- Haskell-Cafe - Haskell.org

ErrorT instance of MonadPlus
by Dmitry Olshansky 26 Feb '10

26 Feb '10

Hello, ErrorT instance of MonadPlus define that in case of fail both arguments of mplus a value of Error will be the Error of second computation: m `mplus` n = ErrorT $ do a <- runErrorT m case a of Left _ -> runErrorT n Right r -> return (Right r) Could it be changed in this way: m `mplus` n = ErrorT $ do a <- runErrorT m case a of Left e -> do b <- runErrorT n case b of Left e' -> return $ Left (e `eplus` e') r -> return r r -> return r where `eplus` could be placed in class Error a: eplus :: a -> a -> a eplus x y = y -- default implementation In this case we could combine Errors more flexible. Best regards, Dmitry

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problem loading package unix-2.3.2.0 while installing hackages on OpenSuse
by gmi 26 Feb '10

26 Feb '10

I have OpenSuse 11.2 running on VMWare and I have recently installed ghc 6.10.4 from the RPM from OpenSuse. Now I want to install cabal. I have downloaded the required packages from hackage and I try to run: runhaskell Setup configure on any of these packages. With option -v I can see an error while linking unix-2.3.2.0. I am no expert on OpenSuse yet. Does anyone know what's happening here? linux-nrvh:/home/gmi/Download/cabal/zlib-0.5.2.0 # runhaskell -f /usr/bin/ghc -v Setup configure --with-compiler=ghc-6.10.4 ... Loading package filepath-1.1.0.2 ... linking ... done. Loading package unix-2.3.2.0 ... Setup: <command line>: can't load .so/.DLL for: dl (libdl.so: cannot open shared object file: No such file or directory) -- View this message in context: http://old.nabble.com/problem-loading-package-unix-2.3.2.0-while-installing… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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hmake and hat
by Joachim Breitner 26 Feb '10

26 Feb '10

Hi, I made hmake compile on Debian again, but it seems it’s not working yet properly. This also affects hat, as it uses hmake to compile. I guess we could just drop hmake and hat, but this is Debian, and we try to avoid dropping stuff that made it into a stable release. Is anyone interested in figuring out whats wrong with hmake and how to make hat compile again? Also, to haskell-cafe, are there any hat users out there? Greetings, Joachim -- Joachim "nomeata" Breitner Debian Developer nomeata(a)debian.org | ICQ# 74513189 | GPG-Keyid: 4743206C JID: nomeata(a)joachim-breitner.de | http://people.debian.org/~nomeata

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Haskell and XML, need some tips from practioners
by Günther Schmidt 26 Feb '10

26 Feb '10

Hi everyone, I had to transform a 136 page word document into an XML-Document so that it can be imported into an application. The word document contained records in nested tables. Anyway through a very, very tedious process of xslt-transformations I finally have the XML document I need. But now I need to amend the attributes of some elements with looked up values from the outside, the lookup-key is a particular attribute value of the nodes. This I cannot do through xslt processing as the information needed is not within the xml document. I was thus going to use Haskell instead of an XSLT-processor for this final step. My question to those with experience of the Haskell-XML tools: which one should I use? Günther

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haddock installation weirdness?
by David Leimbach 26 Feb '10

26 Feb '10

I'm on Mac OS X Snow Leopard, and can't get haddock installed due to the following error: Resolving dependencies... Configuring haddock-2.6.0... Warning: This package indirectly depends on multiple versions of the same package. This is highly likely to cause a compile failure. package haddock-2.6.0 requires Cabal-1.8.0.2 package ghc-6.12.1 requires Cabal-1.8.0.2 package bin-package-db-0.0.0.0 requires Cabal-1.8.0.2 Preprocessing library haddock-2.6.0... Preprocessing executables for haddock-2.6.0... unused terminals: 1 Building haddock-2.6.0... <command line>: cannot satisfy -package-id ghc-6.12.1-b691a185e99c62533666d9a28a9e1988: ghc-6.12.1-b691a185e99c62533666d9a28a9e1988 is unusable due to missing or recursive dependencies: Cabal-1.8.0.2-a08510b9460f1b65f9dee06ed53f0650 bin-package-db-0.0.0.0-0c559ebe951f9972c4e6dfe5ebd4ce6a (use -v for more information) cabal: Error: some packages failed to install: haddock-2.6.0 failed during the building phase. The exception was: ExitFailure 1 When I do a "ghc-pkg list | grep Cabal" I get the following: ghc-pkg list | grep Cabal Cabal-1.8.0.2 Cabal-1.8.0.2 I'm wondering if this means I have a copy in my .cabal, and another from GHC,and if that could be causing a problem? I'm trying to work on the Haddock docs for the NineP package I uploaded the other day, and would rather not have to finish uploading before previewing the result :-) Dave

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hdbc-mysql fails to compile
by Thomas Girod 26 Feb '10

26 Feb '10

Hi there. Looks like hdbc-mysql cannot compile against GHC-6.12 -- confirmed by the hackage build logs. [1] anyone know a way to dodge the problem ? cheers, Tom [1] http://hackage.haskell.org/packages/archive/HDBC-mysql/0.6/logs/failure/ghc…

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Re: [Haskell-cafe] Testing and module export lists
by Magnus Therning 25 Feb '10

25 Feb '10

On Wed, Feb 24, 2010 at 10:41, Arnaud Bailly <arnaud.oqube(a)gmail.com> wrote: > Maybe you just want to test what's in your export list which > represents the public interface of your code. And if you cannot write > a test that exercise private implementation through the public > interface, then maybe there is a design problem... Possibly! The specific case I have is a module for parsing a small language. The module exports a single function, a function that takes a string and returns a tree. Of course the parser is built using parser combinators. I'd like to test each combinator I've built, but I don't want to export them all. The only way I see of achieving this is to split each module in two, one public and one internal, where the public one just re-exports the relevant pieces of the internal one. That's workable, but hardly aesthetically pleasing ;-) /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus＠therning．org http://therning.org/magnus identi.ca|twitter: magthe

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Interested in the Dutch HUG Day (24 April 2010)?
by Sean Leather 25 Feb '10

25 Feb '10

In honor of the first anniversary of the Dutch Haskell Users Group (formed at Hac5 in Utrecht), we are planning a mini symposium to bring together Haskell and functional programming enthusiasts in the Netherlands and surrounding area. *The Dutch HUG Day will be held on Saturday, 24 April.* Please mark that date in your calendar. The remaining details are being worked out. In order to help us with this process, we have created a short survey. If you are even remotely considering attending, please take a moment to fill out a form with your name and the activities that interest you. * http://spreadsheets.google.com/viewform?formkey=dGVuVTdnZEl0ZDE3MEVweUtRdml… Note that this does not commit you to anything. We will only use this information to help plan the event (e.g. determine the number of seats at the location and the types of activities). For more information about the Dutch HUG, visit the wiki page and sign up for the mailing list. * http://www.haskell.org/haskellwiki/Dutch_HUG * http://groups.google.com/group/dutch-hug Thanks for your time! Sincerely, The Dutch HUG Day planning team

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Category Theory woes
by Mark Spezzano 25 Feb '10

25 Feb '10

Hi all, I'm trying to learn Haskell and have come across Monads. I kind of understand monads now, but I would really like to understand where they come from. So I got a copy of Barr and Well's Category Theory for Computing Science Third Edition, but the book has really left me dumbfounded. It's a good book. But I'm just having trouble with the proofs in Chapter 1--let alone reading the rest of the text. Are there any references to things like "Hom Sets" and "Hom Functions" in the literature somewhere and how to use them? The only book I know that uses them is this one. Has anyone else found it frustratingly difficult to find details on easy-to-diget material on Category theory. The Chapter that I'm stuck on is actually labelled Preliminaries and so I reason that if I can't do this, then there's not much hope for me understanding the rest of the book... Maybe there are books on Discrete maths or Algebra or Set Theory that deal more with Hom Sets and Hom Functions? Thanks, Mark Spezzano.

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Heterogeneous Data Structures - Nested Pairs and functional references
by Günther Schmidt 25 Feb '10

25 Feb '10

Hi, I've been thinking a lot lately about heterogeneous and extensible data structures for which HList certainly offers a solution. While HList is implemented through type-level programming I wonder if I can achieve similar results through value-level programming alone. This is where I was thinking of functional references. I wonder if or rather how one could do this: Let's say there was some clever monad ... someMonad = do h1 <- add "twenty" h2 <- add False h3 <- add 16 ..... modify h2 True and get a ("twenty",(True, 16)) back. And while *in* the monad some accessors available. Now come to think of it I think I actually read about this somewhere so I doubt this is truly my idea. Anybody got some thoughts on this? Günther

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