February 2010 - Haskell-Cafe

Function to detect duplicates
by Rafael Gustavo da Cunha Pereira Pinto 26 Feb '10

26 Feb '10

Hi folks, While solving a puzzle, I was posed the problem of finding if there was no duplicates on a list. First I used: noneRepeated=null.(filter (>1)).(map length).group.sort But this seemed very unneficient, so I thought that I could detect the duplicates while sorting, and devised this: import Control.Monad import Data.Maybe noneRepeated=isNothing . (foldl merge (Just [])) . (map sort) . pairs pairs []=[] pairs [x]=[[x]] pairs (x:y:xs)=[x,y]:pairs xs sort []=Just [] sort [x]=Just [x] sort [x,y] | x>y=Just [y,x] | y>x=Just [x,y] | x==y=Nothing merge::(Eq a, Ord a) => Maybe [a]->Maybe [a]->Maybe[a] merge _ Nothing = Nothing merge Nothing _ = Nothing merge (Just []) (Just xs)=Just xs merge (Just xs) (Just [])=Just xs merge (Just (x:xs)) (Just (y:ys)) | x==y = Nothing | x>y = (Just y) +? (merge (Just (x:xs)) (Just ys)) | x<y = (Just x) +? (merge (Just xs) (Just (y:ys))) (+?) = liftM2 (:) My version of the merge sort returns Nothing whenever it finds two equal entries, aborting all subsequent comparisons. I have a few questions for the friendly people at this cafe: 1) Is there any improvement I can make? 2) Can it be parallelized (par, pseq)? Best regards, Rafael

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ANNOUNCE: wxHaskell for ghc-6.12.1
by Daniel Fischer 26 Feb '10

26 Feb '10

Hi all, there's a new suite of wxHaskell on hackage, http://hackage.haskell.org/package/wxdirect-0.12.1.2 http://hackage.haskell.org/package/wxcore-0.12.1.3 http://hackage.haskell.org/package/wx-0.12.1.3 If you have ghc-6.12.1 and wxWidgets-2.8, a simple cabal install wx should give you wx GUI goodness. It built fine on my openSUSE, and I don't see any dependencies which look like they weren't unproblematically cross- platform. Unfortunately I was a little too restrictive with the constraints on containers, so it doesn't work with ghc-6.10, sorry.

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vector to uvector and back again
by stefan kersten 26 Feb '10

26 Feb '10

hi, i've been using the vector [1] library for implementing some signal processing algorithms, but now i'd like to use the statistics [2] package on my data, which is based on the uvector [3] library. is there a (straightforward) way of converting between vectors and uvectors, preferrably O(1)? thanks, <sk> [1] http://hackage.haskell.org/package/vector [2] http://hackage.haskell.org/package/statistics [3] http://hackage.haskell.org/package/uvector

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Proper round-trip HughesPJ/Parsec for Doubles?
by Andy Gimblett 26 Feb '10

26 Feb '10

Hi all, Short version: How can I pretty print and parse values of type Double such that those operations are each other's inverse? Long version: I'm writing and QuickCheck-testing a parser using the approach set out here: http://lstephen.wordpress.com/2007/07/29/parsec-parser-testing-with-quickch… That is, each syntactic category gets a pretty-printer and a parser and an Arbitrary instance, and QuickCheck checks that (parse . prettyPrint) == id, basically. Somewhat unsurprisingly, this sometimes fails for floating point values (I'm using Doubles). Now, I know that floats are in some sense imprecise, and comparing for equality is fraught with peril, but it seems that if x==x then it ought to be at least _possible_ to arrange matters such that (parse . prettyPrint x) == x as well. At worst, pretty-printing the underlying binary representation!? So my feeling is that my parser could be improved. At the moment I'm working around it by defining a type class which checks for equality within some margin of error, and using that instead of Eq - but it's messier than I'd like, so I wondered if there was something obvious I'm missing. As hpaste.org seems to be down, I'll attach a code example here instead. Thanks! -Andy -- Andy Gimblett http://gimbo.org.uk/

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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 25 Feb '10

25 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 25 Feb '10

25 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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