> >>> Hope my commenting on this subject doesn't become my own form of
> >>> *pedantry*.
> >>
> >> In an earlier thread I have explained that one can consider a software
> >> architecture as divided into levels. What is an error in one level (text
> >> editor plugin, web server thread, operating system process) is an
> exception
> >> in the next higher level (text editor, web server, shell respectively).
> This
> >> doesn't reduce the importance to distinguish between errors and
> exceptions
> >> within one level. All approaches so far that I have seen in Haskell just
> mix
> >> exceptions and errors in an arbitrary way.
> >
> > I think we can all appreciate why it would be a bad thing is we treat
> > exceptions as errors. For example, I don't want my program to crash on a
> > file not found.
> >
> > On the other hand, what's so bad about treating errors as exceptions? If
> > instead of the program crashing on an array-out-of-bound or pattern-match
> it
> > throws an exception which can be caught, so what?
> >
> > Michael
> >
> I think the key is in the difference between the user/client and
> programmer/developer. As Henning has been saying, these roles change
> as you go through the different levels of the program, but I see the
> difference between an error and an exception as this: when a problem
> is relevant to the user/client, it's an exception; when it is
> irrelevant to the user/client, it's an error. Suppose you were using
> some sort of exception framework and you got an error from a piece of
> library code (not the head function) saying that "head" had failed
> somewhere. This is absolutely meaningless to a client. It just means
> there's a problem in the library code; it doesn't mean anything is
> amiss in the client's space. The client basically has to throw the
> function out, whether by gracefully aborting the program, disabling
> the plugin, etc. Contrast this with an exception, such as "index not
> in map." This is entirely relevant to the client. All of the code
> knows exactly what is going on; it's just that the index isn't in the
> map. The client can recover from this by, say, substituting a default
> value, adding the index to the map, etc. Now, suppose the client knew
> a priori that the index was *supposed* to be in the map. Now this
> becomes an *error* to the *client* of the client, since there is a bug
> in the first client's code.
> I guess my point is that if I have a function, say, sort :: Ord a =>
> [a] -> [a], and sort calls head somewhere in it, there's no point
> having "sort" throw an exception for "Prelude.head: []" since that
> means nothing to the client. It would make more sense to have an
> InternalError type that just says "OK, sorry client, I screwed up,
> just clean things up as best you can". If really were something that
> the client could have responded to, sort should have caught the head
> error inside the function definition and rethrown a different
> exception; say, SortEmptyListError if your sort function didn't work
> on empty lists (contrived example, sorry).
> Alex
The WrapFailure typeclass in the the failure package makes this kind of
library design fairly straightforward. I think you're making an argument for
treating errors and exceptions the same way.

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