Message: 8 Date: Fri, 27 Mar 2009 10:41:05 -0700 From: Jonathan Cast Subject: Re: [Haskell-cafe] Re: Exception handling in numeric        computations To: Gregory Petrosyan Cc: haskell-cafe@haskell.org Message-ID: <1238175665.20367.12.camel@jcchost> Content-Type: text/plain; charset=utf-8

On Fri, 2009-03-27 at 20:38 +0300, Gregory Petrosyan wrote:

...

On Fri, Mar 27, 2009 at 7:31 PM, Donn Cave wrote:

...

Quoth John Lato ,

...

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.

I have never felt that I really understood that one.

Me too :-)

BTW, John, how often do you encounter _hardware_ issues compared to "errors"?

Can't speak for anyone else, but I usually encounter hardware issues just before I replace the hardware...

This is getting confusing with two "John"'s replying. For myself, I do not encounter hardware issues often at all (unless you count buggy drivers). They are the exceptional condition.

...

Is an "out of memory" thing an error or exception? You will say "exception, for sure", wouldn't you? :-)

No.  GHC possesses an out-of-memory exception that (IIRC) it never throws, because it's simply not worth trying to recover from heap exhaustion.  Maybe 20 years ago it was, but these days a program that manages to exhaust space is almost certainly either buggy or poorly optimized.

I would have said exception, although I agree with the assessment that it's not worth trying to recover from.

An `error' is any condition where the correct response is for the programmer to change the source code :)

...

And if it is a result of applying known-to-be-very-memory-hungry algorithms to non-trivial input? Looks like programmer's error, doesn't it?

See above.

Due to their brevity, I sometimes find it difficult to understand Jonathan's replies. So at the risk of putting words in his mouth, I will take an attempt to explain this further. Keep in mind these are my words, not his, and it's possible I have completely misunderstood his point since I've only been using Haskell intensively for about a year (and I have no formal CS training, and other biographical points apply as well). Running out of memory is an exceptional condition. Doing something that is likely to cause memory exhaustion is a programming error, because it was the wrong solution to the problem. Memory exhaustion, the exceptional condition, is an end result of making the programming error. In this case (but not all), it is possible to treat the symptoms by handling the exception, but the general solution is to avoid the problem in the first place.

...

And I think I can provide lots of similar examples.

If there exists separation between errors and exceptions, it should be very strong and evident — otherwise "casual programmers" like myself will need to stare at the ceiling every time they write something to decide what suits best.

I am not sure when I would every throw an exception in Haskell code, short of re-raising from a handler. Things that I would have used exceptions for in C-sharp can be handled explicitly with Maybe or Either, a style that I greatly prefer. John

Protip: try pacing instead of staring at the ceiling.

jcc