- Haskell-Cafe - Haskell.org

Error Handling
by Thomas L. Bevan 13 Dec '02

13 Dec '02

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Hi, I'm writing a little command line app, but I have a question about error handling. The code will go something like, main = do (a:b:cs) <- getArgs i <- return (read a :: Int) j <- return (read b :: Int) putStr $ i + j How can I catch any possible cast exception? Ideally, I think I would like to use the ErrorT monad as a means of handling a potentially fairly complex set of inputs, but I can see no way of handling these errors neatly within that monad. Tom -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.2.1 (GNU/Linux) iD8DBQE98+VqGse0w80KF2gRAqnMAJ9CMiOWFS+3y2ELu7851MyvTb1aCgCgru+z EtW/mO+WZ0lqtYUECcHyTDE= =r1DY -----END PGP SIGNATURE-----

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Monads [beginner]
by maas＠cin.ufpe.br 13 Dec '02

13 Dec '02

What are Monads? Where can I read about then? I read that there´s some mathematical foundation under this concept, where can read about these theories? +---------------------------------+ | Marcos Aurélio Almeida da Silva | | E-mail: maas(a)cin.ufpe.br | +---------------------------------+

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RE: Haskell related Debian packages
by Simon Marlow 13 Dec '02

13 Dec '02

> Please excuse me if anyone feels that this email isn't > appropriate for this mailing list. Though these are somewhat > Debian-specific questions, I thought that some Haskell folks > could lent their insight. You would be the users after all. Also see the recent discussion on glasgow-haskell-users about a library building/packaging system (which didn't really reach a conclusion). > There aren't many examples of libraries (without any > binaries) that are packaged thus far. I noticed that > ctklight installs stuff in /usr/share/hugs98/extentions, but > it can be used with GHC also, so I am not sure why its done > this way. I don't use hugs too much though, also I don't know > anything about nhc. > > So I'm considering sticking the *.lhs files for HUnit, for > example, in /usr/share/hunit. Then people can alter their > Makefiles to include that directory in either hugs or ghc, > but I see one problem with that: GHC would want to compile > the source files, and stick them in /usr/share/hunit, but > would be unable to (because of permissions problems when > users compile). > > The package could install binary files, but then it would > build-depend on ghc which doesn't exist for several Debian > platforms. So I guess it could check to see if ghc is > installed on the building computer and stick binaries in /usr/lib. I think I would have two versions of each package: one that builds & installs a package for GHC, and one that installs source files for use with Hugs (GHCi can use the GHC package). These really are entirely different beasts, and in most cases will have zero files in common, so I think it makes sense to separate them altogether. That way you don't have to require GHC in order to install the Hugs version of the package, and vice-versa. Cheers, Simon

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Re: ffi
by Martin Huschenbett 12 Dec '02

12 Dec '02

> > $ ghc -ffi -o myprog Main.hs cfile.o > When I try this I also get an error: martin:~/work/prograemmelchen> ghc -ffi -o myprog Main.hs cfile.o ghc-5.02.2: unrecognised flag: -ffi Usage: For basic information, try the `--help' option. Do I use a wrong GHC version? THX, Martin.

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Re: Monads [beginner]
by Benderjgdefault＠aol.com 12 Dec '02

12 Dec '02

In a message dated 12/12/2002 2:45:31 PM Eastern Standard Time, maas(a)cin.ufpe.br writes: > What are Monads? > Where can I read about then? > > I read that there´s some mathematical foundation under this > concept, where > can read about these theories? A large set of articles on monads in the context of Haskell are collected on my bibliography web site: http://haskell.readscheme.org/monads.html There are also a few, more tutorial, articles available at http://haskell.org/bookshelf/ that are not included on my monads page. For more theoretical works-- while still being programming language related--I suggest Eugenio Moggi's site: http://www.disi.unige.it/person/MoggiE/publications.html. Jim Bender

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Haskell related Debian packages
by None 12 Dec '02

12 Dec '02

I'm considering packaging several Haskell libraries for Debian, and wonder what people think about where things should go. Please excuse me if anyone feels that this email isn't appropriate for this mailing list. Though these are somewhat Debian-specific questions, I thought that some Haskell folks could lent their insight. You would be the users after all. There aren't many examples of libraries (without any binaries) that are packaged thus far. I noticed that ctklight installs stuff in /usr/share/hugs98/extentions, but it can be used with GHC also, so I am not sure why its done this way. I don't use hugs too much though, also I don't know anything about nhc. So I'm considering sticking the *.lhs files for HUnit, for example, in /usr/share/hunit. Then people can alter their Makefiles to include that directory in either hugs or ghc, but I see one problem with that: GHC would want to compile the source files, and stick them in /usr/share/hunit, but would be unable to (because of permissions problems when users compile). The package could install binary files, but then it would build-depend on ghc which doesn't exist for several Debian platforms. So I guess it could check to see if ghc is installed on the building computer and stick binaries in /usr/lib. Another option would be to compile packages for GHC in the post-install script if ghc is installed, and stick those installed files in /usr/lib, and the .hs files in /usr/share/hugs98/extentions if hugs is installed. So my question is: if you were / are using Debian, what do you think that a well-behaved Haskell library package should do? Also Debian users might email me privately with suggestions on what are the most important programs to package: I plan to start with the ones I use the most. peace, Isaac

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AW: ffi
by Markus.Schnell＠infineon.com 12 Dec '02

12 Dec '02

> $ cc -c cfile.c > $ ghc -o myprog Main.hs cfile.o won't you need -fffi also? Markus

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ffi
by Martin Huschenbett 11 Dec '02

11 Dec '02

Hi all, I've got a simple question concerning FFI but as I am a newbie I don't know the answet. I've got a C-source-file, maybe something like: //cfile.c void cfun() { /* ... do something ... */ } //EOF and a Haskell-source-File, maybe something like: -- Main.hs module Main where main :: IO () main = do -- do something cfun -- I wan't to call the function written in C here -- do again something -- EOF And now my question is: How do i realize this? Can you please tell me how write a binding and how tu run ghc. Thank you very much, Martin.

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video4linux
by Martin Huschenbett 10 Dec '02

10 Dec '02

Hi all, does anybody know how to read a single image from a video4linux device under Haskell? Thanks for all help, Martin

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Re: AW: Editor Tab Expansion
by Marcin 'Qrczak' Kowalczyk 10 Dec '02

10 Dec '02

Fri, 6 Dec 2002 12:09:46 +0100, Ingo Wechsung <iw(a)contexo.de> pisze: > No. My editor produces the ASCII code for horizontal tab, when I > hit the tab key. Just as it produces the ASCII code for a when I > hit the a key. > > That's how it should be. It shouln't, becase tabs are 8 spaces, which is too much for indentation of programs in most languages. > The real problem is, that nobody could tell from just looking at > the screen, what a program means even if you know the layout rules > very well. Of course I can. If you can't, perhaps you are using a non-standard tab width? Please fix settings of your editor and it will work. > Instead, your text editor displays some space in place of the tab. > Only if the expansion rules of your editor are compatible with > Haskell's may you grasp the source codes meaning. But they are compatible because there is one most universally accepted interpretation of a tab (move to the next multiple of 8 columns). Any other interpretation hampers portability and should be avoided. > Think of a software development team. One member (in India) > uses "convert n leading spaces to tabs", the other (in Japan) > "convert leading tabs to m spaces". Everytime one of them makes a > small change in a file that has been changed by the other before, > the source code control system will have a huge diff. All these problems are caused by people who use a different tab size than 8. With consistent tab size there is no reason to convert between tabs and spaces. -- __("< Marcin Kowalczyk \__/ qrczak(a)knm.org.pl ^^ http://qrnik.knm.org.pl/~qrczak/

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