- Haskell-Cafe - Haskell.org

Re: Exception handling in numeric computations
by John Lato 28 Mar '09

28 Mar '09

> > Message: 8 > Date: Fri, 27 Mar 2009 10:41:05 -0700 > From: Jonathan Cast <jonathanccast(a)fastmail.fm> > Subject: Re: [Haskell-cafe] Re: Exception handling in numeric > computations > To: Gregory Petrosyan <gregory.petrosyan%2Bhaskell(a)gmail.com> > Cc: haskell-cafe(a)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 <donn(a)avvanta.com> wrote: >> > Quoth John Lato <jwlato(a)gmail.com>, >> > >> >> 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 >

1 0

Re: Use unsafePerformIO to catch Exception?
by John Lato 28 Mar '09

28 Mar '09

> From: Jules Bean <jules(a)jellybean.co.uk> > wren ng thornton wrote: >> The type of head should not be [a] -> a + Error, it should be (a:[a]) -> >> a. With the latter type the compiler can ensure the precondition will be >> proved before calling head, thus eliminating erroneous calls. > > Yes, but you know and I know that's not haskell. > > I'm talking about haskell. > > In haskell - a language which does not fully support dependent types - > head is both necessary and useful. > I could follow the rest of this, but I don't understand why 'head' is necessary. Couldn't you always replace it with a case statement, with undefined on [] if necessary? I won't deny that it's extremely useful, though! John Lato

3 3

Internationalization of Haskell code
by Vasyl Pasternak 28 Mar '09

28 Mar '09

In my blog http://progandprog.blogspot.com/2009/03/i18n-and-haskell.html, I've described process of internationalization Haskell programms using GNU GetText. I wrote hgettext tool and Text.I18N.GetText library to help with internationalization (latest sources on http://hgettext.googlecode.com/files/hgettext-0.1.2.tar.gz) A bit later I plan to put it to hackage and wiki (waiting for account). -- Best regards, Vasyl Pasternak

4 3

The infamous evil monomorphism restriction (was: A bit of a shock - Memoizing functions)
by Peter Verswyvelen 28 Mar '09

28 Mar '09

>From a previous email in the beginners list I more or less understood that the monomorphism restriction will not exist anymore in Haskell Prime. Is this correct? On Fri, Mar 27, 2009 at 10:32 PM, Jonathan Cast <jonathanccast(a)fastmail.fm>wrote: > On Fri, 2009-03-27 at 14:26 -0700, Kirk Martinez wrote: > > Your powersOfTwo function, since it gets memoized automatically (is > > this the case for all functions of zero arguments?), > > It is the case for all functions which have zero arguments *at the time > they are presented to the code generator*. The infamous evil > monomorphism restriction arises from the fact that overloaded > expressions, such as > > negative_one = exp(pi * sqrt(-1)) > > look like functions of zero arguments, but are not, and hence do not get > memoized. This behavior was considered sufficiently surprising, when it > was discovered in early Haskell compilers, that the construct was > outlawed from the language entirely. > > jcc > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

1 1

A bit of a shock - Memoizing functions
by Gü?nther Schmidt 27 Mar '09

27 Mar '09

Hi, um, well, I'm not even sure if I have correctly understood this. Some of the memoizing functions, they actually "remember" stuff *between* calls? Günther

9 9

toSelem takes no format parameter
by Kemps-Benedix Torsten 27 Mar '09

27 Mar '09

Sterling, the toSElem function of ToSElem does not take a parameter that allows specification of e.g. the locale that shall be used for rendering a specific attribute. If one wants to change this, one also has to change setAttribute and the like. Do you have any plans to extend HStringTemplate in this way? Schöne Grüße Torsten Kemps-Benedix (Geschäftsführer) SKS Unternehmensberatung GmbH & Co KG Geheimrat-Hummel-Platz 4 D-65239 Hochheim am Main Germany M +49.163.36017.01 T +49.700.36017.000 F +49.700.36017.011 torsten.kemps-benedix(a)sks-ub.de <blocked::mailto:torsten.kemps-benedix@sks-ub.de> http://www.sks-ub.de <blocked::http://www.sks-ub.de/>

2 1

wxHaskell
by Tsunkiet Man 27 Mar '09

27 Mar '09

Hello, I've just installed wxHaskell as follow: - Download wxHaskell from the following page: http://haskell.org/haskellwiki/WxHaskell/Download and downloaded the binairy version for Windows XP. - Registered using the batch file provided. Now I have this small program from my education page that I want to run just to make sure my installation is correct. ( http://www.cs.uu.nl/wiki/pub/FP/Practicum/Balls.hs) I've also got Helium (2003) and winHugs (sept 2006) (I'm so sorry for using the old versions, but I just can't find new versions) My GHC compiler is version: GHC 6.10.1 and my wxHaskell is named like: wxhaskell-bin-msw2.8.9-ghc6.10.1-0.11.0-0.zip. (If you need more information just contact me) Now my error message in winHugs is: And if ERROR file:.\Balls.hs - Can't find imported module "Graphics.UI.WX" And if I run that in Helium I get some error message that the . in "Graphics.UI.WX" is unexpected. (Which means by my views that it didn't import wxHaskell.) And if I run it in ghci it compiles perfect. So I thought let's type in "main" to start it in a interpreter. So I get the following: http://img10.imageshack.us/img10/5118/testzrs.jpg. I don't think that program is running correctly. Can someone point out what went wrong or what I am doing wrong? And some small question, do I always have to compile those programs in order for it to run correctly? Thanks for your time and effort! Tsunkiet Man

2 2

want to upgrade to 6.10.1 ghc
by Vasili I. Galchin 27 Mar '09

27 Mar '09

Hello, I am running Ubuntu Linux. I have forgotten what is the preferred means of upgrading. Download the source and compile it using my current compiler .. 6.8.2? Regards, Vasili

2 1

ANN: HackMail 0.0 -- Procmail + Monads = Awesome!
by Joe Fredette 27 Mar '09

27 Mar '09

I'm happy to announce my very second Hackage upload!* HackMail is a Procmail-alike, though it doesn't (yet) support promail syntax. It dynamically loads a haskell source file (literate or vanilla, eithers okay) and then sits as a daemon watching a directory for new emails. The source file contains a function which sorts email and delivers it to some directory. As well as some configuration data. Not everything is working just yet -- the program is operational, but still lacking some of the features I want, namely a pipe mode, and some other things. It also uses a homebrew email parser, and lacks Haddock and Tests, but I wanted to release it mostly for comments/questions/helpful criticism from interested parties. Also, I wanted to upload something to hackage... It's a good feeling. Questions/Comments/Donations of Money, Vestal Virgins, or Cookies welcome! Package is on Hackage @ http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HackMail Source is on Patch-tag @ http://patch-tag.com/repo/Hackmail /Joe Fredette * (I uploaded a little tool called addLicenseInfo to test out the procedure, so... yah.)

1 0

Arch Linux now provides more than 1000 Haskell libraries and apps
by Don Stewart 27 Mar '09

27 Mar '09

A small milestone in the packaging business: http://archhaskell.wordpress.com/2009/03/27/arch-haskell-news-mar-14-2009-1… More than 1000 Haskell packages packaged up for Arch Linux. Hackage now has 1163 (+41) Haskell packages, of which 1007 (+33) have been natively packaged for Arch in AUR. That’s 33 new packages in two weeks, and lots of updates as well. Read this week's updates on the blog. -- Don

1 0